CiteULike: klouie's saccade

A Neural Correlate of Motivational Conflict in the Superior Colliculus of the Macaque

Masaki Isoda — 2008-07-03T16:23:32-00:00

J Neurophysiol (2 July 2008), 90275.2008.

Behavior is controlled by both external instructions and internal motives, but the actions demanded by each may be different. A common consequence of such a conflict is a delay in decision making and subsequent motor responses. It is unknown, however, what neural mechanisms underlie motivational conflict and associated response delay. To answer this question, we recorded single-neuron activity in the superior colliculus (SC) as macaque monkeys performed a visually guided, asymmetrically rewarded saccade task. A peripheral spot of light at one of two opposing positions was illuminated to indicate a saccade target. In a given block of trials, one position was associated with a big reward and the other with a small reward. The big-reward position was alternated across blocks. Behavioral analyses revealed that small-reward trials created a conflict between the instructed saccade to one position and the internally motivated, yet invalid saccade to the opposite position. We found that movement neurons in the SC temporally exhibited bursting activity after the appearance of the small-reward target opposite their movement field. This transient activity predicted the amount of response delay for upcoming saccades. Our data suggest that motivational conflict activates movement neurons in both colliculi, thereby delaying saccade initiation through intercollicular inhibitory interactions. 10.1152/jn.90275.2008

Coding of Shape and Position in Macaque Lateral Intraparietal Area

Peter Janssen — 2008-06-25T17:31:53-00:00

J. Neurosci., Vol. 28, No. 26. (25 June 2008), pp. 6679-6690.

The analysis of object shape is critical for both object recognition and grasping. Areas in the intraparietal sulcus of the rhesus monkey are important for the visuomotor transformations underlying actions directed toward objects. The lateral intraparietal (LIP) area has strong anatomical connections with the anterior intraparietal area, which is known to control the shaping of the hand during grasping, and LIP neurons can respond selectively to simple two-dimensional shapes. Here we investigate the shape representation in area LIP of awake rhesus monkeys. Specifically, we determined to what extent LIP neurons are tuned to shape dimensions known to be relevant for grasping and assessed the invariance of their shape preferences with regard to changes in stimulus size and position in the receptive field. Most LIP neurons proved to be significantly tuned to multiple shape dimensions. The population of LIP neurons that were tested showed barely significant size invariance. Position invariance was present in a minority of the neurons tested. Many LIP neurons displayed spurious shape selectivity arising from accidental interactions between the stimulus and the receptive field. We observed pronounced differences in the receptive field profiles determined by presenting two different shapes. Almost all LIP neurons showed spatially selective saccadic activity, but the receptive field for saccades did not always correspond to the receptive field as determined using shapes. Our results demonstrate that a subpopulation of LIP neurons encodes stimulus shape. Furthermore, the shape representation in the dorsal visual stream appears to differ radically from the known representation of shape in the ventral visual stream. 10.1523/JNEUROSCI.0499-08.2008

Properties of saccades generated as a choice response.

KM Lee — 2008-06-23T21:50:43-00:00

Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale, Vol. 162, No. 3. (April 2005), pp. 278-286.

Since Hick's original description, many subsequent studies have confirmed the logarithmic relationship that exists between response time and the number of alternatives (NA) for a choice response. In the present study a novel paradigm was used to quantify saccade response time as a function of NA. Normal subjects were required to make a saccade to the remembered location of a visual target whose color was specified by a centrally located cue. The paradigm thus required a stimulus-response transformation similar to that used by Hick. The results show that, when such a transformation was required, a logarithmic relationship was found for saccadic response time. The use of a color-to-location paradigm to study saccade choice response time produced an unexpected additional result that may provide insight into the neural organization of the saccadic system. When the number of alternative choice responses was large (4 or 8), subjects frequently made a two-saccade response instead of a single saccade to the correct location. The first movement in such a sequence was in the correct direction, but was hypometric. A second movement then followed which moved the eyes onto the correct location. These results suggest dynamic dissociations in the mechanisms underlying the triggering of saccades and the specification of their metrics.

Expectation of reward modulates cognitive signals in the basal ganglia.

R Kawagoe — 2007-01-29T21:56:50-00:00

Nat Neurosci, Vol. 1, No. 5. (September 1998), pp. 411-416.

Action is controlled by both motivation and cognition. The basal ganglia may be the site where these kinds of information meet. Using a memory-guided saccade task with an asymmetric reward schedule, we show that visual and memory responses of caudate neurons are modulated by expectation of reward so profoundly that a neuron's preferred direction often changed with the change in the rewarded direction. The subsequent saccade to the target was earlier and faster for the rewarded direction. Our results indicate that the caudate contributes to the determination of oculomotor outputs by connecting motivational values (for example, expectation of reward) to visual information.

Reward-dependent gain and bias of visual responses in primate superior colliculus.

T Ikeda — 2008-05-06T22:49:25-00:00

Neuron, Vol. 39, No. 4. (14 August 2003), pp. 693-700.

Eye movements are often influenced by expectation of reward. Using a memory-guided saccade task with an asymmetric reward schedule, we show that visual responses of monkey SC neurons increase when the visual stimulus indicates an upcoming reward. The increase occurred in two distinct manners: (1) reactively, as an increase in the gain of the visual response when the stimulus indicated an upcoming reward; (2) proactively, as an increase in anticipatory activity when reward was expected in the neuron's response field. These effects were observed mostly in saccade-related SC neurons in the deeper layer which would receive inputs from the cortical eye fields and the basal ganglia. These results, together with recent findings, suggest that the gain modulation may be determined by the inputs from both the cortical eye fields and the basal ganglia, whereas the anticipatory bias may be derived mainly from the basal ganglia.

Orienting of attention.

MI Posner — 2007-05-15T22:44:46-00:00

Q J Exp Psychol, Vol. 32, No. 1. (February 1980), pp. 3-25.

Saccade and blinking evoked by microstimulation of the posterior parietal association cortex of the monkey.

H Shibutani — 2008-05-05T18:35:10-00:00

Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale, Vol. 55, No. 1. (1984), pp. 1-8.

Electrical stimulation with microelectrodes of the posterior parietal association cortex in alert behaving monkeys elicited saccadic eye movements and blinking. The sites in which saccades were elicited by electrical stimulation were concentrated in the anteromedial part of area 7a, especially in the posterior bank of the intraparietal sulcus, in a region which sends efferent projections to the frontal eye field and the superior colliculus, but they were also found in the posterolateral part of area 7a. Compared with the frontal eye fields and the superior colliculus, the threshold current for eliciting saccades was relatively high, on the average 86 microA. Moreover, the elicitation of saccade was inconsistent even with suprathreshold stimulation and suppressed during visual fixation. Latencies of the saccades were relatively long, on the average 50ms; they were longer in the posterolateral part than in the anteromedial part. Direction and amplitude of evoked saccades depended on the site of stimulation, but was independent of eye position in most cases. However, "goal-directed" saccades which depended on initial eye position were elicited in three penetrations in the posterolateral part of area 7a. The threshold of mainly in the lateral part of area 7a. The threshold of blinking was 70 microA and the latency was 50 ms on the average. In contrast to saccades, blinking was elicited constantly with each stimulus even during attentive fixation. We occasionally recorded single unit activity at the site of stimulation with the same electrodes. More than half of the units recorded at the site of blinking responded to approaching visual stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Deficits of visual attention and saccadic eye movements after lesions of parietooccipital cortex in monkeys.

JC Lynch — 2008-05-04T01:43:17-00:00

Journal of neurophysiology, Vol. 61, No. 1. (January 1989), pp. 74-90.

1. Visual attention is often profoundly disturbed in humans after damage to the cortex of the posterior parietal lobe, particularly of the minor hemisphere, with some patients being totally unaware of visual stimuli in the hemifield of extrapersonal space contralateral to the cortical damage. This severe form of visual inattention is usually called contralateral neglect and has occasionally been reported following posterior parietal lesions in monkeys. However, in monkeys, only qualitative observations have been published and those reports are not in agreement concerning the severity of the deficit. The present experiments were designed to measure quantitatively the amount of disruption of selective visual attention which is produced by lesions of posterior parietal and parietooccipital cortical lesions in monkeys. 2. Five monkeys were trained to visually fixate and follow with their gaze a small visual stimulus as it suddenly moved varying distances (8, 16, or 24 degrees) from the midline into the left or right visual hemifields. Two animals then received a unilateral cortical lesion limited to the inferior parietal lobule (IPL). Three animals received unilateral lesions which included both the inferior parietal lobule and a portion of adjacent dorsal prestriate cortex (IPL-PS). 3. Visual inattention is commonly divided into two levels of severity. The more severe form, contralateral neglect, is the complete absence of behavioral response to a stimulus in the visual field contralateral to hemisphere damage. The less severe deficit, usually called visual extinction, is a tendency to ignore the contralateral of two visual stimuli when they appear simultaneously and symmetrically placed with respect to the center of the subject's surroundings. The five monkeys in this study were tested on a stimulus paradigm which simultaneously measured the severity of visual neglect and also the amount and duration of visual extinction which were produced by the cortical lesions. 4. All monkeys displayed contralateral visual extinction after unilateral posterior parietal or parietooccipital lesions. Three of the five monkeys showed a reversal of the visual extinction after a second, symmetrical lesion was placed in the opposite hemisphere. No monkey showed evidence of full-blown contralateral neglect after lesions limited to the parietooccipital cortex, either in the formal testing situation or during informal neurological examinations. The severity of the observed inattention did not appear to be related to the size of the cortical lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

Response of neurons in the lateral intraparietal area to a distractor flashed during the delay period of a memory-guided saccade.

KD Powell — 2008-04-30T14:51:07-00:00

Journal of neurophysiology, Vol. 84, No. 1. (July 2000), pp. 301-310.

Recent experiments raised the possibility that the lateral intraparietal area (LIP) might be specialized for saccade planning. If this was true, one would expect a decreased sensitivity to irrelevant visual stimuli appearing late in the delay period of a memory-guided delayed-saccade task to a target outside the neurons' receptive fields. We trained two monkeys to perform a standard memory-guided delayed-saccade task and a distractor task in which a stimulus flashed for 200 ms at a predictable time 300-100 ms before the end of the delay period. We used two locations, one in the most active part of the receptive field and another well outside the receptive field. We used six kinds of trials randomly intermixed: simple delayed-saccade trials into or away from the receptive field and distractor trials with saccade target and distractor both in the receptive field, both out of the receptive field, or one at each location. This enabled us to study the response to the distractor as a function of the monkey's preparation of a memory-guided delayed-saccade task. We had assumed that the preparation of a saccade away from the receptive field would result in an attenuation of the response to the distractor, i.e., a distractor at the location of the saccade goal would evoke a greater response than when it appeared at a location far from the saccade goal. Instead we found that neurons exhibited either a normal or an enhanced visual response to the distractor during the memory period when the target flashed outside the receptive field. When the distractor flashed at the location of the saccade target, the response to the distractor was either unchanged or diminished. The response to a distractor away from the target location of a memory-guided saccade was even greater than the response to the same target when it was the target for the memory-guided saccade task. Immediate presaccadic activity did not distinguish between a saccade to the receptive field when there was no distractor and a distractor in the receptive field when the monkey made a saccade elsewhere. Nonetheless the distractor had no significant effect on the saccade latency, accuracy, or velocity despite the brisk response it evoked immediately before the saccade. We suggest that these results are inconsistent with a role for LIP in the specific generation of saccades, but they are consistent with a role for LIP in the generation of visual attention.

A microcircuit model of the frontal eye fields.

J Heinzle — 2007-12-21T21:45:52-00:00

J Neurosci, Vol. 27, No. 35. (29 August 2007), pp. 9341-9353.

The cortical control of eye movements is highly sophisticated. Not only can eye movements be made to the most salient target in a visual scene, but they can also be controlled by top-down rules as is required for visual search or reading. The cortical area called frontal eye fields (FEF) has been shown to play a key role in the visual to oculomotor transformations in tasks requiring an eye movement pattern that is not completely reactive, but follows a previously learned rule. The layered, local cortical circuit, which provides the anatomical substrate for all cortical computation, has been studied extensively in primary sensory cortex. These studies led to the concept of a "canonical circuit" for neocortex (Douglas et al., 1989; Douglas and Martin, 1991), which proposes that all areas of neocortex share a common basic circuit. However, it has not ever been explored whether in principle the detailed canonical circuit derived from cat area 17 (Binzegger et al., 2004) could implement the quite different functions of prefrontal cortex. Here, we show that the canonical circuit can, with a few modifications, model the primate FEF. The spike-based network of integrate-and-fire neurons was tested in tasks that were used in electrophysiological experiments in behaving macaque monkeys. The dynamics of the model matched those of neurons observed in the FEF, and the behavioral results matched those observed in psychophysical experiments. The close relationship between the model and the cortical architecture allows a detailed comparison of the simulation results with physiological data and predicts details of the anatomical circuit of the FEF.

Temporal Patterning of Saccadic Eye Movement Signals

Daniel Kimmel — 2007-07-19T13:42:35-00:00

J. Neurosci., Vol. 27, No. 29. (18 July 2007), pp. 7619-7630.

Electrical microstimulation is used widely in experimental neurophysiology to examine causal links between specific brain areas and their behavioral functions and is used clinically to treat neurological and psychiatric disorders in patients. Typically, microstimulation is applied to local brain regions as a train of equally spaced current pulses. We were interested in the sensitivity of a neural circuit to a train of variably spaced pulses, as is observed in physiological spike trains. We compared the effect of fixed, decelerating, accelerating, and randomly varying microstimulation patterns on the likelihood and metrics of eye movements evoked from the frontal eye field of monkeys, while holding the mean interpulse interval constant. Our results demonstrate that the pattern of microstimulation pulses strongly influences the probability of evoking a saccade, as well as the metrics of the saccades themselves. Specifically, the pattern most closely resembling physiological spike trains (accelerating pattern) was most effective at evoking a saccade, three times more so than the least effective decelerating pattern. A saccade-triggered average of effective random trains confirmed the positive relationship between accelerating rate and efficacy. These results have important implications for the use of electrical microstimulation in both experimental and clinical settings and suggest a means to study the role of temporal pattern in the encoding of behavioral and cognitive functions. 10.1523/JNEUROSCI.0386-07.2007

Neuronal switching of sensorimotor transformations for antisaccades.

M Zhang — 2007-05-10T21:48:07-00:00

Nature, Vol. 408, No. 6815. (8 2000), pp. 971-975.

The influence of cognitive context on orienting behaviour can be explored using the mixed memory-prosaccade, memory-antisaccade task. A symbolic cue, such as the colour of a visual stimulus, instructs the subject to make a brief, rapid eye movement (a saccade) either towards the stimulus (prosaccade) or in the opposite direction (antisaccade). Thus, the appropriate sensorimotor transformation must be switched on to execute the instructed task. Despite advances in our understanding of the neuronal processing of antisaccades, it remains unclear how the brain selects and computes the sensorimotor transformation leading to an antisaccade. Here we show that area LIP of the posterior parietal cortex is involved in these processes. LIP's population activity turns from the visual direction to the motor direction during memory-antisaccade trials. About one-third of the visual neurons in LIP produce a brisk, transient discharge in certain memory-antisaccade trials. We call this discharge 'paradoxical' because its timing is visual-like but its direction is motor. The paradoxical discharge shows, first, that switching occurs already at the level of visual cells, as previously proposed by Schlag-Rey and colleagues; and second, that this switching is accomplished very rapidly, within 50 ms from the arrival of the visual signals in LIP.

Progression in neuronal processing for saccadic eye movements from parietal cortex area lip to superior colliculus.

M Paré — 2007-05-07T16:13:08-00:00

J Neurophysiol, Vol. 85, No. 6. (June 2001), pp. 2545-2562.

Neurons in both the lateral intraparietal area (LIP) of the monkey parietal cortex and the intermediate layers of the superior colliculus (SC) are activated well in advance of the initiation of saccadic eye movements. To determine whether there is a progression in the covert processing for saccades from area LIP to SC, we systematically compared the discharge properties of LIP output neurons identified by antidromic activation with those of SC neurons collected from the same monkeys. First, we compared activity patterns during a delayed saccade task and found that LIP and SC neurons showed an extensive overlap in their responses to visual stimuli and in their sustained activity during the delay period. The saccade activity of LIP neurons was, however, remarkably weaker than that of SC neurons and never occurred without any preceding delay activity. Second, we assessed the dependence of LIP and SC activity on the presence of a visual stimulus by contrasting their activity in delayed saccade trials in which the presentation of the visual stimulus was either sustained (visual trials) or brief (memory trials). Both the delay and the presaccadic activity levels of the LIP neuronal sample significantly depended on the sustained presence of the visual stimulus, whereas those of the SC neuronal sample did not. Third, we examined how the LIP and SC delay activity relates to the future production of a saccade using a delayed GO/NOGO saccade task, in which a change in color of the fixation stimulus instructed the monkey either to make a saccade to a peripheral visual stimulus or to withhold its response and maintain fixation. The average delay activity of both LIP and SC neuronal samples significantly increased by the advance instruction to make a saccade, but LIP neurons were significantly less dependent on the response instruction than SC neurons, and only a minority of LIP neurons was significantly modulated. Thus despite some overlap in their discharge properties, the neurons in the SC intermediate layers showed a greater independence from sustained visual stimulation and a tighter relationship to the production of an impending saccade than the LIP neurons supplying inputs to the SC. Rather than representing the transmission of one processing stage in parietal cortex area LIP to a subsequent processing stage in SC, the differences in neuronal activity that we observed suggest instead a progressive evolution in the neuronal processing for saccades.

A pathway in primate brain for internal monitoring of movements.

MA Sommer — 2006-11-09T16:29:50-00:00

Science, Vol. 296, No. 5572. (24 May 2002), pp. 1480-1482.

It is essential to keep track of the movements we make, and one way to do that is to monitor correlates, or corollary discharges, of neuronal movement commands. We hypothesized that a previously identified pathway from brainstem to frontal cortex might carry corollary discharge signals. We found that neuronal activity in this pathway encodes upcoming eye movements and that inactivating the pathway impairs sequential eye movements consistent with loss of corollary discharge without affecting single eye movements. These results identify a pathway in the brain of the primate Macaca mulatta that conveys corollary discharge signals.

Population coding of saccadic eye movements by neurons in the superior colliculus

Choongkil Lee — 2006-11-03T15:46:34-00:00

Nature, Vol. 332, No. 6162. (24 March 1988), pp. 357-360.

Movement selection in advance of action in the superior colliculus.

PW Glimcher — 2005-02-22T21:50:09-00:00

Nature, Vol. 355, No. 6360. (6 February 1992), pp. 542-545.

The primate superior colliculus contains a map of saccadic eye movements. Saccades are high-velocity eye movements to selected targets in the visual field, but little is known about the neural mechanisms responsible for target selection or the related problem of choosing a particular movement from the oculomotor repertoire. Two classes of neurons have been described in the superior colliculus which show bursts of activity before the saccade: discrete bursters display a vigorous pre-saccadic burst and prelude bursters show low-frequency activity as a prelude to burst onset. We have designed experiments to test whether prelude activity is related to saccade selection. Our tasks use a cue to specify which of two physically identical visual stimuli is the goal of an impending saccade. This cue is spatially and temporally isolated from the potential targets as well as from visual cues signalling movement initiation. Our results show that prelude activity occurs shortly after information is available for correct saccade selection and, more importantly, the activity is predictive of saccade choice. The results thus suggest that the superior colliculus participates in the process of saccade selection.

Neural mechanisms underlying target selection with saccadic eye movements.

PH Schiller — 2006-10-25T15:32:39-00:00

Prog Brain Res, Vol. 149 (2005), pp. 157-171.

In exploring the visual scene we make about three saccadic eye movements per second. During each fixation, in addition to analyzing the object at which we are looking, a decision has to be made as to where to look next. Although we perform this task with the greatest of ease, the computations to perform the task are complex and involve numerous brain structures. We have applied several investigative tools that include single-cell recordings, microstimulation, pharmacological manipulations and lesions to learn more about the neural control of visually guided eye saccadic movements. Electrical stimulation of the superior colliculus (SC), areas V1 and V2, the lateral intraparietal sulcus (LIP), the frontal eye fields (FEF) and the medial eye fields (MEF) produces saccadic eye movements at low current levels. After ablation of the SC, electrical microstimulation of V1, V2, and LIP no longer elicits saccadic eye movements whereas stimulation of the FEF and MEF continues to be effective. Ablation of the SC but not of the FEF eliminates short-latency saccadic eye movements to visual targets called "express saccades," whereas lesions of the FEF selectively interfere with target selection. Bilateral removal of both the SC and the FEF causes major, long lasting deficits: all visually elicited saccadic eye movements are eliminated. In intact monkeys, subthreshold electrical microstimulation of the FEF and MEF as well as the lower layers of V1 and V2 and of some subregions of LIP greatly facilitates the choice of targets presented in the receptive fields of the stimulated neurons. By contrast, stimulation of the upper layers of V1 and V2 and other sub-regions of LIP produces a dramatic interference in target selection. Examination of the role of inhibitory circuits in eye-movement generation reveals that local infusion of muscimol, a GABA (gamma-aminobutyric acid) agonist, or bicuculline, a GABA antagonist, interferes with target selection in V1. On the other hand, infusion of bicuculline into the FEF produces facilitation in target choice and irrepressible saccades. It appears therefore that inhibitory circuits play a central role in visual analysis in V1 and in the generation of saccadic eye movements in the FEF. It is proposed that two major streams can be discerned in visually guided eye-movement control, the posterior from occipital and parietal cortex that reaches the brainstem via the SC and the anterior from the FEF and MEF that has direct access to the brainstem oculomotor centers.

Role of the basal ganglia in the control of purposive saccadic eye movements.

O Hikosaka — 2006-09-18T19:23:44-00:00

Physiol Rev, Vol. 80, No. 3. (July 2000), pp. 953-978.

In addition to their well-known role in skeletal movements, the basal ganglia control saccadic eye movements (saccades) by means of their connection to the superior colliculus (SC). The SC receives convergent inputs from cerebral cortical areas and the basal ganglia. To make a saccade to an object purposefully, appropriate signals must be selected out of the cortical inputs, in which the basal ganglia play a crucial role. This is done by the sustained inhibitory input from the substantia nigra pars reticulata (SNr) to the SC. This inhibition can be removed by another inhibition from the caudate nucleus (CD) to the SNr, which results in a disinhibition of the SC. The basal ganglia have another mechanism, involving the external segment of the globus pallidus and the subthalamic nucleus, with which the SNr-SC inhibition can further be enhanced. The sensorimotor signals carried by the basal ganglia neurons are strongly modulated depending on the behavioral context, which reflects working memory, expectation, and attention. Expectation of reward is a critical determinant in that the saccade that has been rewarded is facilitated subsequently. The interaction between cortical and dopaminergic inputs to CD neurons may underlie the behavioral adaptation toward purposeful saccades.

Reward-predicting activity of dopamine and caudate neurons--a possible mechanism of motivational control of saccadic eye movement.

R Kawagoe — 2005-08-01T15:57:23-00:00

J Neurophysiol, Vol. 91, No. 2. (February 2004), pp. 1013-1024.

Recent studies have suggested that the basal ganglia are related to motivational control of behavior. To study how motivational signals modulate motor signals in the basal ganglia, we examined activity of midbrain dopamine (DA) neurons and caudate (CD) projection neurons while monkeys were performing a one-direction-rewarded version (1DR) of memory-guided saccade task. The cue stimulus indicated the goal position for an upcoming saccade and the presence or absence of reward after the trial. Among four monkeys we studied, three were sensitive to reward such that saccade velocity was significantly higher in the rewarded trials than in the nonrewarded trials; one monkey was insensitive to reward. In the reward-sensitive monkeys, both DA and CD neurons responded differentially to reward-indicating and no-reward-indicating cues. Thus DA neurons responded with excitation to a reward-indicating cue and with inhibition to a no-reward-indicating cue. A group of CD neurons responded to the cue in their response fields (mostly contralateral) and the cue response was usually enhanced when it indicated reward. In the reward-insensitive monkey, DA neurons showed no response to the cue, while the cue responses of CD neurons were not modulated by reward. Many CD neurons in the reward-sensitive monkeys, but not the reward-insensitive monkey, showed precue activity. These results suggest that DA neurons, with their connection to CD neurons, modulate the spatially selective signals in CD neurons in the reward-predicting manner and CD neurons in turn modulate saccade parameters with their polysynaptic connections to the oculomotor brain stem.

Role of primate substantia nigra pars reticulata in reward-oriented saccadic eye movement.

M Sato — 2006-09-18T14:26:16-00:00

J Neurosci, Vol. 22, No. 6. (15 March 2002), pp. 2363-2373.

To test the hypothesis that the basal ganglia are related to reward-oriented saccades, we examined activity of substantia nigra pars reticulata (SNr) neurons by using a one-direction-rewarded version of the memory-guided saccade task (1DR). Many SNr neurons changed (decreased or increased) their activity after and before a visual cue (post-cue and pre-cue activity). Post-cue decreases or increases tended to be larger to a contralateral cue. They were often modulated prospectively by the presence or absence of reward, either positively (enhanced in the rewarded condition) or negatively (enhanced in the nonrewarded condition). The positive reward modulation was more common among decreasing type neurons, whereas no such preference was observed among increasing type neurons. The reward-contingent decrease in SNr neuronal activity would facilitate rewarded saccades by inducing disinhibition in superior colliculus (SC) neurons. In contrast, the increase in SNr activity would suppress a saccade less selectively (rewarded or nonrewarded) by augmenting inhibition of SC neurons. The post-cue activity was often preceded by anticipatory pre-cue activity. Most typically, post-cue decrease was preceded by pre-cue decrease, selectively when the contralateral side was rewarded. This would reinforce the reward-oriented nature of SNr neuronal activity. The decreases and increases in SNr activity may be derived directly and indirectly, respectively, from the caudate (CD), where neurons show reward-contingent pre-cue and post-cue activity. These results suggest that the CD-SNr-SC mechanism would promote saccades oriented to reward.

Functional properties of monkey caudate neurons. I. Activities related to saccadic eye movements.

O Hikosaka — 2006-09-14T16:34:56-00:00

J Neurophysiol, Vol. 61, No. 4. (April 1989), pp. 780-798.

1. We recorded single cell activities in the caudate nucleus of the monkeys trained to perform a series of visuomotor tasks. In the first part of this paper, we summarize the types and locations of neurons in the monkey caudate nucleus. In the second part, we report the characteristics of neurons related to saccadic eye movements. 2. Neurons were classified into two types in terms of spontaneous discharge pattern. A majority of the neurons (2,287/2,559, 89%) had very low-frequency discharges (mostly less than 1 Hz). The rest (n = 272) showed irregular-tonic discharges (3-8 Hz) with broad spikes. 3. Of 2,559 neurons tested, 867 showed spike activity related to some aspects of the tasks; 502 neurons showed discharges in response to environmental changes outside, not in relation to, the tasks. None of the neurons responsive in or outside the tasks belonged to the irregular-tonic type. 4. The task-related activities were classified as: Saccade-related, Visual, Auditory, Cognitive, Fixation-related, and Reward-related. The activities detected outside the tasks were classified into: Visual, Auditory, Movement-related, Reward-related, and Other. Few neurons had both task-related and task-unrelated activities. 5. The locations of recorded neurons were determined using a coordinate system based on the anterior and posterior commissures. Task-related neurons were clustered longitudinally in the central part of the caudate. Neurons responsive outside the tasks were more widely distributed; specifically, auditory neurons were in the medial part, whereas movement-related neurons were in the lateral part. The irregular-tonic neurons were dispersed all over the caudate. 6. The monkey was trained to fixate on a spot of light on the screen and, when the spot moved, to follow it by making a saccade. A visually guided saccade occurred when the spot moved to another location without a time gap (saccade task). A memory-guided saccade occurred when the spot first disappeared and after a time gap reappeared at a fixed location (saccade with gap task). By delivering a cue stimulus while the monkey was fixating, a memory-guided saccade was elicited to a randomly chosen location (delayed saccade task).(ABSTRACT TRUNCATED AT 400 WORDS)

LIP responses to a popout stimulus are reduced if it is overtly ignored

Anna Ipata — 2006-07-26T12:30:16-00:00

Nature Neuroscience, Vol. 9, No. 8. (02 July 2006), pp. 1071-1076.

Selection and Maintenance of Saccade Goals in the Human Frontal Eye Fields.

Clayton E Curtis — 2006-02-11T09:25:27-00:00

J Neurophysiol (8 February 2006)

In a delayed-response task, response selection marks an important transition from sensory to motor processing. Using event-related fMRI, we imaged the human brain during performance of a novel delayed-saccade task that isolated response selection from visual encoding and motor execution. The frontal eye-fields (FEF) and intraparietal sulcus (IPS) both showed robust contra-lateralized activity time-locked to response selection. Moreover, response selection affected delay-period activity differently in these regions; it persisted throughout the memory delay period following response selection in the FEF, but not IPS. Our results indicate that the FEF and IPS both make important but distinct contributions to spatial working memory. The mechanism that the FEF uses to support spatial working memory is tied to the selection and prospective coding of saccade goals, while the role of the IPS may be more tied to retrospective coding of sensory representations.

Look and see: how the brain moves your eyes about.

PH Schiller — 2006-06-05T16:05:43-00:00

Prog Brain Res, Vol. 134 (2001), pp. 127-142.

Two major cortical streams are involved in the generation of visually guided saccadic eye movements: the anterior and the posterior. The anterior stream from the frontal and medial eye fields has direct access to brainstem oculomotor centers. The posterior stream from the occipital cortices reaches brainstem oculomotor centers through the superior colliculus. The parietal cortex interconnects with both streams. Our findings suggest that the posterior stream plays an unique role in the execution of rapid, short-latency eye movements called 'express saccades'. Both the anterior and posterior streams play a role in the selection of targets to which saccades are to be generated, but do so in different ways. Areas V1, V2 and LIP contribute to decisions involved in where to look as well as where not to look. In addition, area LIP is involved in decisions about how long to maintain fixation prior to the execution of a saccade. Area V4 does not appear to be directly involved in eye-movement generation. In the anterior stream, the frontal eye fields, and to a lesser extent the medial eye fields, are involved in the correct execution of saccades subsequent to decisions made about where to look and where not to look.

Shape Representations and Visual Guidance of Saccadic Eye Movements

Tirin Moore — 2006-04-12T20:31:00-00:00

Science, Vol. 285, No. 5435. (17 September 1999), pp. 1914-1917.

10.1126/science.285.5435.1914

Changes in visual receptive fields with microstimulation of frontal cortex.

KM Armstrong — 2006-06-02T14:44:10-00:00

Neuron, Vol. 50, No. 5. (1 June 2006), pp. 791-798.

The influence of attention on visual cortical neurons has been described in terms of its effect on the structure of receptive fields (RFs), where multiple stimuli compete to drive neural responses and ultimately behavior. We stimulated the frontal eye field (FEF) of passively fixating monkeys and produced changes in V4 responses similar to known effects of voluntary attention. Subthreshold FEF stimulation enhanced visual responses at particular locations within the RF and altered the interaction between pairs of RF stimuli to favor those aligned with the activated FEF site. Thus, we could influence which stimulus drove the responses of individual V4 neurons. These results suggest that spatial signals involved in saccade preparation are used to covertly select among multiple stimuli appearing within the RFs of visual cortical neurons.

Selective gating of visual signals by microstimulation of frontal cortex.

T Moore — 2006-06-01T20:29:41-00:00

Nature, Vol. 421, No. 6921. (23 January 2003), pp. 370-373.

Several decades of psychophysical and neurophysiological studies have established that visual signals are enhanced at the locus of attention. What remains a mystery is the mechanism that initiates biases in the strength of visual representations. Recent evidence argues that, during spatial attention, these biases reflect nascent saccadic eye movement commands. We examined the functional interaction of saccade preparation and visual coding by electrically stimulating sites within the frontal eye fields (FEF) and measuring its effect on the activity of neurons in extrastriate visual cortex. Here we show that visual responses in area V4 could be enhanced after brief stimulation of retinotopically corresponding sites within the FEF using currents below that needed to evoke saccades. The magnitude of the enhancement depended on the effectiveness of receptive field stimuli as well as on the presence of competing stimuli outside the receptive field. Stimulation of non-corresponding FEF representations could suppress V4 responses. The results suggest that the gain of visual signals is modified according to the strength of spatially corresponding eye movement commands.

The effects of frontal eye field and dorsomedial frontal cortex lesions on visually guided eye movements.

PH Schiller — 2006-05-31T23:02:02-00:00

Nat Neurosci, Vol. 1, No. 3. (July 1998), pp. 248-253.

In the frontal lobe of primates, two areas play a role in visually guided eye movements: the frontal eye fields (FEF) and the medial eye fields (MEF) in dorsomedial frontal cortex. Previously, FEF lesions have revealed only mild deficits in saccadic eye movements that recovered rapidly. Deficits in eye movements after MEF ablation have not been shown. We report the effects of ablating these areas singly or in combination, using tests in which animals were trained to make saccadic eye movements to paired or multiple targets presented at various temporal asynchronies. FEF lesions produced large and long-lasting deficits on both tasks. Sequences of eye movements made to successively presented targets were also impaired. Much smaller deficits were observed after MEF lesions. Our findings indicate a major, long-lasting loss in temporal ordering and processing speed for visually guided saccadic eye movement generation after FEF lesions and a significant but smaller and shorter-lasting loss after MEF lesions.

Electrical microstimulation distinguishes distinct saccade-related areas in the posterior parietal cortex.

P Thier — 2005-05-03T05:36:54-00:00

J Neurophysiol, Vol. 80, No. 4. (October 1998), pp. 1713-1735.

Electrical microstimulation (0.1-ms bipolar pulses at 500 Hz, current strength usually between 100 and 200 microA) was used to delineate saccade-related areas in the posterior parietal cortex of monkeys. Stimulation-induced saccades were found to be restricted to the lateral intraparietal area (area LIP) in the intraparietal sulcus (IPS) and a region on the medial aspect of the parietal lobe (area MP, medial parietal area), close to the caudal end of the cingulate sulcus, whereas stimulation of area 7a did not evoke eye movements. Two different types of evoked saccades were observed. Modified vector saccades, whose amplitude was modified by the position of the eyes at stimulation onset were the hallmark of sites in area LIP and area MP. The same sites were characterized by a propensity of single units active in the memory and presaccadic response segments of the memory saccade paradigm. Goal-directed saccades driving the eyes toward a circumscribed region relative to the head were largely restricted to a small strip of cortex on the lateral bank and the floor of the IPS (the intercalated zone), separating the representation of upward and downward directed saccades in LIP. Unlike stimulation in LIP or MP, stimulation in the intercalated zone gave rise to head, pinnae, facial, and shoulder movements accompanying the evoked saccades. We propose that the amplitude modification of vector saccades characterizing LIP and MP may reflect a spatially distributed head-centered coding scheme for saccades. On the other hand, the goal-directed saccades found in the intercalated zone could indicate the use of a spatially much more localized representation of desired location in head-centered space.

Activity in the lateral intraparietal area predicts the goal and latency of saccades in a free-viewing visual search task.

AE Ipata — 2006-05-24T19:22:01-00:00

J Neurosci, Vol. 26, No. 14. (5 April 2006), pp. 3656-3661.

The purpose of saccadic eye movements is to facilitate vision, by placing the fovea on interesting objects in the environment. Eye movements are not made for reward, and they are rarely restricted. Despite this, most of our knowledge about the neural genesis of eye movements comes from experiments in which specific eye movements are rewarded or restricted. Such experiments have demonstrated that activity in the lateral intraparietal (LIP) area of the monkey correlates with the monkey's planning of a memory-guided saccade or deciding where, on the basis of motion information, to make a saccade. However, other experiments have shown that neural activity in LIP can easily be dissociated from the generation of saccadic eye movements, especially when sophisticated behavioral paradigms dissociate the monkey's locus of attention from the goal of an intended saccade. In this study, we trained monkeys to report the results of a visual search task by making a nontargeting hand movement. Once the task began, the monkeys were entirely free to move their eyes, and rewards were not contingent on the monkeys making specific eye movements. We found that neural activity in LIP predicted not only the goal of the monkey's saccades but also their saccadic latencies.

A model of saccade generation based on parallel processing and competitive inhibition.

JM Findlay — 2006-05-22T21:02:47-00:00

Behav Brain Sci, Vol. 22, No. 4. (August 1999)

During active vision, the eyes continually scan the visual environment using saccadic scanning movements. This target article presents an information processing model for the control of these movements, with some close parallels to established physiological processes in the oculomotor system. Two separate pathways are concerned with the spatial and the temporal programming of the movement. In the temporal pathway there is spatially distributed coding and the saccade target is selected from a "salience map." Both pathways descend through a hierarchy of levels, the lower ones operating automatically. Visual onsets have automatic access to the eye control system via the lower levels. Various centres in each pathway are interconnected via reciprocal inhibition. The model accounts for a number of well-established phenomena in target-elicited saccades: the gap effect, express saccades, the remote distractor effect, and the global effect. High-level control of the pathways in tasks such as visual search and reading is discussed; it operates through spatial selection and search selection, which generally combine in an automated way. The model is examined in relation to data from patients with unilateral neglect.

A visual salience map in the primate frontal eye field.

KG Thompson — 2006-05-22T15:24:16-00:00

Prog Brain Res, Vol. 147 (2005), pp. 251-262.

Models of attention and saccade target selection propose that within the brain there is a topographic map of visual salience that combines bottom-up and top-down influences to identify locations for further processing. The results of a series of experiments with monkeys performing visual search tasks have identified a population of frontal eye field (FEF) visually responsive neurons that exhibit all of the characteristics of a visual salience map. The activity of these FEF neurons is not sensitive to specific features of visual stimuli; but instead, their activity evolves over time to select the target of the search array. This selective activation reflects both the bottom-up intrinsic conspicuousness of the stimuli and the top-down knowledge and goals of the viewer. The peak response within FEF specifies the target for the overt gaze shift. However, the selective activity in FEF is not in itself a motor command because the magnitude of activation reflects the relative behavioral significance of the different stimuli in the visual scene and occurs even when no saccade is made. Identifying a visual salience map in FEF validates the theoretical concept of a salience map in many models of attention. In addition, it strengthens the emerging view that FEF is not only involved in producing overt gaze shifts, but is also important for directing covert spatial attention.

Role of Dopamine in the Primate Caudate Nucleus in Reward Modulation of Saccades

Kae Nakamura — 2006-05-18T13:44:16-00:00

J. Neurosci., Vol. 26, No. 20. (17 May 2006), pp. 5360-5369.

Expected reward impacts behavior and neuronal activity in brain areas involved in sensorimotor processes. However, where and how reward signals affect sensorimotor signals is unclear. Here, we show evidence that reward-dependent modulation of behavior depends on normal dopamine transmission in the striatum. Monkeys performed a visually guided saccade task in which expected reward gain was different depending on the position of the target. Saccadic reaction times were reliably shorter on large-reward trials than on small-reward trials. When position-reward contingency was switched, the reaction time difference changed rapidly. Injecting dopamine D1 antagonist into the caudate significantly attenuated the reward-dependent saccadic reaction time changes. Conversely, injecting D2 antagonist into the same region enhanced the reward-dependent changes. These results suggest that reward-dependent changes in saccadic eye movements depend partly on dopaminergic modulation of neuronal activity in the caudate nucleus. 10.1523/JNEUROSCI.4853-05.2006

Eye position effects on visual, memory, and saccade-related activity in areas LIP and 7a of macaque.

RA Andersen — 2005-12-08T14:22:57-00:00

J Neurosci, Vol. 10, No. 4. (April 1990), pp. 1176-1196.

We studied the effect of eye position on the light-sensitive, memory, and saccade-related activities of neurons of the lateral intraparietal area and area 7a in the posterior parietal cortex of rhesus monkeys. A majority of the cells showed significant effects of eye position, for each of the 3 types of response. The direction tuning of the light-sensitive, memory and saccade responses did not change with eye position but the magnitude of the response did. Since previous work showed a similar effect for the light-sensitive response of area 7a neurons (Andersen and Mountcastle, 1983; Andersen et al., 1985b), the present results indicate that this modulating effect of eye position may be a general one, as it is found in 3 types of responses in 2 cortical areas. Gain fields were mapped by measuring the effect of eye position on the magnitude of the response at 9 different eye positions for each neuron. The gain fields were usually planar or largely planar for all 3 types of response in both areas, indicating that the magnitude of the response usually varies linearly with both horizontal and vertical eye position. A similar observation was made previously for the gain fields of the light-sensitive response of area 7a neurons (Andersen et al., 1985b). Although gain fields sloped in all directions for the population of cells, the gain field slopes of the light-sensitive, memory and saccade responses for individual cells were usually similar. It is proposed that these eye position effects play an important role in making coordinate transformations for visually guided movement.

The role of the lateral intraparietal area of the monkey in the generation of saccades and visuospatial attention.

ME Goldberg — 2005-11-21T14:25:55-00:00

Ann N Y Acad Sci, Vol. 956 (April 2002), pp. 205-215.

The brain cannot monitor or react towards the entire world at a given time. Instead, using the process of attention, it selects objects in the world for further analysis. Neuronal activity in the monkey intraparietal area has the properties appropriate for a neuronal substrate of attention: instead of all objects being represented in the parietal cortex, only salient objects are. Such objects can be salient because of their physical properties (recently flashed objects or moving objects) or because they can be made important to the animal by virtue of a task. Although lateral intraparietal area (LIP) neurons respond through the delay period of a memory-guided saccade, they also respond in an enhanced manner to distractors flashed during the delay period of a memory-guided saccade being generated to a position outside the receptive field. This activity parallels the monkey's psychophysical attentional process: attention is ordinarily pinned at the goal of a memory-guided saccade, but it shifts briefly to the locus of a task-irrelevant distractor flashed briefly during the delay period and then returns to the goal. Although neurons in LIP have been implicated as being directly involved in the generation of saccadic eye movements, their activity does not predict where, when, or if a saccade will occur. The ensemble of activity in LIP, however, does accurately describe the locus of attention.

The updating of the representation of visual space in parietal cortex by intended eye movements.

JR Duhamel — 2005-08-16T15:36:51-00:00

Science, Vol. 255, No. 5040. (3 January 1992), pp. 90-92.

Every eye movement produces a shift in the visual image on the retina. The receptive field, or retinal response area, of an individual visual neuron moves with the eyes so that after an eye movement it covers a new portion of visual space. For some parietal neurons, the location of the receptive field is shown to shift transiently before an eye movement. In addition, nearly all parietal neurons respond when an eye movement brings the site of a previously flashed stimulus into the receptive field. Parietal cortex both anticipates the retinal consequences of eye movements and updates the retinal coordinates of remembered stimuli to generate a continuously accurate representation of visual space.

The supplementary motor area encodes reward expectancy in eye movement tasks.

Michael Campos — 2005-04-26T00:56:55-00:00

J Neurophysiol (20 April 2005)

Neural activity signifying the expectation of reward has been found recently in many parts of the brain, including midbrain and cortical structures. These signals can facilitate goal-directed behavior or the learning of new skills based on reinforcements. Here we show that neurons in the supplementary motor area (SMA), an area concerned with movements of the body and limbs, also carry a reward expectancy signal in the post-saccadic period of oculomotor tasks. While the monkeys performed blocks of memory-guided and object-based saccades, the neurons discharged a burst after a ~200 ms delay following the target acquiring saccade in the memory task, but often fired concurrently with the target acquiring saccade in the object task. The hypothesis that this post-saccadic bursting activity reflects the expectation of a reward was tested with a series of manipulations to the memory-guided saccade task. It was found that, while the timing of the bursting activity corresponds to a visual feedback stimulus, the visual feedback is not required for the neurons to discharge a burst. Second, blocks of no-reward trials reveal an extinction of the bursting activity as the monkeys come to understand that they would not be rewarded for properly generated saccades. Finally, the delivery of unexpected rewards confirmed that, in many of the neurons, the activity is not related to a motor plan to acquire the reward (e.g. licking). Thus we conclude that reward expectancy is represented by the activity of SMA neurons, even in the context of an oculomotor task. These results suggest that the reward expectancy signal is broadcast over a large extent of motor cortex, and may facilitate the learning of new, coordinated behavior between different body parts.

Saccadic eye movements cause compression of time as well as space

Concetta Morrone — 2005-07-01T19:24:04-00:00

Nature Neuroscience, Vol. 8, No. 7. (19 June 2005), pp. 950-954.

Saccades exhibit abrupt transition between reactive and predictive; predictive saccade sequences have long-term correlations.

M Shelhamer — 2005-06-08T21:01:24-00:00

J Neurophysiol, Vol. 90, No. 4. (October 2003), pp. 2763-2769.

To compensate for neural delays, organisms require predictive motor control. We investigated the transition between reaction and prediction in saccades (rapid eye movements) to periodically paced targets. Tracking at low frequencies (0.2-0.3 Hz) is reactive (eyes lag target) and at high frequencies (0.9-1.0 Hz) is predictive (eyes anticipate target); there is an abrupt rather than smooth transition between the two modes (a "phase transition," as found in bistable physical systems). These behaviors represent stable modes of the oculomotor control system, with attendant rapid switching between the neural pathways underlying the different modes. Furthermore, predictive saccades exhibit long-term correlations (slow decay of the autocorrelation function, manifest as a 1/f alpha spectrum). This indicates that predictive trials are not independent. The findings have implications for the understanding of predictive motor control: predictive performance during a given trial is influenced by a feedback process that takes into account the latency of previous trials.

Sequences of predictive saccades are correlated over a span of approximately 2 s and produce a fractal time series.

M Shelhamer — 2005-06-07T16:04:26-00:00

J Neurophysiol, Vol. 93, No. 4. (April 2005), pp. 2002-2011.

We previously demonstrated that there is an abrupt (rather than smooth) transition between reactive and predictive modes of eye-movement tracking of target lights (a phase transition). We also found evidence that the sequence of eye movements in the reactive mode was independent, whereas those in the predictive mode were correlated and possibly formed a random fractal sequence. Here we confirm the finding of fractal structure by quantifying the rate of decay of nonlinear forecasting when applied to these data. We also estimate the window over which consecutive trials are correlated and show that the duration of this window is fixed in time rather than number of trials. These results have implications for the neural mechanisms that drive predictive movements.

The neural selection and control of saccades by the frontal eye field.

JD Schall — 2005-04-18T14:20:00-00:00

Philos Trans R Soc Lond B Biol Sci, Vol. 357, No. 1424. (29 August 2002), pp. 1073-1082.

Recent research has provided new insights into the neural processes that select the target for and control the production of a shift of gaze. Being a key node in the network that subserves visual processing and saccade production, the frontal eye field (FEF) has been an effective area in which to monitor these processes. Certain neurons in the FEF signal the location of conspicuous or meaningful stimuli that may be the targets for saccades. Other neurons control whether and when the gaze shifts. The existence of distinct neural processes for visual selection and saccade production is necessary to explain the flexibility of visually guided behaviour.

Contextual modulation of substantia nigra pars reticulata neurons.

A Handel — 2005-02-22T21:48:50-00:00

J Neurophysiol, Vol. 83, No. 5. (May 2000), pp. 3042-3048.

Neurons in the substantia nigra pars reticulata (SNr) are known to encode saccadic eye movements within some, but not all, behavioral contexts. However, the precise contextual factors that effect the modulations of nigral activity are still uncertain. To further examine the effect of behavioral context on the SNr, we recorded the activity of 72 neurons while monkeys made saccades during a delayed saccade task and during periods of free viewing. We quantified and compared the movement fields of each neuron for saccades made under three different conditions: 1) spontaneous saccades, which shifted gaze during periods of free viewing when no stimuli were presented and no reinforcements were delivered; 2) fixational saccades, which brought gaze into alignment with a fixation target at the start of a delayed saccade trial, were necessary for trial completion, but were not directly followed by reinforcement; and 3) terminal saccades, which brought gaze into alignment with a visual target at the end of a delayed saccade trial and were directly followed by reinforcement. For three of the four SNr neuron classes, saccade-related modulations were only present before terminal saccades. For the fourth class, discrete pausers, saccade-related modulations were substantially larger for terminal saccades than for fixational saccades, and modulations were absent for spontaneous saccades. These results and other recent work on the basal ganglia suggest that some saccade-related signals in the SNr may be influenced by the reinforcement associated with a particular saccadic eye movement.

Single neurons in posterior cingulate cortex of behaving macaque: eye movement signals.

CR Olson — 2005-02-21T20:40:15-00:00

J Neurophysiol, Vol. 76, No. 5. (November 1996), pp. 3285-3300.

1. Posterior cingulate cortex, although widely regarded as a part of the limbic system, is connected most strongly to parietal and frontal areas with sensory, motor, and cognitive functions. To gain insight into the functional nature of posterior cingulate cortex, we have recorded from its neurons in monkeys performing oculomotor tasks known to activate parietal and frontal neurons. We have found that posterior cingulate neurons fire during periods of ocular fixation at a rate determined by the angle of gaze and by the size and direction of the preceding eye movement. 2. The activity of 530 posterior cingulate neurons was monitored while rhesus macaque monkeys made visually guided eye movements to spots projected on a tangent screen. 3. In 150/530 neurons, a statistically significant shift in the rate of discharge occurred around the time of onset of saccadic eye movements. The preponderant form of response was an increase in activity (142/150 neurons). 4. In 142 neurons exhibiting significant excitation after saccades in at least one direction, the level of discharge was analyzed as a function of time relative to onset of the saccade. Across the neuronal population as a whole, activity increased sharply at the moment of onset of the saccade, rising to a maximum after 200 ms and then declining slowly. The net level of discharge remained well above presaccadic baseline even after > 1 s of postsaccadic fixation. 5. In 63 neurons, the postsaccadic rate of discharge was analyzed relative to the angle of the eye in the orbit by monitoring neuronal activity while the monkey executed saccades of uniform direction and amplitude to four targets spaced at 16-deg intervals along a line. The postsaccadic firing level was significantly dependent on orbital angle in 44/63 neurons. 6. In 45 neurons, the postsaccadic rate of discharge was analyzed relative to saccade direction by monitoring neuronal activity while the monkey executed 16-deg saccades to a constant target from diametrically opposed starting points. The postsaccadic level of activity was significantly dependent on saccade direction in 20/ 45 neurons. 7. In 58 neurons, the postsaccadic rate of discharge was analyzed relative to saccade amplitude by monitoring neuronal activity while the monkey executed saccades, which varied in amplitude (4, 8, 16, and 32 deg) but which were constant in direction and brought the eye to bear on a constant endpoint. The postsaccadic level of activity was significantly dependent on saccade amplitude in 24/58 neurons. In all neurons exhibiting significant amplitude-dependence, stronger firing accompanied larger saccades. 8. The activity of 10 neurons was monitored during smooth pursuit eye movements (20 deg/s upward, downward, leftward, and rightward). The level of firing varied as a function of both the position of the eye (9 neurons) and the velocity of the eye (6 neurons). 9. We conclude that posterior cingulate neurons monitor eye movements and eye position. It is unlikely that they participate in the generation of eye movements because their shifts of discharge follow the onset of the movements. Eye-movement-related signals in posterior cingulate cortex may reflect the participation of this area in assigning spatial coordinates to retinal images.

Saccadic probability influences motor preparation signals and time to saccadic initiation.

MC Dorris — 2005-02-20T19:52:17-00:00

J Neurosci, Vol. 18, No. 17. (1 September 1998), pp. 7015-7026.

One must be prudent when selecting potential saccadic targets because the eyes can only move to one location at a time, yet movements must occur quickly enough to permit interaction with a rapidly changing world. This process of efficiently acquiring relevant targets may be aided by advanced planning of a movement toward an upcoming target whose location is gathered via environmental cues or situational experience. We studied how saccadic reaction times (SRTs) and early pretarget neuronal activity covaried as a function of saccadic probability. Monkeys performed a saccadic task in which the probability of the required saccade being directed into the response field of a neuron varied systematically between blocks of trials. We recorded simultaneously the early pretarget activity of saccade-related neurons in the intermediate layers of the superior colliculus. We found that, as the likelihood of the saccade being generated into the response field of the neuron increased, the level of neuronal activity preceding target presentation also increased. Our data suggest that this early activity codes motor preparation because its activity was related to not only the metrics but also the timing of the saccade, with 94% (29/31) of the neurons tested having significant negative correlations between discharge rate and SRT. This view is supported by cases in which exceptionally high levels of pretarget activity were associated with anticipatory saccades into the response field of a neuron that occurred in advance of the target being presented. This study demonstrates how situational experience can expedite motor behavior via the advanced preparation of motor programs.

Modulation of neuronal activity in superior colliculus by changes in target probability.

MA Basso — 2005-02-20T19:47:18-00:00

J Neurosci, Vol. 18, No. 18. (15 September 1998), pp. 7519-7534.

Complex visual scenes require that a target for an impending saccadic eye movement be selected from a larger number of possible targets. We investigated whether changing the probability that a visual stimulus would be selected as the target for a saccade altered activity of monkey superior colliculus (SC) neurons in two experiments. First, we changed the number of possible targets on each trial. Second, we kept the visual display constant and presented a single saccade target repeatedly so that target probability was established over time. Buildup neurons in the SC, those with delay period activity, showed a consistent reduction in activity as the probability of the saccade decreased, independent of the visual stimulus configuration. Other SC neurons, fixation and burst, were largely unaffected by the changes in saccade target probability. Because we had monkeys making saccades to many locations within the visual field, we could examine activity associated with saccades outside of the movement field of neurons. We found the activity of buildup neurons to be similar across the SC, before the target was identified, and reduced when the number of possible targets increased. The results of our experiments are consistent with a role for this activity in establishing a motor set. We found, consistent with this interpretation, that the activity of these neurons was predictive of the latency of a saccadic eye movement and not other saccade parameters such as end point or peak velocity.

Modulation of neuronal activity by target uncertainty.

MA Basso — 2005-02-20T19:48:01-00:00

Nature, Vol. 389, No. 6646. (4 September 1997), pp. 66-69.

Visual scenes are composed of many elements and although we can appreciate a scene as a whole, we can only move our eyes to one element of the scene at a time. As visual scenes become more complex, the number of potential targets in the scene increases, and the uncertainty that any particular one will be selected for an eye movement also increases. How motor systems accommodate this target uncertainty remains unknown. The activities of neurons in both the cerebral cortex and superior colliculus are modulated by this selection process. We reasoned that activity associated with target uncertainty should be evident in the saccadic motor system at the final stages of neural processing, in the superior colliculus. By systematically changing the number of stimuli from which a selection must be made and recording from superior colliculus neurons, we found that as the target uncertainty increased, the neural activity preceding target selection decreased. These results indicate that neurons within the final common pathway for movement generation are active well in advance of the selection of a particular movement. This early activity varies with the probability that a particular movement will be selected.

Nonspatial saccade-specific activation in area LIP of monkey parietal cortex.

AR Dickinson — 2005-02-17T19:53:01-00:00

J Neurophysiol, Vol. 90, No. 4. (October 2003), pp. 2460-2464.

We present evidence that neurons in the lateral intraparietal area (LIP) of monkey posterior parietal cortex (PPC) are activated by the instruction to make an eye movement, even in the complete absence of a spatial target. This study employed a visually guided motor task that dissociated the type of movement to make (saccade or reach) from the location where the movement was to be made. Using this task, animals were instructed to prepare a specific type of movement prior to knowing the spatial location of the movement target. We found that 25% of the LIP neurons recorded in two animals were activated significantly more by the instruction to prepare a saccade than by the instruction to prepare a reach. This finding indicates that LIP is involved in more than merely spatial attention and provides further evidence for nonspatial effector-specific signal processing in the dorsal stream.