CiteULike: j-ito's vision

Brain Circuits for the Internal Monitoring of Movements

Marc Sommer — 2008-06-19T15:46:34-00:00

Annual Review of Neuroscience, Vol. 31, No. 1. (2008), pp. 317-338.

Each movement we make activates our own sensory receptors, thus causing a problem for the brain: the spurious, movement-related sensations must be discriminated from the sensory inputs that really matter, those representing our environment. Here we consider circuits for solving this problem in the primate brain. Such circuits convey a copy of each motor command, known as a corollary discharge (CD), to brain regions that use sensory input. In the visual system, CD signals may help to produce a stable visual percept from the jumpy images resulting from our rapid eye movements. A candidate pathway for providing CD for vision ascends from the superior colliculus to the frontal cortex in the primate brain. This circuit conveys warning signals about impending eye movements that are used for planning subsequent movements and analyzing the visual world. Identifying this circuit has provided a model for studying CD in other primate sensory systems and may lead to a better understanding of motor and mental disorders.

A quantitative theory of immediate visual recognition.

T Serre — 2008-06-17T15:15:35-00:00

Progress in brain research, Vol. 165 (2007), pp. 33-56.

Human and non-human primates excel at visual recognition tasks. The primate visual system exhibits a strong degree of selectivity while at the same time being robust to changes in the input image. We have developed a quantitative theory to account for the computations performed by the feedforward path in the ventral stream of the primate visual cortex. Here we review recent predictions by a model instantiating the theory about physiological observations in higher visual areas. We also show that the model can perform recognition tasks on datasets of complex natural images at a level comparable to psychophysical measurements on human observers during rapid categorization tasks. In sum, the evidence suggests that the theory may provide a framework to explain the first 100-150 ms of visual object recognition. The model also constitutes a vivid example of how computational models can interact with experimental observations in order to advance our understanding of a complex phenomenon. We conclude by suggesting a number of open questions, predictions, and specific experiments for visual physiology and psychophysics.

Transcranial magnetic stimulation over posterior parietal cortex disrupts transsaccadic memory of multiple objects.

SL Prime — 2008-07-10T15:29:38-00:00

The Journal of neuroscience : the official journal of the Society for Neuroscience, Vol. 28, No. 27. (2 July 2008), pp. 6938-6949.

The posterior parietal cortex (PPC) plays a role in spatial updating of goals for eye and arm movements across saccades, but less is known about its role in updating perceptual memory. We reported previously that transsaccadic memory has a capacity for storing the orientations of three to four Gabor patches either within a single fixation (fixation task) or between separate fixations (saccade task). Here, we tested the role of the PPC in transsaccadic memory in eight subjects by simultaneously applying single-pulse transcranial magnetic stimulation (TMS) over the right and left PPC, over several control sites, and comparing these to behavioral controls with no TMS. In TMS trials, we randomly delivered pulses at one of three different time intervals around the time of the saccade, or at an equivalent time in the fixation task. Controls confirmed that subjects could normally retain at least three visual features. TMS over the left PPC and a control site had no significant effect on this performance. However, TMS over the right PPC disrupted memory performance in both tasks. This TMS-induced effect was most disruptive in the saccade task, in particular when stimulation coincided more closely with saccade timing. Here, the capacity to compare presaccadic and postsaccadic features was reduced to one object, as expected if the spatial aspect of memory was disrupted. This finding suggests that right PPC plays a role in the spatial processing involved in transsaccadic memory of visual features. We propose that this process uses saccade-related feedback signals similar to those observed in spatial updating.

Biased competition through variations in amplitude of γ -oscillations

Magteld Zeitler — 2008-07-03T11:41:36-00:00

Journal of Computational Neuroscience, Vol. 25, No. 1. (2008), pp. 89-107.

Abstract Experiments in visual cortex have shown that the firing rate of a neuron in response to the simultaneous presentation of a preferred and non-preferred stimulus within the receptive field is intermediate between that for the two stimuli alone (stimulus competition). Attention directed to one of the stimuli drives the response towards the response induced by the attended stimulus alone (selective attention). This study shows that a simple feedforward model with fixed synaptic conductance values can reproduce these two phenomena using synchronization in the gamma-frequency range to increase the effective synaptic gain for the responses to the attended stimulus. The performance of the model is robust to changes in the parameter values. The model predicts that the phase locking between presynaptic input and output spikes increases with attention.

Au naturel.

GB Stanley — 2008-06-28T16:11:07-00:00

Neuron, Vol. 58, No. 4. (22 May 2008), pp. 467-469.

Although adaptation is a ubiquitous property of neurons in the early visual pathway, the functional consequences in the natural visual environment are unknown. In this issue of Neuron, Mante et al. show, through a comprehensive set of in vivo experiments in the visual thalamus, that the basic functional mechanisms of adaptation that have been well studied with artificial probes capture the neuronal response in the natural environment and are predictable from properties of the visual scene that may be represented by local neural ensembles.

Postnatal Development of Onset Transient Responses in Macaque V1 and V2 Neurons.

Bin Zhang — 2008-06-28T16:05:43-00:00

Journal of neurophysiology (25 June 2008)

Vision of newborn infants is limited by immaturities in their visual brain. In adult primates the transient onset discharges of visual cortical neurons are thought to be intimately involved with capturing the rapid succession of brief images in visual scenes. Here we sought to determine the responsiveness and quality of transient responses in individual neurons of the primary visual cortex (V1) and visual area 2 (V2) of infant monkeys. We show that the transient component of neuronal firing to 640 millisecond stationary gratings was as robust and as reliable as in adults only 2 weeks after birth while the sustained component was more sluggish in infants than in adults. Thus, the cortical circuitry supporting onset transient responses is functionally mature near birth, and our findings predict that neonates, known for their 'impoverished vision', are capable of initiating relatively mature fixating eye movements and of performing in detection of simple objects far better than traditionally thought.

SYNCHRONIZATION OF NEURONAL RESPONSES IN PRIMARY VISUAL CORTEX OF MONKEYS VIEWING NATURAL IMAGES.

Pedro E Maldonado — 2008-06-25T10:26:41-00:00

Journal of neurophysiology (18 June 2008)

When inspecting visual scenes, primates perform on average four saccadic eye movements per second which implies that scene segmentation, feature binding and identification of image components is accomplished in less than 200ms. Thus, individual neurons can contribute only a small number of discharges for these complex computations, suggesting that information is encoded not only in the discharge rate but also in the timing of action potentials. While monkeys inspected natural scenes we registered with multi-electrodes from primary visual cortex, the discharges of simultaneously recorded neurons. Relating these signals to eye movements, revealed that discharge rates peaked around 90ms after fixation onset and then decreased to near baseline levels within 200ms. Unitary event analysis revealed that preceding this increase in firing, there was an episode of enhanced response synchronization during which discharges of spatially distributed cells coincided within 5ms windows significantly more often than predicted by the discharge rates. This episode started 30ms after fixation onset and ended by the time discharge rates had reached their maximum. When the animals scanned a blank screen a small change in firing rate but no excess synchronization was observed. The short latency of the stimulation related synchronization phenomena suggests a fast acting mechanism for the coordination of spike timing that may contribute to the basic operations of scene segmentation.

Refinement of the retinogeniculate pathway.

William Guido — 2008-06-19T09:01:45-00:00

The Journal of physiology (12 June 2008)

Much of our present understanding about the mechanisms contributing to the activity dependent refinement of sensory connections comes from experiments done in the retinogeniculate pathway. In recent years the mouse has emerged as a model system of study. This review outlines the major changes in connectivity that occur in this species and a potential mechanism that can account for such remodeling. During early postnatal life when spontaneous activity of retinal ganglion cells sweeps across the retina in waves, retinal projections from the two eyes to the dorsal lateral geniculate nucleus (LGN) segregate to form non-overlapping eye specific domains. There is a loss of binocular innervation, a pruning of excitatory inputs from a dozen or more to one or two, and the emergence of inhibitory circuitry. Many of these changes underlie the development of precise eye specific visual maps and receptive field structure of LGN neurons. Retinal activity plays a major role both in the induction and maintenance of this refinement. The activity dependent influx of Ca(2+) through L-type channels and associated activation of CREB signaling may underlie the pruning and stabilization of developing retinogeniculate connections.

Neuronal mechanisms of visual stability

Robert Wurtz — 2008-06-10T07:03:03-00:00

Vision Research, Vol. In Press, Corrected Proof

Human vision is stable and continuous in spite of the incessant interruptions produced by saccadic eye movements. These rapid eye movements serve vision by directing the high resolution fovea rapidly from one part of the visual scene to another. They should detract from vision because they generate two major problems: displacement of the retinal image with each saccade and blurring of the image during the saccade. This review considers the substantial advances in understanding the neuronal mechanisms underlying this visual stability derived primarily from neuronal recording and inactivation studies in the monkey, an excellent model for systems in the human brain. For the first problem, saccadic displacement, two neuronal candidates are salient. First are the neurons in frontal and parietal cortex with shifting receptive fields that provide anticipatory activity with each saccade and are driven by a corollary discharge. These could provide the mechanism for a retinotopic hypothesis of visual stability and possibly for a transsaccadic memory hypothesis, The second neuronal mechanism is provided by neurons whose visual response is modulated by eye position (gain field neurons) or are largely independent of eye position (real position neurons), and these neurons could provide the basis for a spatiotopic hypothesis. For the second problem, saccadic suppression, visual masking and corollary discharge are well established mechanisms, and possible neuronal correlates have been identified for each.

Functional Mechanisms Shaping Lateral Geniculate Responses to Artificial and Natural Stimuli

Valerio Mante — 2008-05-26T08:21:46-00:00

Neuron, Vol. 58, No. 4. (22 May 2008), pp. 625-638.

Summary Functional models of the early visual system should predict responses not only to simple artificial stimuli but also to sequences of complex natural scenes. An ideal testbed for such models is the lateral geniculate nucleus (LGN). Mechanisms shaping LGN responses include the linear receptive field and two fast adaptation processes, sensitive to luminance and contrast. We propose a compact functional model for these mechanisms that operates on sequences of arbitrary images. With the same parameters that fit the firing rate responses to simple stimuli, it predicts the bulk of the firing rate responses to complex stimuli, including natural scenes. Further improvements could result by adding a spiking mechanism, possibly one capable of bursts, but not by adding mechanisms of slow adaptation. We conclude that up to the LGN the responses to natural scenes can be largely explained through insights gained with simple artificial stimuli.

Saccades to a Remembered Location Elicit Spatially Specific Activation in the Human Retinotopic Visual Cortex.

Joy J Geng — 2008-06-03T10:07:25-00:00

Journal of cognitive neuroscience (29 May 2008)

Abstract The possible impact upon the human visual cortex from saccades to remembered target locations was investigated using functional magnetic resonance imaging (fMRI). A specific location in the upper-right or upper-left visual quadrant served as the saccadic target. After a delay of 2400 msec, an auditory signal indicated whether to execute a saccade to that location (go trial) or to cancel the saccade and remain centrally fixated (no-go). Group fMRI analysis revealed activation specific to the remembered target location for executed saccades, in the contralateral lingual gyrus. No-go trials produced similar, albeit significantly reduced, effects. Individual retinotopic mapping confirmed that on go trials, quadrant-specific activations arose in those parts of ventral V1, V2, and V3 that coded the target location for the saccade, whereas on no-go trials, only the corresponding parts of V2 and V3 were significantly activated. These results indicate that a spatial-motor saccadic task (i.e., making an eye movement to a remembered location) is sufficient to activate the retinotopic visual cortex spatially corresponding to the target location, and that this activation is also present (though reduced) when no saccade is executed. We discuss the implications of finding that saccades to remembered locations can affect the early visual cortex, not just those structures conventionally associated with eye movements, in relation to recent ideas about attention, spatial working memory, and the notion that recently activated representations can be "refreshed" when needed.

Low-Frequency Local Field Potentials and Spikes in Primary Visual Cortex Convey Independent Visual Information

Andrei Belitski — 2008-06-02T10:17:41-00:00

J. Neurosci., Vol. 28, No. 22. (28 May 2008), pp. 5696-5709.

Local field potentials (LFPs) reflect subthreshold integrative processes that complement spike train measures. However, little is yet known about the differences between how LFPs and spikes encode rich naturalistic sensory stimuli. We addressed this question by recording LFPs and spikes from the primary visual cortex of anesthetized macaques while presenting a color movie. We then determined how the power of LFPs and spikes at different frequencies represents the visual features in the movie. We found that the most informative LFP frequency ranges were 1-8 and 60-100 Hz. LFPs in the range of 12-40 Hz carried little information about the stimulus, and may primarily reflect neuromodulatory inputs. Spike power was informative only at frequencies <12 Hz. We further quantified "signal correlations" (correlations in the trial-averaged power response to different stimuli) and "noise correlations" (trial-by-trial correlations in the fluctuations around the average) of LFPs and spikes recorded from the same electrode. We found positive signal correlation between high-gamma LFPs (60-100 Hz) and spikes, as well as strong positive signal correlation within high-gamma LFPs, suggesting that high-gamma LFPs and spikes are generated within the same network. LFPs <24 Hz shared strong positive noise correlations, indicating that they are influenced by a common source, such as a diffuse neuromodulatory input. LFPs <40 Hz showed very little signal and noise correlations with LFPs >40 Hz and with spikes, suggesting that low-frequency LFPs reflect neural processes that in natural conditions are fully decoupled from those giving rise to spikes and to high-gamma LFPs. 10.1523/JNEUROSCI.0009-08.2008

Transient Induced Gamma-Band Response in EEG as a Manifestation of Miniature Saccades

Shlomit Yuval-Greenberg — 2008-05-16T06:19:08-00:00

Neuron, Vol. 58, No. 3. (8 May 2008), pp. 429-441.

Summary The induced gamma-band EEG response (iGBR) recorded on the scalp is widely assumed to reflect synchronous neural oscillation associated with object representation, attention, memory, and consciousness. The most commonly reported EEG iGBR is a broadband transient increase in power at the gamma range ~200-300 ms following stimulus onset. A conspicuous feature of this iGBR is the trial-to-trial poststimulus latency variability, which has been insufficiently addressed. Here, we show, using single-trial analysis of concomitant EEG and eye tracking, that this iGBR is tightly time locked to the onset of involuntary miniature eye movements and reflects a saccadic "spike potential." The time course of the iGBR is related to an increase in the rate of saccades following a period of poststimulus saccadic inhibition. Thus, whereas neuronal gamma-band oscillations were shown conclusively with other methods, the broadband transient iGBR recorded by scalp EEG reflects properties of miniature saccade dynamics rather than neuronal oscillations.

The Effects of Visual Stimulation and Selective Visual Attention on Rhythmic Neuronal Synchronization in Macaque Area V4

Pascal Fries — 2008-05-16T06:11:15-00:00

J. Neurosci., Vol. 28, No. 18. (30 April 2008), pp. 4823-4835.

Selective attention lends relevant sensory input priority access to higher-level brain areas and ultimately to behavior. Recent studies have suggested that those neurons in visual areas that are activated by an attended stimulus engage in enhanced gamma-band (30-70 Hz) synchronization compared with neurons activated by a distracter. Such precise synchronization could enhance the postsynaptic impact of cells carrying behaviorally relevant information. Previous studies have used the local field potential (LFP) power spectrum or spike-LFP coherence (SFC) to indirectly estimate spike synchronization. Here, we directly demonstrate zero-phase gamma-band coherence among spike trains of V4 neurons. This synchronization was particularly evident during visual stimulation and enhanced by selective attention, thus confirming the pattern inferred from LFP power and SFC. We therefore investigated the time course of LFP gamma-band power and found rapid dynamics consistent with interactions of top-down spatial and feature attention with bottom-up saliency. In addition to the modulation of synchronization during visual stimulation, selective attention significantly changed the prestimulus pattern of synchronization. Attention inside the receptive field of the recorded neuronal population enhanced gamma-band synchronization and strongly reduced alpha-band (9-11 Hz) synchronization in the prestimulus period. These results lend further support for a functional role of rhythmic neuronal synchronization in attentional stimulus selection. 10.1523/JNEUROSCI.4499-07.2008

LFP power spectra in V1 cortex: the graded effect of stimulus contrast.

JA Henrie — 2008-04-18T07:24:00-00:00

Journal of neurophysiology, Vol. 94, No. 1. (July 2005), pp. 479-490.

We recorded local field potentials (LFPs) and single-unit activity simultaneously in the macaque primary visual cortex (V1) and studied their responses to drifting sinusoidal gratings that were chosen to be "optimal" for the single units. Over all stimulus conditions, the LFP spectra have much greater power in the low-frequency band (< or = 10 Hz) than higher frequencies and can be described as "1/f." Analysis of the total power limited to the low, gamma (25-90 Hz), or broad (8-240 Hz) frequency bands of the LFP as a function of stimulus contrast indicates that the LFP power gradually increases with stimulus strength across a wide band in a manner roughly comparable to the increase in the simultaneously recorded spike activity. However, the low-frequency band power remains approximately constant across all stimulus contrasts. More specifically the gamma-band LFP power increases differentially more with respect to baseline than either higher or lower bands as stimulus contrast increases. At the highest stimulus contrasts, we report as others have previously, that the power spectrum of the LFP typically contains an obvious peak in the gamma-frequency band. The gamma-band peak emerges from the overall broadband enhancement in LFP power at stimulus contrasts where most single units' responses have begun to saturate. The temporal/spectral structures of the LFP located in the gamma band-which become most evident at the highest contrasts-provide additional constraints on potential mechanisms underlying the stimulus response properties of spiking neurons in V1.

Neuronal activity in the primary visual cortex of the cat freely viewing natural images.

PE Maldonado — 2008-04-14T08:53:08-00:00

Neuroscience, Vol. 144, No. 4. (23 February 2007), pp. 1536-1543.

Many studies have now demonstrated that neurons in the visual cortex of cats and monkeys change their activity when stimuli are presented beyond their classical receptive field, and that these responses are not readily apparent from their receptive field properties. However few studies have been conducted to investigate the discharge properties of neurons in the visual cortex of animals when they are allow to freely view natural images. We employ tetrodes, which enable simultaneous and separable recordings of small numbers of neighboring neurons, to record 102 single units from 59 sites from areas 17 and 18 of two alert cats. While the animals viewed either natural images or black screens, they made frequent saccadic eye movements and gaze fixations. Fixations onto an image's location increased neuronal firing peaking at 80-100 ms after the fixation onset, to then decrease steadily with time despite continuous fixation. Saccades trigger a fast decrease in firing rate for both images and darkness. When we examined the incidence of correlated firing, we observed significant synchrony during the initial phases of visual fixations when the animals viewed natural scenes. Such synchrony was absent during saccadic eye movements and during eye movements in darkness. Our data revealed that scanning of natural scenes is associated with a rapid succession of distinct fixation-related activation patterns that included transient rate changes and excess coincident firing. The transient nature of these synchronization phenomena suggests a fast acting mechanism, which is in good agreement with the evidence that basic operations of scene analysis must be accomplished within a few tens of milliseconds in primary visual cortex.

Entrainment of neuronal oscillations as a mechanism of attentional selection.

P Lakatos — 2008-04-10T01:01:54-00:00

Science (New York, N.Y.), Vol. 320, No. 5872. (4 April 2008), pp. 110-113.

Whereas gamma-band neuronal oscillations clearly appear integral to visual attention, the role of lower-frequency oscillations is still being debated. Mounting evidence indicates that a key functional property of these oscillations is the rhythmic shifting of excitability in local neuronal ensembles. Here, we show that when attended stimuli are in a rhythmic stream, delta-band oscillations in the primary visual cortex entrain to the rhythm of the stream, resulting in increased response gain for task-relevant events and decreased reaction times. Because of hierarchical cross-frequency coupling, delta phase also determines momentary power in higher-frequency activity. These instrumental functions of low-frequency oscillations support a conceptual framework that integrates numerous earlier findings.

The temporal resolution of neural codes: does response latency have a unique role?

MW Oram — 2008-03-17T16:43:31-00:00

Philos Trans R Soc Lond B Biol Sci, Vol. 357, No. 1424. (29 August 2002), pp. 987-1001.

This article reviews the nature of the neural code in non-human primate cortex and assesses the potential for neurons to carry two or more signals simultaneously. Neurophysiological recordings from visual and motor systems indicate that the evidence for a role for precisely timed spikes relative to other spike times (ca. 1-10 ms resolution) is inconclusive. This indicates that the visual system does not carry a signal that identifies whether the responses were elicited when the stimulus was attended or not. Simulations show that the absence of such a signal reduces, but does not eliminate, the increased discrimination between stimuli that are attended compared with when the stimuli are unattended. The increased accuracy asymptotes with increased gain control, indicating limited benefit from increasing attention. The absence of a signal identifying the attentional state under which stimuli were viewed can produce the greatest discrimination between attended and unattended stimuli. Furthermore, the greatest reduction in discrimination errors occurs for a limited range of gain control, again indicating that attention effects are limited. By contrast to precisely timed patterns of spikes where the timing is relative to other spikes, response latency provides a fine temporal resolution signal (ca. 10 ms resolution) that carries information that is unavailable from coarse temporal response measures. Changes in response latency and changes in response magnitude can give rise to different predictions for the patterns of reaction times. The predictions are verified, and it is shown that the standard method for distinguishing executive and slave processes is only valid if the representations of interest, as evidenced by the neural code, are known. Overall, the data indicate that the signalling evident in neural signals is restricted to the spike count and the precise times of spikes relative to stimulus onset (response latency). These coding issues have implications for our understanding of cognitive models of attention and the roles of executive and slave systems.

Signal timing across the macaque visual system.

MT Schmolesky — 2008-04-03T08:33:28-00:00

Journal of neurophysiology, Vol. 79, No. 6. (June 1998), pp. 3272-3278.

The onset latencies of single-unit responses evoked by flashing visual stimuli were measured in the parvocellular (P) and magnocellular (M) layers of the dorsal lateral geniculate nucleus (LGNd) and in cortical visual areas V1, V2, V3, V4, middle temporal area (MT), medial superior temporal area (MST), and in the frontal eye field (FEF) in individual anesthetized monkeys. Identical procedures were carried out to assess latencies in each area, often in the same monkey, thereby permitting direct comparisons of timing across areas. This study presents the visual flash-evoked latencies for cells in areas where such data are common (V1 and V2), and are therefore a good standard, and also in areas where such data are sparse (LGNd M and P layers, MT, V4) or entirely lacking (V3, MST, and FEF in anesthetized preparation). Visual-evoked onset latencies were, on average, 17 ms shorter in the LGNd M layers than in the LGNd P layers. Visual responses occurred in V1 before any other cortical area. The next wave of activation occurred concurrently in areas V3, MT, MST, and FEF. Visual response latencies in areas V2 and V4 were progressively later and more broadly distributed. These differences in the time course of activation across the dorsal and ventral streams provide important temporal constraints on theories of visual processing.

Measurements of Simultaneously Recorded Spiking Activity and Local Field Potentials Suggest that Spatial Selection Emerges in the Frontal Eye Field.

IE Monosov — 2008-03-03T15:48:38-00:00

Neuron, Vol. 57, No. 4. (28 February 2008), pp. 614-625.

The frontal eye field (FEF) participates in selecting the location of behaviorally relevant stimuli for guiding attention and eye movements. We simultaneously recorded local field potentials (LFPs) and spiking activity in the FEF of monkeys performing memory-guided saccade and covert visual search tasks. We compared visual latencies and the time course of spatially selective responses in LFPs and spiking activity. Consistent with the view that LFPs represent synaptic input, visual responses appeared first in the LFPs followed by visual responses in the spiking activity. However, spatially selective activity identifying the location of the target in the visual search array appeared in the spikes about 30 ms before it appeared in the LFPs. Because LFPs reflect dendritic input and spikes measure neuronal output in a local brain region, this temporal relationship suggests that spatial selection necessary for attention and eye movements is computed locally in FEF from spatially nonselective inputs.

Phase-of-Firing Coding of Natural Visual Stimuli in Primary Visual Cortex.

Marcelo A Montemurro — 2008-03-12T00:24:39-00:00

Curr Biol (5 March 2008)

We investigated the hypothesis that neurons encode rich naturalistic stimuli in terms of their spike times relative to the phase of ongoing network fluctuations rather than only in terms of their spike count. We recorded local field potentials (LFPs) and multiunit spikes from the primary visual cortex of anaesthetized macaques while binocularly presenting a color movie. We found that both the spike counts and the low-frequency LFP phase were reliably modulated by the movie and thus conveyed information about it. Moreover, movie periods eliciting higher firing rates also elicited a higher reliability of LFP phase across trials. To establish whether the LFP phase at which spikes were emitted conveyed visual information that could not be extracted by spike rates alone, we compared the Shannon information about the movie carried by spike counts to that carried by the phase of firing. We found that at low LFP frequencies, the phase of firing conveyed 54% additional information beyond that conveyed by spike counts. The extra information available in the phase of firing was crucial for the disambiguation between stimuli eliciting high spike rates of similar magnitude. Thus, phase coding may allow primary cortical neurons to represent several effective stimuli in an easily decodable format.

Rapid Neural Coding in the Retina with Relative Spike Latencies

Tim Gollisch — 2008-02-22T10:01:31-00:00

Science, Vol. 319, No. 5866. (22 February 2008), pp. 1108-1111.

Natural vision is a highly dynamic process. Frequent body, head, and eye movements constantly bring new images onto the retina for brief periods, challenging our understanding of the neural code for vision. We report that certain retinal ganglion cells encode the spatial structure of a briefly presented image in the relative timing of their first spikes. This code is found to be largely invariant to stimulus contrast and robust to noisy fluctuations in response latencies. Mechanistically, the observed response characteristics result from different kinetics in two retinal pathways ("ON" and "OFF") that converge onto ganglion cells. This mechanism allows the retina to rapidly and reliably transmit new spatial information with the very first spikes emitted by a neural population. 10.1126/science.1149639

Transient Cortical Excitation at the Onset of Visual Fixation.

Csaba Rajkai — 2007-05-13T21:38:43-00:00

Cereb Cortex (10 May 2007)

Primates actively examine the visual world by rapidly shifting gaze (fixation) over the elements in a scene. Despite this fact, we typically study vision by presenting stimuli with gaze held constant. To better understand the dynamics of natural vision, we examined how the onset of visual fixation affects ongoing neuronal activity in the absence of visual stimulation. We used multiunit activity and current source density measurements to index neuronal firing patterns and underlying synaptic processes in macaque V1. Initial averaging of neural activity synchronized to the onset of fixation suggested that a brief period of cortical excitation follows each fixation. Subsequent single-trial analyses revealed that 1) neuronal oscillation phase transits from random to a highly organized state just after the fixation onset, 2) this phase concentration is accompanied by increased spectral power in several frequency bands, and 3) visual response amplitude is enhanced at the specific oscillatory phase associated with fixation. We hypothesize that nonvisual inputs are used by the brain to increase cortical excitability at fixation onset, thus "priming" the system for new visual inputs generated at fixation. Despite remaining mechanistic questions, it appears that analysis of fixation-related responses may be useful in studying natural vision.

Different Processing Phases for Features, Figures, and Selective Attention in the Primary Visual Cortex.

Pieter R Roelfsema — 2007-12-10T15:29:41-00:00

Neuron, Vol. 56, No. 5. (6 December 2007), pp. 785-792.

Our visual system imposes structure onto images that usually contain a diversity of surfaces, contours, and colors. Psychological theories propose that there are multiple steps in this process that occur in hierarchically organized regions of the cortex: early visual areas register basic features, higher areas bind them into objects, and yet higher areas select the objects that are relevant for behavior. Here we test these theories by recording from the primary visual cortex (area V1) of monkeys. We demonstrate that the V1 neurons first register the features (at a latency of 48 ms), then segregate figures from the background (after 57 ms), and finally select relevant figures over irrelevant ones (after 137 ms). We conclude that the psychological processing stages map onto distinct time episodes that unfold in the visual cortex after the presentation of a new stimulus, so that area V1 may contribute to all these processing steps.

Ultra-rapid categorisation in non-human primates.

P Girard — 2008-02-11T11:25:44-00:00

Anim Cogn (8 February 2008)

The visual system of primates is remarkably efficient for analysing information about objects present in complex natural scenes. Recent work has demonstrated that they perform this at very high speeds. In a choice saccade task, human subjects can initiate a first reliable saccadic eye movement response to a target (the image containing an animal) in only 120 ms after image onset. Such fast responses impose severe time constraints if one considers neuronal responses latencies in high-level ventral areas of the macaque monkey. The question then arises: are non-human primates able to perform the task? Two rhesus macaque monkeys (Macaca mulatta) were trained to perform the same forced-choice categorization task as the one used in humans. Both animals performed the task with a high accuracy and generalized to new stimuli that were introduced everyday: accuracy levels were comparable both with new and well-known images (84% vs. 94%). More importantly, reaction times were extremely fast (minimum reaction time 100 ms and median reaction time 152 ms). Given that typical single units onset times in Inferotemporal cortex (IT) are about as long as the shortest behavioural responses measured here, we conclude that visual processing involved in ultra rapid categorisations might be based on rather simple shape cue analysis that can be achieved in areas such as extrastriate cortical area V4. The present paper demonstrates for the first time, that rhesus macaque monkeys (Macaca mulatta) are able to match human performance in a forced-choice saccadic categorisation task of animals in natural scenes.

Saccadic eye movement related potentials.

F Jagla — 2008-01-21T08:36:47-00:00

Physiol Res, Vol. 56, No. 6. (2007), pp. 707-713.

The saccadic eye movement related potentials (SEMRPs) enable to study brain mechanisms of the sensorimotor integration. SEMRPs provide insight into various cognitive mechanisms related to planning, programming, generation and execution of the saccadic eye movements. SEMRPs can be used to investigate pathophysiological mechanisms of several disorders of the central nervous system. Here we shortly summarize basic findings concerning the significance of SEMRP components, their relationship to the functional brain asymmetry and visual attention level as well as changes related to certain neuropsychological disorders.

Macaque V1 Activity During Natural Vision: Effects of Natural Scenes and Saccades.

Sean P Macevoy — 2008-01-15T02:33:47-00:00

J Neurophysiol (12 December 2007)

Natural vision takes place within the context of rich varied stimuli and frequent eye movements. In this study, we examined how scene complexity and saccades combine to sculpt the temporal response patterns of V1 neurons. To bridge the gap between conventional and free-viewing experiments, we compared neural responses across four paradigms ranging from less to more natural. An optimal bar stimulus was either flashed into a receptive field (RF) or brought into it via saccade, and was embedded in either a natural scene or a uniform gray background. Responses to flashed bars tended to be higher with a uniform rather than natural background. The most novel result is that responses evoked by stimuli brought into the RF via saccades were enhanced compared to stimuli flashed during steady fixation. No single factor appears to account entirely for this surprising effect, but there were small contributions from fixational saccades and residual activity carried over from the previous fixation. We also found a negative correlation with cells' response "history", in that a larger response on one fixation was associated with a lower response on the subsequent fixation. The effects of the natural background and saccades exhibited a significant non-linear interaction, with the suppressive effects of the natural background less for stimuli entering RFs with saccades. Together, these results suggest that even responses to standard optimal stimuli are difficult to predict under conditions similar to natural vision, and further demonstrate the importance of naturalistic experimental paradigms to the study of visual processing in V1.

Differences in saccade-evoked brain activation patterns with eyes open or eyes closed in complete darkness.

K Hüfner — 2008-01-11T03:17:12-00:00

Exp Brain Res (9 January 2008)

In this study we attempted to differentiate distinct components of the saccade network, namely cortical ocular motor centers and parieto-occipital brain regions, by means of a "minimal design" approach. Using a blocked design fMRI paradigm we evaluated the BOLD changes in a 2 x 2 factorial design experiment which was performed in complete darkness: while looking straight ahead with eyes open (OPEN) or closed (CLOSED) as well as during the execution of self-initiated horizontal to-and-fro saccades with the eyes open (SACCopen) or closed (SACCclosed). Eye movements were monitored outside the scanner via electro-oculography and during scanning using video-oculography. Unintentional eye-drifts did not differ during OPEN and CLOSED and saccade frequencies, and amplitudes did not vary significantly between the two saccade conditions. The main findings of the functional imaging study were as follows: (1) Saccades with eyes open or closed in complete darkness lead to distinct differences in brain activation patterns. (2) A parieto-occipital brain region including the precuneus, superior parietal lobule, posterior part of the intraparietal sulcus (IPS), and cuneus was relatively deactivated during saccades performed with eyes closed but not during saccades with eyes open or when looking straight ahead. This could indicate a preparatory state for updating spatial information, which is active during saccades with eyes open even without actual visual input. The preparatory state is suppressed when the eyes are closed during the saccades. (3) Selected ocular motor areas, not including the parietal eye field (PEF), show a stronger activation during SACCclosed than during SACCopen. The increased effort involved in performing saccades with eyes closed, perhaps due to the unusualness of the task, may be the cause of this increased activation.

The temporal impulse response underlying saccadic decisions.

CJ Ludwig — 2007-11-23T19:02:04-00:00

J Neurosci, Vol. 25, No. 43. (26 October 2005), pp. 9907-9912.

Models of perceptual decision making often assume that sensory evidence is accumulated over time in favor of the various possible decisions, until the evidence in favor of one of them outweighs the evidence for the others. Saccadic eye movements are among the most frequent perceptual decisions that the human brain performs. We used stochastic visual stimuli to identify the temporal impulse response underlying saccadic eye movement decisions. Observers performed a contrast search task, with temporal variability in the visual signals. In experiment 1, we derived the temporal filter observers used to integrate the visual information. The integration window was restricted to the first approximately 100 ms after display onset. In experiment 2, we showed that observers cannot perform the task if there is no useful information to distinguish the target from the distractor within this time epoch. We conclude that (1) observers did not integrate sensory evidence up to a criterion level, (2) observers did not integrate visual information up to the start of the saccadic dead time, and (3) variability in saccade latency does not correspond to variability in the visual integration period. Instead, our results support a temporal filter model of saccadic decision making. The temporal impulse response identified by our methods corresponds well with estimates of integration times of V1 output neurons.

Selective activation of visual cortex neurons by fixational eye movements: implications for neural coding.

DM Snodderly — 2007-11-23T18:55:42-00:00

Vis Neurosci, Vol. 18, No. 2. (r 2001), pp. 259-277.

During normal vision, when subjects attempt to fix their gaze on a small stimulus feature, small fixational eye movements persist. We have recorded the impulse activity of single neurons in primary visual cortex (V1) of macaque monkeys while their fixational eye movements moved the receptive-field activating region (AR) over and around a stationary stimulus. Three types of eye movement activation were found. (1) Saccade cells discharged when a fixational saccade moved the AR onto the stimulus, off the stimulus, or across the stimulus. (2) Position/drift cells discharged during the intersaccadic (drift) intervals and were not activated by saccades that swept the AR across the stimulus without remaining on it. To activate these neurons, it was essential that the AR be placed on the stimulus and many of these cells were selective for the sign of contrast. They had smaller ARs than the other cell types. (3) Mixed cells fired bursts of activity immediately following saccades and continued to fire at a lower rate during intersaccadic intervals. The tendency of each neuron to fire transient bursts or sustained trains of impulses following saccades was strongly correlated with the transiency of its response to stationary flashed stimuli. For one monkey, an extraretinal influence accompanying fixational saccades was identified. During natural viewing, the different eye movement classes probably make different contributions to visual processing. Position/drift neurons are well suited for coding spatial details of the visual scene because of their small AR size and their selectivity for sign of contrast and retinal position. However, saccade neurons transmit information that is ambiguous with respect to the spatial details of the retinal image because they are activated whether the AR lands on a stimulus contour, or the AR leaves or crosses the contour and lands in another location. Saccade neurons may be involved in constructing a stable world in spite of incessant retinal image motion, as well as in suppressing potentially confusing input associated with saccades.

Dynamics of neuronal sensitivity in visual cortex and local feature discrimination.

V Dragoi — 2006-11-12T05:19:53-00:00

Nat Neurosci, Vol. 5, No. 9. (September 2002), pp. 883-891.

A striking aspect of natural scenes is that image features such as line orientation are strongly correlated at neighboring spatial locations but not at distant locations. Thus, during the viewing of a scene, eye movements are often accompanied by a change in the orientation structure of the image. How does this behavior influence the discrimination of local features and their encoding by visual cortical neurons? Here we examined the perceived changes in orientation induced by brief exposure to oriented image patterns in monkeys and humans, and then used reverse correlation to investigate dynamic changes in neuronal sensitivity in the primary visual cortex (V1) of behaving monkeys. Whereas brief adaptation to an oriented grating impaired identification of nearby orientations by broadening orientation selectivity and changing the preferred orientation of individual V1 neurons, it actually enhanced the identification of orthogonal orientations by sharpening neuronal selectivity. Hence, successive exposure to image patches of dissimilar spatial structure enhances both the ability to discriminate local features and the encoding of these features by V1 neurons.

Image structure at the center of gaze during free viewing.

V Dragoi — 2006-11-12T05:21:50-00:00

J Cogn Neurosci, Vol. 18, No. 5. (May 2006), pp. 737-748.

It is generally believed that the visual system is adapted to the statistics of the visual world. Measuring and understanding these statistics require precise knowledge of the structure of the signals reaching fovea during image scanning. However, despite the fact that eye movements cause retinal stimulation to change several times in a second, it is implicitly assumed that images are sampled uniformly during natural viewing. By analyzing the eye movements of three rhesus monkeys freely viewing natural scenes, we report here significant anisotropy in stimulus statistics at the center of gaze. We find that fixation on an image patch is more likely to be followed by a saccade to a nearby patch of similar orientation structure or by a saccade to a more distant patch of largely dissimilar orientation structure. Furthermore, we show that orientation-selective neurons in the primary visual cortex (V1) can take advantage of eye movement statistics to selectively improve their discrimination performance.

Visually evoked cortical potentials in awake cats during saccadic eye movements.

S Chakraborty — 2007-11-15T16:04:54-00:00

Exp Brain Res, Vol. 122, No. 2. (September 1998), pp. 203-213.

Visually evoked potentials (VEPs) measured under conditions of retinal image stabilization that minimized the influences of visual masking and smearing were averaged from electroencephalographic records measured from striate cortex of three cats. The amplitudes of the VEPs increased around saccade initiation. The grating-evoked potentials obtained at different times relative to the saccade exhibited changes in waveform shape that could be attributed to a saccade-evoked potential. The changes in the shape of the waveform were reasonably accounted for by the summation of the grating-evoked potential (produced when the cat did not make a saccade) and an appropriately timed saccade-evoked potential. The fundamental amplitudes of the residual potentials were computed and found to vary across the time course of the saccade. These observations suggest that there are other influences besides visual masking that are exerted early in the visual pathway to modulate visual processing during saccadic eye movements. A corollary discharge process is the most likely candidate to exert these influences.

Phasic electrical activity in the brain associated with eye movement in waking cats.

K Sakai — 2007-11-06T09:39:53-00:00

Brain Res, Vol. 56 (29 June 1973), pp. 135-150.

Periodic activity in the visual system of the cat.

M Laufer — 2007-11-06T09:31:07-00:00

Vision Res, Vol. 7, No. 3. (March 1967), pp. 215-229.

Effects of saccades on the activity of neurons in the cat lateral geniculate nucleus.

D Lee — 2007-11-05T15:04:17-00:00

J Neurophysiol, Vol. 79, No. 2. (February 1998), pp. 922-936.

Effects of saccades on individual neurons in the cat lateral geniculate nucleus (LGN) were examined under two conditions: during spontaneous saccades in the dark and during stimulation by large, uniform flashes delivered at various times during and after rewarded saccades made to small visual targets. In the dark condition, a suppression of activity began 200-300 ms before saccade start, peaked approximately 100 ms before saccade start, and smoothly reversed to a facilitation of activity by saccade end. The facilitation peaked 70-130 ms after saccade end and decayed during the next several hundred milliseconds. The latency of the facilitation was related inversely to saccade velocity, reaching a minimum for saccades with peak velocity >70-80 degrees /s. Effects of saccades on visually evoked activity were remarkably similar: a facilitation began at saccade end and peaked 50-100 ms later. When matched for saccade velocity, the time courses and magnitudes of postsaccadic facilitation for activity in the dark and during visual stimulation were identical. The presaccadic suppression observed in the dark condition was similar for X and Y cells, whereas the postsaccadic facilitation was substantially stronger for X cells, both in the dark and for visually evoked responses. This saccade-related regulation of geniculate transmission appears to be independent of the conditions under which the saccade is evoked or the state of retinal input to the LGN. The change in activity from presaccadic suppression to postsaccadic facilitation amounted to an increase in gain of geniculate transmission of approximately 30%. This may promote rapid central registration of visual inputs by increasing the temporal contrast between activity evoked by an image near the end of a fixation and that evoked by the image immediately after a saccade.

EEG of striate cortex in blind monkeys: effects of eye movements and sleep.

H Sakakura — 2007-11-05T15:01:37-00:00

Arch Ital Biol, Vol. 114, No. 1. (February 1976), pp. 23-48.

After control studies, using electrodes permanently implanted in the central visual system, squirrel monkeys and macaques were in most instances blinded by acute glaucoma. This permitted subsequent observation of eye movements. Ocular nystagmus developed in all cases. Beginning immediately upon recovery from anesthesia, and persisting for at least 1 year, the EEG of the striate cortex was characterized by totally flat periods up to several seconds in duration which were ended abruptly by a sharp "spike" trailed in turn by a ragged high voltage, slow pattern for another second or two. The great majority of these "spikes" from the blind striate cortex occurred within 60-200 msec after a saccadic eye movement, made either in nystagmus or attempted fixation. They were not dependent upon proprioception from the extraocular muscles. It is suggested that they represent a "corollary discharge" for movement of the eyes. The blind striate cortex was judged to be hyperexcitable on the basis of these saccade-associated "spikes", not often observable in intact monkeys, and from the increase both in response evoked by electrical stimulation of optic radiation and amplitude of the EEG in sleep.

Influence of eye movements on geniculo-striate excitability in the cat.

WR Adey — 2007-11-05T14:54:33-00:00

J Physiol, Vol. 235, No. 3. (December 1973), pp. 805-821.

1. The excitability of the geniculo-striate pathway during a saccadic eye movement was studied in alert cats with chronically implanted electrodes. Excitability was assessed by the amplitude of post-synaptic components of field responses in both lateral geniculate nucleus and visual cortex to electrical stimulation of the optic chiasm. Modifications in amplitude were evaluated during the period following eye movements, by triggering a stimulator from potential shifts in the electrooculograms and altering delays in the stimulus pulse.2. The post-synaptic component of the geniculate response was markedly depressed for about 150 msec, reaching a trough at approximately 100 msec after the initiation of an eye movement. This effect was dependent on the visual environment and was not observed in complete darkness. A similar depression occurred when the visual field was abruptly moved by retinal impulses generated by a quick displacement of the image of the visual world associated with an eye movement. The depression reflected a reduction of cellular discharge to the orthodromic volley and hence a suppression of the transmission of visual information through the lateral geniculate nucleus. This may be a mechanism for saccadic suppression.3. The post-synaptic components of the cortical response were enhanced for about 200 msec, reaching a peak at approximately 150 msec after the initiation of an eye movement. Although this facilitation occurred also in complete darkness, it did not occur when the visual field was abruptly shifted while the eyes were stationary. The fact that it occurred with eye movements and exclusively in the post-synaptic components suggests that it was caused by signals from a system closely related to eye movements. This may be a manifestation of the corollary mechanism.

Saccade-related activity in areas 18 and 21a of cats freely viewing complex scenes.

GU Moeller — 2007-11-05T14:43:25-00:00

Neuroreport, Vol. 18, No. 5. (26 March 2007), pp. 401-404.

Although saccadic eye movements can radically change the retinal image, perceptually their impact is surprisingly small. Here, we investigate possible neuronal correlates of saccadic suppression in cats freely viewing natural stimuli. By comparing changes attributable to saccadic events with passive stimulus changes, we find that during saccades: (i) evoked and induced activity is reduced in areas 18 and 21a by equal amounts, (ii) the variability of neuronal activity with stimulus category is abolished in both areas, and (iii) the high-power transient induced by stimulus change is not observed. These results present electrophysiological evidence for saccadic suppression at the level of primary and higher visual cortex under natural conditions.

Changes in visual perception at the time of saccades.

J Ross — 2007-11-05T14:36:59-00:00

Trends Neurosci, Vol. 24, No. 2. (February 2001), pp. 113-121.

We frequently reposition our gaze by making rapid ballistic eye movements that are called saccades. Saccades pose problems for the visual system, because they generate rapid, large-field motion on the retina and change the relationship between the object position in external space and the image position on the retina. The brain must ignore the one and compensate for the other. Much progress has been made in recent years in understanding the effects of saccades on visual function and elucidating the mechanisms responsible for them. Evidence suggests that saccades trigger two distinct neural processes: (1) a suppression of visual sensitivity, specific to the magnocellular pathway, that dampens the sensation of motion and (2) a gross perceptual distortion of visual space in anticipation of the repositioning of gaze. Neurophysiological findings from several laboratories are beginning to identify the neural substrates involved in these effects.

Topography of visually evoked brain activity during eye movements: lambda waves, saccadic suppression, and discrimination performance.

W Skrandies — 2007-11-05T14:25:27-00:00

Int J Psychophysiol, Vol. 27, No. 1. (July 1997), pp. 15-27.

Eye movement-related brain activity was studied in 14 subjects by recording EEG topographically in 16 channels over the occipital brain areas. Potential fields obtained with or without the simultaneous presentation of a visual stimulus during the time course of horizontal saccades were compared. Without visual stimulation, eye movements were followed at a mean latency of about 65 ms by a lateralized occipital dominant component whose topography was determined by the direction of the saccade but whose latency was independent of the time course of the eye movements. This component was reminiscent of lambda waves, however, it could also be elicited in complete darkness. When stimuli were presented during saccades, component latencies increased significantly, and there were also topographic changes in the evoked potential fields. Negative centroids were located more anteriorly and positive ones more posteriorly on the scalp when compared to brain activity recorded with stable eye positions and visual stimulation. All subjects reported no suppression of visual stimuli when presented during saccades occurred. This was confirmed by testing the discrimination performance of an independent group of 27 subjects. Our data show that the execution of saccades elicits electrophysiological patterns of activation in the visual cortex even without visual input. The increase of component latency observed during saccades as well as topographical differences suggest that visual information is processed by different neuronal elements during saccadic eye movements.

Selective suppression of the magnocellular visual pathway during saccadic eye movements.

DC Burr — 2007-05-18T12:53:17-00:00

Nature, Vol. 371, No. 6497. (6 October 1994), pp. 511-513.

Visual scientists have long sought to explain why the world remains stable during saccades, the ballistic eye-movements that continually displace the retinal image at fast but resolvable velocities. An early suggestion was that vision may be actively suppressed during saccades, but experimental support has been variable. Here we present evidence that saccadic suppression does occur, but that it is selective for patterns modulated in luminance at low spatial frequencies. Patterns of higher spatial frequency, and equiluminant patterns (modulated only in colour) at all spatial frequencies were not suppressed during saccades, but actually enhanced. The selectivity of the suppression suggests that it is confined to the colour-blind magnocellular stream (which provides the dominant input to motion centres and areas involved with attention), where it could dull the otherwise disturbing sense of fast low-spatial-frequency image motion. Masking studies suggest that the suppression precedes the site of contrast masking and may therefore occur early in visual processing, possibly as early as the lateral geniculate nucleus.

Influence of saccadic eye movements on geniculostriate excitability in normal monkeys.

JR Bartlett — 2007-11-05T14:16:38-00:00

Exp Brain Res, Vol. 25 (28 July 1976), pp. 487-509.

Using permanently implanted electrodes in squirrel monkeys and macaques, transmission through the lateral geniculate nucleus (LGN) was assayed from the amplitude of potentials evoked in optic radiation by and electrical pulse applied to optic tract. Averaging of either individually or machine selected potentials, elicited at 0.3, 1.0, 20 or 50 HZ, in all cases showed a decrease in transmission ranging from 5-60% in the period after saccadic eye movements made ad libitum. The suppression was greater in a patterned visual environment than in diffuse illumination, which in turn was greater than that occurring following saccades in the dark. Demonstration of the effect in darkness always required data averaging and never exceeded 20%. The effect was consistently greater in the magnocellular than parvocellular component. Suppresion was often abruptly terminated and replaced by a facilitation of 5-15% about 100 msec after saccade detection. Comparable effects were observed for excitability of striate cortex tested by a stimulus pulse applied to optic radiation. In addition, sharply demarcated potentials inherently arising in LGN and striate cortex were found in association with saccades made even in total darkness. Neglecting a possible but dubious contribution from eye muscle proprioceptors, the experiments establish the existence of a centrally originating modulation of visual processing at both LGN and striate cortex in ralation to saccadic eye movement in primates. This modulation may partially underlie the phenomenon of "saccadic suppression" and hasten the acquistion of a meaningful visualsample immediately following an ocular saccade. It remains uncertain as to how it may relate to similar or greater effects accompanying changes in alertness, or to fluctuations of unknown origin occurring sometimes semirhythmically at 0.05-0.03 HZ (Fig 7).

Large-Scale Gamma-Band Phase Synchronization and Selective Attention.

Sam M Doesburg — 2007-06-15T19:04:05-00:00

Cereb Cortex (7 June 2007)

Explaining the emergence of a coherent conscious percept and an intentional agent from the activity of distributed neurons is key to understanding how the brain produces higher cognitive processes. Gamma-band synchronization has been proposed to be a mechanism for the functional integration of neural populations that together form a transitory, large-scale, task- and/or percept-specific network. The operation of this mechanism in the context of attention orienting entails that cortical regions representing attended locations should show more gamma-band synchronization with other cortical areas than would those representing unattended locations. This increased synchronization should be apparent in the same time frame as that of the deployment of attention to a particular location. In order to observe this effect, we made electroencephalogram recordings while subjects attended to one side or the other of the visual field (which we confirmed by event-related potential analysis) and calculated phase-locking statistics between the signals recorded at relevant electrode pairs. We observed increased gamma-band phase synchronization between visual cortex contralateral to the attended location and other, widespread, cortical areas approximately 240-380 ms after the directional cue was presented, confirming the prediction of a large-scale gamma synchronous network oriented to the cued location.

Compression and reflection of visually evoked cortical waves.

W Xu — 2007-07-06T18:19:24-00:00

Neuron, Vol. 55, No. 1. (5 July 2007), pp. 119-129.

Neuronal interactions between primary and secondary visual cortical areas are important for visual processing, but the spatiotemporal patterns of the interaction are not well understood. We used voltage-sensitive dye imaging to visualize neuronal activity in rat visual cortex and found visually evoked waves propagating from V1 to other visual areas. A primary wave originated in the monocular area of V1 and was "compressed" when propagating to V2. A reflected wave initiated after compression and propagated backward into V1. The compression occurred at the V1/V2 border, and local GABA(A) inhibition is important for the compression. The compression/reflection pattern provides a two-phase modulation: V1 is first depolarized by the primary wave, and then V1 and V2 are simultaneously depolarized by the reflected and primary waves, respectively. The compression/reflection pattern only occurred for evoked waves and not for spontaneous waves, suggesting that it is organized by an internal mechanism associated with visual processing.

Spontaneous Activity Associated with Primary Visual Cortex: A Resting-State fMRI Study.

Kun Wang — 2007-07-06T06:42:48-00:00

Cereb Cortex (29 June 2007)

Brain functions during the resting state have attracted considerable attention in the past several years. However, little has been known about spontaneous activity in the sensory cortices in the task-free state. This study used functional magnetic resonance imaging (fMRI) to investigate the existence of spontaneous activity in the primary visual areas (PVA) of normal-sighted subjects and to explore the physiological implications of such activity. Our results revealed that we were able to detect spontaneous activity, which was nonrandom in that it was distinctly clustered both temporally and spatially in the PVA of each subject. In addition, the neural network associated with the PVA-related spontaneous activity included the visual association areas, the precuneus, the precentral/postcentral gyrus, the middle frontal gyrus, the fusiform gyrus, the inferior/middle temporal gyrus, and the parahippocampal gyrus. After considering the functions of these regions, we speculated that the PVA-related spontaneous activity may be associated with memory-related mental imagery and/or visual memory consolidation processes. These findings confirm the presence of spontaneous activity in the PVA and related brain areas. This confirmation supports the perspective that brain is a system intrinsically operating on its own, and sensory information interacts with rather than determines the operation of the system.

Surfing a spike wave down the ventral stream

Rufin Vanrullen — 2007-10-12T08:46:13-00:00

Vision Research, Vol. 42, No. 23. (October 2002), pp. 2593-2615.

Rate coding versus temporal order coding: what the retinal ganglion cells tell the visual cortex.

R Van Rullen — 2006-11-19T17:48:15-00:00

Neural Comput, Vol. 13, No. 6. (June 2001), pp. 1255-1283.

It is often supposed that the messages sent to the visual cortex by the retinal ganglion cells are encoded by the mean firing rates observed on spike trains generated with a Poisson process. Using an information transmission approach, we evaluate the performances of two such codes, one based on the spike count and the other on the mean interspike interval, and compare the results with a rank order code, where the first ganglion cells to emit a spike are given a maximal weight. Our results show that the rate codes are far from optimal for fast information transmission and that the temporal structure of the spike train can be efficiently used to maximize the information transfer rate under conditions where each cell needs to fire only one spike.

Temporal codes and sparse representations: A key to understanding rapid processing in the visual system

Rudy Guyonneau — 2006-02-18T07:30:44-00:00

Journal of Physiology-Paris, Vol. 98, No. 4-6. ( 2004), pp. 487-497.

Where neural information processing is concerned, there is no debate about the fact that spikes are the basic currency for transmitting information between neurons. How the brain actually uses them to encode information remains more controversial. It is commonly assumed that neuronal firing rate is the key variable, but the speed with which images can be analysed by the visual system poses a major challenge for rate-based approaches. We will thus expose here the possibility that the brain makes use of the spatio-temporal structure of spike patterns to encode information. We then consider how such rapid selective neural responses can be generated rapidly through spike-timing-dependent plasticity (STDP) and how these selectivities can be used for visual representation and recognition. Finally, we show how temporal codes and sparse representations may very well arise one from another and explain some of the remarkable features of processing in the visual system.

Ultra-rapid object detection with saccadic eye movements: visual processing speed revisited.

H Kirchner — 2007-10-12T06:48:41-00:00

Vision Res, Vol. 46, No. 11. (May 2006), pp. 1762-1776.

Previous ultra-rapid go/no-go categorization studies with manual responses have demonstrated the remarkable speed and efficiency with which humans process natural scenes. Using a forced-choice saccade task we show here that when two scenes are simultaneously flashed in the left and right hemifields, human participants can reliably make saccades to the side containing an animal in as little as 120 ms. Low level differences between target and distractor images were unable to account for these exceptionally fast responses. The results suggest a very fast and unexpected route linking visual processing in the ventral stream with the programming of saccadic eye movements.