CiteULike: Tag kristina

Anterior cingulate cortex activity can be independent of response conflict in Stroop-like tasks.

Ardi Roelofs — 2006-09-13T18:43:56-00:00

Proc Natl Acad Sci U S A (5 September 2006)

Cognitive control includes the ability to formulate goals and plans of action and to follow these while facing distraction. Previous neuroimaging studies have shown that the presence of conflicting response alternatives in Stroop-like tasks increases activity in dorsal anterior cingulate cortex (ACC), suggesting that the ACC is involved in cognitive control. However, the exact nature of ACC function is still under debate. The prevailing conflict detection hypothesis maintains that the ACC is involved in performance monitoring. According to this view, ACC activity reflects the detection of response conflict and acts as a signal that engages regulative processes subserved by lateral prefrontal brain regions. Here, we provide evidence from functional MRI that challenges this view and favors an alternative view, according to which the ACC has a role in regulation itself. Using an arrow-word Stroop task, subjects responded to incongruent, congruent, and neutral stimuli. A critical prediction made by the conflict detection hypothesis is that ACC activity should be increased only when conflicting response alternatives are present. Our data show that ACC responses are larger for neutral than for congruent stimuli, in the absence of response conflict. This result demonstrates the engagement of the ACC in regulation itself. A computational model of Stroop-like performance instantiating a version of the regulative hypothesis is shown to account for our findings.

Attentional modulation in the human visual cortex: the time-course of the BOLD response and its implications.

AT Smith — 2006-03-10T18:14:24-00:00

Neuroimage, Vol. 29, No. 1. (1 January 2006), pp. 328-334.

Throughout the visual areas of the brain, the sensory response to a stimulus is enhanced by attending to the stimulus. Neurophysiological studies in primates show that such enhancement is marked in posterior parietal cortex and some anterior occipital areas, but much more modest in the earliest processing stages, such as the primary visual cortex (V1). In contrast, human fMRI studies show large and robust attentional modulation in all visual areas, including V1. We investigate the possibility that, in the case of fMRI, the BOLD (blood oxygen level dependent) response may be increased not only by local attention-related increases in neural activity, but also by local blood-flow increases caused by remote control systems that anticipate an impending need for oxygen at the attended location. Such changes could be much more rapid than the rather slow response to oxygenation change that typifies the BOLD response. We have employed a paradigm that isolates the component of the BOLD response due to attentional modulation and the component due to the mere presence of a visual stimulus. The results show that the temporal profiles of the BOLD responses in human V1 to the onset of a stimulus and to the onset of attention are extremely similar. The time-course of the attention-related BOLD response is not consistent with the action of remote, anticipatory control mechanisms and suggests that the modulatory effect of attention seen in human V1 with fMRI probably reflects genuine changes in local neural activity that are considerably larger than in non-human primates.

Direct neurophysiological evidence for spatial suppression surrounding the focus of attention in vision.

JM Hopf — 2006-03-10T17:10:19-00:00

Proc Natl Acad Sci U S A, Vol. 103, No. 4. (24 January 2006), pp. 1053-1058.

The spatial focus of attention has traditionally been envisioned as a simple spatial gradient of enhanced activity that falls off monotonically with increasing distance. Here, we show with high-density magnetoencephalographic recordings in human observers that the focus of attention is not a simple monotonic gradient but instead contains an excitatory peak surrounded by a narrow inhibitory region. To demonstrate this center-surround profile, we asked subjects to focus attention onto a color pop-out target and then presented probe stimuli at various distances from the target. We observed that the electromagnetic response to the probe was enhanced when the probe was presented at the location of the target, but the probe response was suppressed in a narrow zone surrounding the target and then recovered at more distant locations. Withdrawing attention from the pop-out target by engaging observers in a demanding foveal task eliminated this pattern, confirming a truly attention-driven effect. These results indicate that neural enhancement and suppression coexist in a spatially structured manner that is optimal to attenuate the most deleterious noise during visual object identification.

Behavior-dependent short-term assembly dynamics in the medial prefrontal cortex.

Shigeyoshi Fujisawa — 2008-06-09T14:29:32-00:00

Nature neuroscience (30 May 2008)

Although short-term plasticity is believed to play a fundamental role in cortical computation, empirical evidence bearing on its role during behavior is scarce. Here we looked for the signature of short-term plasticity in the fine-timescale spiking relationships of a simultaneously recorded population of physiologically identified pyramidal cells and interneurons, in the medial prefrontal cortex of the rat, in a working memory task. On broader timescales, sequentially organized and transiently active neurons reliably differentiated between different trajectories of the rat in the maze. On finer timescales, putative monosynaptic interactions reflected short-term plasticity in their dynamic and predictable modulation across various aspects of the task, beyond a statistical accounting for the effect of the neurons' co-varying firing rates. Seeking potential mechanisms for such effects, we found evidence for both firing pattern-dependent facilitation and depression, as well as for a supralinear effect of presynaptic coincidence on the firing of postsynaptic targets.

Development of hemodynamic responses and functional connectivity in rat somatosensory cortex.

Matthew T Colonnese — 2007-12-03T15:42:44-00:00

Nat Neurosci (25 November 2007)

Functional magnetic resonance imaging (fMRI) is a valuable method for probing postnatal circuit refinement and plasticity. However, its use during early development has been hindered by uncertainty as to the nature of neurovascular coupling in young individuals. Here we used somatosensory stimulation in rats to determine age-related parameters of the blood oxygenation level-dependent (BOLD) signal from its apparent inception on postnatal day 13 to adulthood. By comparing fMRI measurements with electrophysiological recordings, we determined that the regional BOLD response in these animals undergoes a systematic decline in latency and growth in amplitude over this period. We found no evidence of negative BOLD at any age. Maturation of hemodynamic responses correlated with age-dependent increases in susceptibility to inhibition of carbonic anhydrase. With knowledge of the infant BOLD response characteristics, we showed that interhemispheric and higher-order cortical stimulus responses are enhanced during the first several weeks after birth.

Evidence for modality-independent order coding in working memory.

Ann Depoorter — 2008-07-15T14:41:49-00:00

Quarterly journal of experimental psychology (2006) (30 April 2008)

The aim of the present study was to investigate the representation of serial order in working memory, more specifically whether serial order is coded by means of a modality-dependent or a modality-independent order code. This was investigated by means of a series of four experiments based on a dual-task methodology in which one short-term memory task was embedded between the presentation and recall of another short-term memory task. Two aspects were varied in these memory tasks-namely, the modality of the stimulus materials (verbal or visuo-spatial) and the presence of an order component in the task (an order or an item memory task). The results of this study showed impaired primary-task recognition performance when both the primary and the embedded task included an order component, irrespective of the modality of the stimulus materials. If one or both of the tasks did not contain an order component, less interference was found. The results of this study support the existence of a modality-independent order code.

Breakdown of Functional Connectivity in Frontoparietal Networks Underlies Behavioral Deficits in Spatial Neglect

Biyu He — 2007-03-18T00:15:04-00:00

Neuron, Vol. 53, No. 6. (15 March 2007), pp. 905-918.

Summary Spatial neglect is a syndrome following stroke manifesting attentional deficits in perceiving and responding to stimuli in the contralesional field. We examined brain network integrity in patients with neglect by measuring coherent fluctuations of fMRI signals (functional connectivity). Connectivity in two largely separate attention networks located in dorsal and ventral frontoparietal areas was assessed at both acute and chronic stages of recovery. Connectivity in the ventral network, part of which directly lesioned, was diffusely disrupted and showed no recovery. In the structurally intact dorsal network, interhemispheric connectivity in posterior parietal cortex was acutely disrupted but fully recovered. This acute disruption, and disrupted connectivity in specific pathways in the ventral network, strongly correlated with impaired attentional processing across subjects. Lastly, disconnection of the white matter tracts connecting frontal and parietal cortices was associated with more severe neglect and more disrupted functional connectivity. These findings support a network view in understanding neglect.

Musical experience shapes human brainstem encoding of linguistic pitch patterns.

Patrick C M Wong — 2007-03-19T17:36:45-00:00

Nat Neurosci (11 March 2007)

Music and speech are very cognitively demanding auditory phenomena generally attributed to cortical rather than subcortical circuitry. We examined brainstem encoding of linguistic pitch and found that musicians show more robust and faithful encoding compared with nonmusicians. These results not only implicate a common subcortical manifestation for two presumed cortical functions, but also a possible reciprocity of corticofugal speech and music tuning, providing neurophysiological explanations for musicians' higher language-learning ability.

The neural bases of momentary lapses in attention

DH Weissman — 2006-06-28T17:36:05-00:00

Nature Neuroscience, Vol. 9, No. 7. (11 June 2006), pp. 971-978.

Examining the relationship between free recall and immediate serial recall: the serial nature of recall and the effect of test expectancy.

P Bhatarah — 2008-03-11T20:25:55-00:00

Mem Cognit, Vol. 36, No. 1. (January 2008), pp. 20-34.

In two experiments, we examined the relationship between free recall and immediate serial recall (ISR), using a within-subjects (Experiment 1) and a between-subjects (Experiment 2) design. In both experiments, participants read aloud lists of eight words and were precued or postcued to respond using free recall or ISR. The serial position curves were U-shaped for free recall and showed extended primacy effects with little or no recency for ISR, and there was little or no difference between recall for the precued and the postcued conditions. Critically, analyses of the output order showed that although the participants started their recall from different list positions in the two tasks, the degree to which subsequent recall was serial in a forward order was strikingly similar. We argue that recalling in a serial forward order is a general characteristic of memory and that performance on ISR and free recall is underpinned by common memory mechanisms.

Proactive interference from items previously stored in visual working memory.

T Makovski — 2008-03-11T20:22:56-00:00

Mem Cognit, Vol. 36, No. 1. (January 2008), pp. 43-52.

This study investigates the fate of information that was previously stored in visual working memory but that is no longer needed. Previous research has found inconsistent results, with some showing effective release of irrelevant information and others showing proactive interference. Using change detection tasks of colors or shapes, we show that participants tend to falsely classify a changed item as "no change" if it matches one of the memory items on the preceding trial. The interference is spatially specific: Memory for the preceding trial interferes more if it matches the feature value and the location of a test item than if it does not. Interference results from retaining information in visual working memory, since it is absent when items on the preceding trials are passively viewed, or are attended but not memorized. We conclude that people cannot fully eliminate unwanted visual information from current working memory tasks.

Neural repetition suppression reflects fulfilled perceptual expectations.

Christopher Summerfield — 2008-08-07T08:44:32-00:00

Nature neuroscience (1 August 2008)

Stimulus-evoked neural activity is attenuated on stimulus repetition (repetition suppression), a phenomenon that is attributed to largely automatic processes in sensory neurons. By manipulating the likelihood of stimulus repetition, we found that repetition suppression in the human brain was reduced when stimulus repetitions were improbable (and thus, unexpected). Our data suggest that repetition suppression reflects a relative reduction in top-down perceptual 'prediction error' when processing an expected, compared with an unexpected, stimulus.

Psychophysical and neurometric detection performance under stimulus uncertainty.

Maik C Stüttgen — 2008-08-11T14:03:16-00:00

Nature neuroscience (1 August 2008)

Signal detection theoretical analyses of spike counts have revealed that some cortical neurons can exceed psychophysical sensitivity in cases where a sensory signal is specified exactly. It is not known whether this finding holds in the more natural situation where signal occurrence is temporally uncertain. We investigated the ability of rat barrel cortex neurons to detect faint and transient whisker deflections occurring at unspecified times. The progression from fully specified stimuli to temporal uncertainty degraded neuronal sensitivity such that it seems highly unlikely that single neurons can provide the basis for decoding uncertain perceptual events. However, modeling the sensitivity of neuronal pools on basis of spike timing precision across several neurons in an optimal encoding window of 25 ms showed that the subject's perceptual sensitivity could be based on the occurrence of coincident spikes from four to five neurons.

Switching from automatic to controlled action by monkey medial frontal cortex

Masaki Isoda — 2007-01-27T11:53:28-00:00

Nature Neuroscience, Vol. 10, No. 2. (21 January 2007), pp. 240-248.

Figure-ground mechanisms provide structure for selective attention.

Fangtu T Qiu — 2007-10-10T01:22:43-00:00

Nat Neurosci (7 October 2007)

Attention depends on figure-ground organization: figures draw attention, whereas shapes of the ground tend to be ignored. Recent research has revealed mechanisms for figure-ground organization in the visual cortex, but how these mechanisms relate to the attention process remains unclear. Here we show that the influences of figure-ground organization and volitional (top-down) attention converge in single neurons of area V2 in Macaca mulatta. Although we found assignment of border ownership for attended and for ignored figures, attentional modulation was stronger when the attended figure was located on the neuron's preferred side of border ownership. When the border between two overlapping figures was placed in the receptive field, responses depended on the side of attention, and enhancement was generally found on the neuron's preferred side of border ownership. This correlation suggests that the neural network that creates figure-ground organization also provides the interface for the top-down selection process.

Intrinsic Fluctuations within Cortical Systems Account for Intertrial Variability in Human Behavior

Michael Fox — 2007-10-11T01:47:28-00:00

Neuron, Vol. 56, No. 1. (4 October 2007), pp. 171-184.

Summary The resting brain is not silent, but exhibits organized fluctuations in neuronal activity even in the absence of tasks or stimuli. This intrinsic brain activity persists during task performance and contributes to variability in evoked brain responses. What is unknown is if this intrinsic activity also contributes to variability in behavior. In the current fMRI study, we identify a relationship between human brain activity in the left somatomotor cortex and spontaneous trial-to-trial variability in button press force. We then demonstrate that 74% of this brain-behavior relationship is attributable to ongoing fluctuations in intrinsic activity similar to those observed during resting fixation. In addition to establishing a functional and behavioral significance of intrinsic brain activity, these results lend new insight into the origins of variability in human behavior.

Prefrontal Acetylcholine Release Controls Cue Detection on Multiple Timescales

Vinay Parikh — 2007-10-12T09:59:56-00:00

Neuron, Vol. 56, No. 1. (4 October 2007), pp. 141-154.

Summary Cholinergic neurons originating from the basal forebrain innervate the entire cortical mantle. Choline-sensitive microelectrodes were used to measure the synaptic release of cortical acetylcholine (ACh) at a subsecond resolution in rats performing a task involving the detection of cues. Cues that were detected, defined behaviorally, evoked transient increases in cholinergic activity (at the scale of seconds) in the medial prefrontal cortex (mPFC), but not in a nonassociational control region (motor cortex). In trials involving missed cues, cholinergic transients were not observed. Cholinergic deafferentation of the mPFC, but not motor cortex, impaired cue detection. Furthermore, decreases and increases in precue cholinergic activity predicted subsequent cue detection or misses, respectively. Finally, cue-evoked cholinergic transients were superimposed over slower (at the timescale of minutes) changes in cholinergic activity. Cortical cholinergic neurotransmission is regulated on multiple timescales to mediate the detection of behaviorally significant cues and to support cognitive performance.

On the flexibility and the fallibility of associative memory.

KJ Malmberg — 2007-08-14T13:50:12-00:00

Mem Cognit, Vol. 35, No. 3. (April 2007), pp. 545-556.

We report the results of four experiments in which we explored the flexibility and fallibility of associative recognition memory. In each experiment, pairs were studied one or more times, and the task was to discriminate intact from rearranged pairs. The critical findings are that the pattern of false alarm rates was dependent on the nature of the recognition procedure (e.g., ratings vs. yes-no) and the situation in which the task was performed. The specific pattern of findings suggest that subjects adopt different recognition strategies in order to achieve a desired level of performance in the most efficient manner possible by varying the degree to which they base their decisions on familiarity versus recollected information. Implications for theories of recognition memory are discussed.

New Experiences Enhance Coordinated Neural Activity in the Hippocampus

Sen Cheng — 2008-01-24T01:06:18-00:00

Neuron, Vol. 57, No. 2. (24 January 2008), pp. 303-313.

Summary The acquisition of new memories for places and events requires synaptic plasticity in the hippocampus, and plasticity depends on temporal coordination among neurons. Spatial activity in the hippocampus is relatively disorganized during the initial exploration of a novel environment, however, and it is unclear how neural activity during the initial stages of learning drives synaptic plasticity. Here we show that pairs of CA1 cells that represent overlapping novel locations are initially more coactive and more precisely coordinated than are cells representing overlapping familiar locations. This increased coordination occurrs specifically during brief, high-frequency events (HFEs) in the local field potential that are similar to ripples and is not associated with better coordination of place-specific neural activity outside of HFEs. As novel locations become more familiar, correlations between cell pairs decrease. Thus, hippocampal neural activity during learning has a unique structure that is well suited to induce synaptic plasticity and to allow for rapid storage of new memories.

Learning the value of information in an uncertain world.

Timothy E J Behrens — 2007-08-14T13:37:12-00:00

Nat Neurosci (5 August 2007)

Our decisions are guided by outcomes that are associated with decisions made in the past. However, the amount of influence each past outcome has on our next decision remains unclear. To ensure optimal decision-making, the weight given to decision outcomes should reflect their salience in predicting future outcomes, and this salience should be modulated by the volatility of the reward environment. We show that human subjects assess volatility in an optimal manner and adjust decision-making accordingly. This optimal estimate of volatility is reflected in the fMRI signal in the anterior cingulate cortex (ACC) when each trial outcome is observed. When a new piece of information is witnessed, activity levels reflect its salience for predicting future outcomes. Furthermore, variations in this ACC signal across the population predict variations in subject learning rates. Our results provide a formal account of how we weigh our different experiences in guiding our future actions.

Sound source location modulates the irrelevant-sound effect.

A Buchner — 2008-05-27T14:32:25-00:00

Memory & cognition, Vol. 36, No. 3. (April 2008), pp. 617-628.

Participants memorized lists of visually presented digits in silence or while ignoring distractor sounds that either came from the front and thus from the direction in which participants' attention was oriented, or from behind. Distractor sounds impaired recall performance, but the largest impairment was observed when the sound source was directionally close to the frontal visual target display. The results are consistent with the assumption of cross-modal attentional links in models of attention, and they are problematic for explanations of the irrelevant-sound effect within working memory models that do not specify an explicit role of attention in the maintenance of information for immediate serial recall.

Behavioral Shifts and Action Valuation in the Anterior Cingulate Cortex

Rene Quilodran — 2008-01-24T15:58:11-00:00

Neuron, Vol. 57, No. 2. (24 January 2008), pp. 314-325.

Summary Rapid optimization of behavior requires decisions about when to explore and when to exploit discovered resources. The mechanisms that lead to fast adaptations and their interaction with action valuation are a central issue. We show here that the anterior cingulate cortex (ACC) encodes multiple feedbacks devoted to exploration and its immediate termination. In a task that alternates exploration and exploitation periods, the ACC monitored negative and positive outcomes relevant for different adaptations. In particular, it produced signals specific of the first reward, i.e., the end of exploration. Those signals disappeared in exploitation periods but immediately transferred to the initiation of trials--a transfer comparable to learning phenomena observed for dopaminergic neurons. Importantly, these were also observed for high gamma oscillations of local field potentials shown to correlate with brain imaging signal. Thus, mechanisms of action valuation and monitoring of events/actions are combined for rapid behavioral regulation.

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation.

NK Logothetis — 2007-09-10T13:45:08-00:00

Neuron, Vol. 55, No. 5. (6 September 2007), pp. 809-823.

To combine insights obtained from electric field potentials (LFPs) and neuronal spiking activity (MUA) we need a better understanding of the relative spatial summation of these indices of neuronal activity. Compared to MUA, the LFP has greater spatial coherence, resulting in lower spatial specificity and lower stimulus selectivity. A differential propagation of low- and high-frequency electric signals supposedly underlies this phenomenon, which could result from cortical tissue specifically attenuating higher frequencies, i.e., from a frequency-dependent impedance spectrum. Here we directly measure the cortical impedance spectrum in vivo in monkey primary visual cortex. Our results show that impedance is independent of frequency, is homogeneous and tangentially isotropic within gray matter, and can be theoretically predicted assuming a pure-resistive conductor. We propose that the spatial summation of LFP and MUA is determined by the size of these signals' generators and the nature of neural events underlying them, rather than by biophysical properties of gray matter.

Auditory recognition without identification.

AM Cleary — 2008-02-18T20:07:50-00:00

Mem Cognit, Vol. 35, No. 8. (December 2007), pp. 1869-1877.

When visual recognition test items are unidentifiable--through fragmentation, for example--participants can discriminate between unidentifiable items that were presented recently and those that were not. The present study extends this recognition without identification phenomenon to the auditory modality. In several experiments, participants listened to words and were then presented with spoken recognition test items that were embedded in white noise. Participants attempted to identify each spoken word through the white noise, then rated the likelihood that the word was studied. Auditory recognition without identification was found: Participants discriminated between studied and unstudied words in the absence of an ability to identify them through white noise, even when the voice changed from male to female and when the study list was presented visually. The effect was also found when identification was hindered through the isolation of particular phonemes, suggesting that phoneme information may be present in memory traces for recently spoken words.

Specialized Circuits from Primary Visual Cortex to V2 and Area MT.

JJ Nassi — 2007-09-10T13:44:13-00:00

Neuron, Vol. 55, No. 5. (6 September 2007), pp. 799-808.

Primary visual cortex recombines inputs from magnocellular (M) and parvocellular (P) streams to create functionally specialized outputs. Understanding these input-output relationships is complicated by the fact that layer 4B, which provides outputs to dorsal visual areas, contains multiple cell types. Using a modified rabies virus that expresses green fluorescent protein, we show that layer 4B neurons projecting to MT are a majority spiny stellate, whereas those projecting to V2 are overwhelmingly pyramidal. Regardless of cell type, MT-projecting neurons have larger cell bodies, more dendritic length, and are deeper within layer 4B. Furthermore, MT-projecting pyramidal neurons are located preferentially underneath cytochrome oxidase blobs, indicating that MT-projecting neurons of both types restrict their dendrites to M-recipient zones. We conclude that MT-projecting layer 4B neurons are specialized for the fast transmission of information from the M pathway, while V2-projecting neurons are likely to mediate slower computations involving mixed M and P signals.

Cerebral Responses to Change in Spatial Location of Unattended Sounds.

Leon Y Deouell — 2007-09-24T13:54:51-00:00

Neuron, Vol. 55, No. 6. (20 September 2007), pp. 985-996.

The neural basis of spatial processing in the auditory cortex has been controversial. Human fMRI studies suggest that a part of the planum temporale (PT) is involved in auditory spatial processing, but it was recently argued that this region is active only when the task requires voluntary spatial localization. If this is the case, then this region cannot harbor an ongoing spatial representation of the acoustic environment. In contrast, we show in three fMRI experiments that a region in the human medial PT is sensitive to background auditory spatial changes, even when subjects are not engaged in a spatial localization task, and in fact attend the visual modality. During such times, this area responded to rare location shifts, and even more so when spatial variation increased, consistent with spatially selective adaptation. Thus, acoustic space is represented in the human PT even when sound processing is not required by the ongoing task.

Synchronized Firing among Retinal Ganglion Cells Signals Motion Reversal.

G Schwartz — 2007-09-24T13:52:34-00:00

Neuron, Vol. 55, No. 6. (20 September 2007), pp. 958-969.

We show that when a moving object suddenly reverses direction, there is a brief, synchronous burst of firing within a population of retinal ganglion cells. This burst can be driven by either the leading or trailing edge of the object. The latency is constant for movement at different speeds, objects of different size, and bright versus dark contrasts. The same ganglion cells that signal a motion reversal also respond to smooth motion. We show that the brain can build a pure reversal detector using only a linear filter that reads out synchrony from a group of ganglion cells. These results indicate that not only can the retina anticipate the location of a smoothly moving object, but that it can also signal violations in its own prediction. We show that the reversal response cannot be explained by models of the classical receptive field and suggest that nonlinear receptive field subunits may be responsible.

Neuromodulators control the polarity of spike-timing-dependent synaptic plasticity.

GH Seol — 2007-09-24T13:51:51-00:00

Neuron, Vol. 55, No. 6. (20 September 2007), pp. 919-929.

Near coincidental pre- and postsynaptic action potentials induce associative long-term potentiation (LTP) or long-term depression (LTD), depending on the order of their timing. Here, we show that in visual cortex the rules of this spike-timing-dependent plasticity are not rigid, but shaped by neuromodulator receptors coupled to adenylyl cyclase (AC) and phospholipase C (PLC) signaling cascades. Activation of the AC and PLC cascades results in phosphorylation of postsynaptic glutamate receptors at sites that serve as specific "tags" for LTP and LTD. As a consequence, the outcome (i.e., whether LTP or LTD) of a given pattern of pre- and postsynaptic firing depends not only on the order of the timing, but also on the relative activation of neuromodulator receptors coupled to AC and PLC. These findings indicate that cholinergic and adrenergic neuromodulation associated with the behavioral state of the animal can control the gating and the polarity of cortical plasticity.

Early visual deprivation impairs multisensory interactions in humans.

Lisa Putzar — 2007-09-24T13:48:25-00:00

Nat Neurosci (16 September 2007)

Animal studies have shown that visual deprivation during the first months of life permanently impairs the interactions between sensory systems. Here we report an analogous effect for humans who had been deprived of pattern vision for at least the first five months of their life as a result of congenital binocular cataracts. These patients showed reduced audio-visual interactions in later life, although their visual performance in control tasks was unimpaired. Thus, adequate (multisensory) input during the first months of life seems to be a prerequisite in humans, as well as in animals, for the full development of cross-modal interactions.

Two hierarchically organized neural systems for object information in human visual cortex.

Christina S Konen — 2008-01-21T17:09:20-00:00

Nat Neurosci (13 January 2008)

The primate visual system is broadly organized into two segregated processing pathways, a ventral stream for object vision and a dorsal stream for space vision. Here, evidence from functional brain imaging in humans demonstrates that object representations are not confined to the ventral pathway, but can also be found in several areas along the dorsal pathway. In both streams, areas at intermediate processing stages in extrastriate cortex (V4, V3A, MT and V7) showed object-selective but viewpoint- and size-specific responses. In contrast, higher-order areas in lateral occipital and posterior parietal cortex (LOC, IPS1 and IPS2) responded selectively to objects independent of image transformations. Contrary to the two-pathways hypothesis, our findings indicate that basic object information related to shape, size and viewpoint may be represented similarly in two parallel and hierarchically organized neural systems in the ventral and dorsal visual pathways.

Visual working memory for global, object, and part-based information.

MD Patterson — 2007-09-17T14:55:33-00:00

Mem Cognit, Vol. 35, No. 4. (June 2007), pp. 738-751.

We investigated visual working memory for novel objects and parts of novel objects. After a delay period, participants showed strikingly more accurate performance recognizing a single whole object than the parts of that object. This bias to remember whole objects, rather than parts, persisted even when the division between parts was clearly defined and the parts were disconnected from each other so that, in order to remember the single whole object, the participants needed to mentally combine the parts. In addition, the bias was confirmed when the parts were divided by color. These experiments indicated that holistic perceptual-grouping biases are automatically used to organize storage in visual working memory. In addition, our results suggested that the bias was impervious to top-down consciously directed control, because when task demands were manipulated through instruction and catch trials, the participants still recognized whole objects more quickly and more accurately than their parts. This bias persisted even when the whole objects were novel and the parts were familiar. We propose that visual working memory representations depend primarily on the global configural properties of whole objects, rather than part-based representations, even when the parts themselves can be clearly perceived as individual objects. This global configural bias beneficially reduces memory load on a capacity-limited system operating in a complex visual environment, because fewer distinct items must be remembered.

A Neural Representation of Prior Information during Perceptual Inference

Christopher Summerfield — 2008-07-31T12:46:28-00:00

Neuron, Vol. 59, No. 2. (31 July 2008), pp. 336-347.

Summary Perceptual inference is biased by foreknowledge about what is probable or possible. How prior expectations are neurally represented during visual perception, however, remains unknown. We used functional magnetic resonance imaging to measure brain activity in humans judging simple visual stimuli. Perceptual decisions were either biased in favor of a single alternative (A/~A decisions) or taken without bias toward either choice (A/B decisions). Extrastriate and anterior temporal lobe regions were more active during A/~A than A/B decisions, suggesting multiple representations of prior expectations within the visual hierarchy. Forward connectivity was increased when expected and observed perception diverged ("prediction error" signals), whereas prior expectations fed backward from higher to lower regions. Finally, the coincidence between expected and observed perception activated orbital prefrontal regions, perhaps reflecting the reinforcement of prior expectations. These data support computational and quantitative models proposing that a visual percept emerges from converging bottom-up and top-down signals.

Strength-based mirror effects in item and associative recognition: evidence for within-list criterion changes.

WE Hockley — 2007-09-17T14:52:31-00:00

Mem Cognit, Vol. 35, No. 4. (June 2007), pp. 679-688.

Strength-based mirror effects occur when the hit rate is higher and the false alarm rate is lower following strongly encoded study lists than when following more weakly encoded study lists. In Experiments 1A and 1B, strength-based mirror effects were observed in separate tests of single item and associative recognition for random word pairs. In Experiment 2, strength-based mirror effects were again seen when item and associative recognition were tested together. Finally, in Experiments 3 and 4, opposing strength-based mirror effects were observed for item and associative recognition when individual words and word pairs were presented at different rates in the same study lists. Strength-based mirror effects could result from participants' adopting a more conservative decision criterion following strong lists than following weak ones. If this is the case for both item and associative recognition, the present results demonstrate that subjects can adopt different response criteria for different recognition tasks and can alternate between them on a trial-by-trial basis.

Multisensory interplay reveals crossmodal influences on 'sensory-specific' brain regions, neural responses, and judgments.

J Driver — 2008-01-14T19:02:19-00:00

Neuron, Vol. 57, No. 1. (10 January 2008), pp. 11-23.

Although much traditional sensory research has studied each sensory modality in isolation, there has been a recent explosion of interest in causal interplay between different senses. Various techniques have now identified numerous multisensory convergence zones in the brain. Some convergence may arise surprisingly close to low-level sensory-specific cortex, and some direct connections may exist even between primary sensory cortices. A variety of multisensory phenomena have now been reported in which sensory-specific brain responses and perceptual judgments concerning one sense can be affected by relations with other senses. We survey recent progress in this multisensory field, foregrounding human studies against the background of invasive animal work and highlighting possible underlying mechanisms. These include rapid feedforward integration, possible thalamic influences, and/or feedback from multisensory regions to sensory-specific brain areas. Multisensory interplay is more prevalent than classic modular approaches assumed, and new methods are now available to determine the underlying circuits.

Direct evidence for a role of working memory in the attentional blink.

EG Akyürek — 2007-09-17T14:47:59-00:00

Mem Cognit, Vol. 35, No. 4. (June 2007), pp. 621-627.

Theories of selective attention often have a central memory component, which is commonly thought to be limited in some way and is thereby a potential bottleneck in the attentional process. There have been only a few attempts to validate this assertion, and they have produced mixed results. This study presents a specific examination of the link between working memory and attention by engaging active rather than passive memory operations. Two experiments are reported that provide evidence for the involvement of working memory in the attentional blink (AB) phenomenon. Memory loads of increasing size had a detrimental effect on attentional performance within the blink-sensitive interval, but not beyond. Speeded response requirements proved to modulate the AB, but were independent from the memory load effect. Theoretical implications for current models of selective attention are discussed.

The roles of encoding, retrieval, and awareness in change detection.

MR Beck — 2007-09-17T14:46:43-00:00

Mem Cognit, Vol. 35, No. 4. (June 2007), pp. 610-620.

In the experiment reported here, we examined the processes by which expected (probable) changes are detected more frequently than are unexpected (improbable) changes (the change probability effect; Beck, Angelone, & Levin, 2004). The change probability effect may be caused by a bias toward probable changes during encoding of the prechange aspect, during retrieval of the prechange aspect, or during activation of an explicit response to the change. Participants performed a change detection task for probable and improbable changes while their eye movements were tracked. Change detection performance was superior for probable changes, but long-term memory performance was equivalent for both probable and improbable changes. Therefore, although both probable and improbable prechange aspects were encoded, probable prechange aspects were more likely to be retrieved during change detection. Implicit change detection was also greater for probable changes than for improbable changes, suggesting that the change probability effect is the result of a bias during the retrieval and comparison stage of change detection. The stimuli used in the change detection task may be downloaded from www.psychonomic.org/archive.

Only some spatial patterns of fMRI response are read out in task performance.

Mark A Williams — 2007-05-14T15:56:06-00:00

Nat Neurosci (7 May 2007)

Classification methods show that the spatial pattern of a functional magnetic resonance imaging response across the cortex contains category information, but whether such patterns are used, or 'read out', in behavioral performance remains untested. We show that although the spatial pattern in both the retinotopic and lateral occipital cortex (LOC) in humans contains category information, only in the LOC is the pattern stronger for correct than for incorrect trials. Thus, some, but not all, spatial patterns are read out during task performance.

The mnemonic effects of recall on immediate retention.

JS Nairne — 2007-06-07T14:47:12-00:00

Mem Cognit, Vol. 35, No. 1. (January 2007), pp. 191-199.

In three experiments, we investigated the mnemonic effects of an initial recall on later recall in an immediate memory setting. Recall is generally assumed to interfere with the recall of subsequent items (output interference), but previous experiments have failed to control for the confounding effects of time. In the experiments reported here, the passage of time was held constant on all trials; what varied was whether an additional item was recalled (or simply presented) during the retention interval. The results revealed clear evidence of recall's mnemonic effects, but output interference seemed strongest when the initial recall was of an item that followed the target item in the memory list. When participants initially recalled an item immediately preceding the target, target recall improved. This pattern of results places constraints on current models of immediate retention.

Neuronal oscillations and multisensory interaction in primary auditory cortex.

P Lakatos — 2007-01-22T18:22:45-00:00

Neuron, Vol. 53, No. 2. (18 January 2007), pp. 279-292.

Recent anatomical, physiological, and neuroimaging findings indicate multisensory convergence at early, putatively unisensory stages of cortical processing. The objective of this study was to confirm somatosensory-auditory interaction in A1 and to define both its physiological mechanisms and its consequences for auditory information processing. Laminar current source density and multiunit activity sampled during multielectrode penetrations of primary auditory area A1 in awake macaques revealed clear somatosensory-auditory interactions, with a novel mechanism: somatosensory inputs appear to reset the phase of ongoing neuronal oscillations, so that accompanying auditory inputs arrive during an ideal, high-excitability phase, and produce amplified neuronal responses. In contrast, responses to auditory inputs arriving during the opposing low-excitability phase tend to be suppressed. Our findings underscore the instrumental role of neuronal oscillations in cortical operations. The timing and laminar profile of the multisensory interactions in A1 indicate that nonspecific thalamic systems may play a key role in the effect.

Examining recognition criterion rigidity during testing using a biased-feedback technique: evidence for adaptive criterion learning.

S Han — 2008-07-14T14:36:56-00:00

Memory & cognition, Vol. 36, No. 4. (June 2008), pp. 703-715.

Recognition models often assume that subjects use specific evidence values (decision criteria) to adaptively parse continuous memory evidence into response categories (e.g., "old," "new"). Although explicit pretest instructions influence criterion placement, these criteria appear extremely resistant to change once testing begins. We tested criterion sensitivity to local feedback using a novel biased-feedback technique designed to tacitly encourage certain errors by indicating they are the correct choices. Experiment 1 demonstrated that fully correct feedback had little effect on criterion placement, whereas biased feedback during Experiments 2 and 3 yielded prominent, durable, and adaptive criterion shifts, with observers reporting that they were unaware of the manipulation in Experiment 3. These data suggest that recognition criteria can be easily modified during testing through a form of feedback learning that operates independently of stimulus characteristics and observers' awareness of the nature of the manipulation. This mechanism may be fundamentally different from criterion shifts following explicit instructions and warnings, or shifts linked to manipulations of stimulus characteristics combined with feedback highlighting those manipulations.

Feature-based attention modulates orientation-selective responses in human visual cortex.

T Liu — 2007-07-23T13:52:20-00:00

Neuron, Vol. 55, No. 2. (19 July 2007), pp. 313-323.

How does feature-based attention modulate neural responses? We used adaptation to quantify the effect of feature-based attention on orientation-selective responses in human visual cortex. Observers were adapted to two superimposed oblique gratings while attending to one grating only. We measured the magnitude of attention-induced orientation-selective adaptation both psychophysically, by the behavioral tilt aftereffect, and physiologically, using fMRI response adaptation. We found evidence for orientation-selective attentional modulation of neuronal responses-a lower fMRI response for the attended than the unattended orientation-in multiple visual areas, and a significant correlation between the magnitude of the tilt aftereffect and that of fMRI response adaptation in V1, the earliest site of orientation coding. These results show that feature-based attention can selectively increase the response of neuronal subpopulations that prefer the attended feature, even when the attended and unattended features are coded in the same visual areas and share the same retinotopic location.

Feature-Based Attentional Modulations in the Absence of Direct Visual Stimulation.

John T Serences — 2007-07-23T13:50:50-00:00

Neuron, Vol. 55, No. 2. (19 July 2007), pp. 301-312.

When faced with a crowded visual scene, observers must selectively attend to behaviorally relevant objects to avoid sensory overload. Often this selection process is guided by prior knowledge of a target-defining feature (e.g., the color red when looking for an apple), which enhances the firing rate of visual neurons that are selective for the attended feature. Here, we used functional magnetic resonance imaging and a pattern classification algorithm to predict the attentional state of human observers as they monitored a visual feature (one of two directions of motion). We find that feature-specific attention effects spread across the visual field-even to regions of the scene that do not contain a stimulus. This spread of feature-based attention to empty regions of space may facilitate the perception of behaviorally relevant stimuli by increasing sensitivity to attended features at all locations in the visual field.

Attention to stimulus features shifts spectral tuning of V4 neurons during natural vision.

SV David — 2008-08-18T13:59:08-00:00

Neuron, Vol. 59, No. 3. (14 August 2008), pp. 509-521.

Previous neurophysiological studies suggest that attention can alter the baseline or gain of neurons in extrastriate visual areas but that it cannot change tuning. This suggests that neurons in visual cortex function as labeled lines whose meaning does not depend on task demands. To test this common assumption, we used a system identification approach to measure spatial frequency and orientation tuning in area V4 during two attentionally demanding visual search tasks, one that required fixation and one that allowed free viewing during search. We found that spatial attention modulates response baseline and gain but does not alter tuning, consistent with previous reports. In contrast, feature-based attention often shifts neuronal tuning. These tuning shifts are inconsistent with the labeled-line model and tend to enhance responses to stimulus features that distinguish the search target. Our data suggest that V4 neurons behave as matched filters that are dynamically tuned to optimize visual search.

Metamemorial influences in recognition memory: pictorial encoding reduces conjunction errors.

ME Lloyd — 2007-10-09T13:44:47-00:00

Mem Cognit, Vol. 35, No. 5. (July 2007), pp. 1067-1073.

Two experiments are presented that explore the role of the distinctiveness heuristic (e.g., Schacter, Israel, & Racine, 1999) on rates of conjunction errors as a function of encoding condition. The results of Experiment 1 demonstrate a reliable reduction of conjunction errors when participants study pictures relative to both reading words aloud and silently. Experiment 2 demonstrates that the nature of the pictures presented during the study phase is important for reducing conjunction errors. Collectively, the experiments demonstrate that participants can use the distinctiveness heuristic in addition to recall-to-reject strategies to avoid conjunction errors. These findings add to a growing body of literature that suggests that participants are able to use expectations for memory to guide their recognition decisions.

Attention alters spatial integration in macaque V1 in an eccentricity-dependent manner.

Mark Roberts — 2007-10-03T03:49:10-00:00

Nat Neurosci (30 September 2007)

Attention can selectively enhance neuronal responses and exclude external noise, but the neuronal computations that underlie these effects remain unknown. At the neuronal level, noise exclusion might result in altered spatial integration properties. We tested this proposal by recording neuronal activity and length tuning in neurons of the primary visual cortex of the macaque when attention was directed toward or away from stimuli presented in each neuron's classical receptive field. For cells with central-parafoveal receptive fields, attention reduced spatial integration, as demonstrated by a reduction in preferred stimulus length and in the size of the spatial summation area. Conversely, in cells that represented more peripheral locations, attention increased spatial integration by increasing the cell's summation area. This previously unknown dichotomy between central and peripheral vision could support accurate analysis of attended foveal objects and target selection for impending eye movements to peripheral objects.

Malleable templates: reshaping our crystallized skills to create new outcomes

Scott Grafton — 2008-02-27T13:30:30-00:00

Nature Neuroscience, Vol. 11, No. 3., pp. 248-249.

Lateral Sharpening of Cortical Frequency Tuning by Approximately Balanced Inhibition

Guangying Wu — 2008-04-10T11:36:49-00:00

Neuron, Vol. 58, No. 1. (10 April 2008), pp. 132-143.

Summary Cortical inhibition plays an important role in shaping neuronal processing. The underlying synaptic mechanisms remain controversial. Here, in vivo whole-cell recordings from neurons in the rat primary auditory cortex revealed that the frequency tuning curve of inhibitory input was broader than that of excitatory input. This results in relatively stronger inhibition in frequency domains flanking the preferred frequencies of the cell and a significant sharpening of the frequency tuning of membrane responses. The less selective inhibition can be attributed to a broader bandwidth and lower threshold of spike tonal receptive field of fast-spike inhibitory neurons than nearby excitatory neurons, although both types of neurons receive similar ranges of excitatory input and are organized into the same tonotopic map. Thus, the balance between excitation and inhibition is only approximate, and intracortical inhibition with high sensitivity and low selectivity can laterally sharpen the frequency tuning of neurons, ensuring their highly selective representation.

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.

One-Dimensional Dynamics of Attention and Decision Making in LIP

Surya Ganguli — 2008-04-14T13:04:51-00:00

Neuron, Vol. 58, No. 1. (10 April 2008), pp. 15-25.

Summary Where we allocate our visual spatial attention depends upon a continual competition between internally generated goals and external distractions. Recently it was shown that single neurons in the macaque lateral intraparietal area (LIP) can predict the amount of time a distractor can shift the locus of spatial attention away from a goal. We propose that this remarkable dynamical correspondence between single neurons and attention can be explained by a network model in which generically high-dimensional firing-rate vectors rapidly decay to a single mode. We find direct experimental evidence for this model, not only in the original attentional task, but also in a very different task involving perceptual decision making. These results confirm a theoretical prediction that slowly varying activity patterns are proportional to spontaneous activity, pose constraints on models of persistent activity, and suggest a network mechanism for the emergence of robust behavioral timing from heterogeneous neuronal populations.

Expression of long-term depression underlies visual recognition memory.

S Griffiths — 2008-05-05T14:20:25-00:00

Neuron, Vol. 58, No. 2. (24 April 2008), pp. 186-194.

The modifications occurring in the brain during learning and memory are still poorly understood but may involve long-lasting changes in synaptic transmission (synaptic plasticity). In perirhinal cortex, a lasting decrement in neuronal responsiveness is associated with visual familiarity discrimination, leading to the hypothesis that long-term depression (LTD)-like synaptic plasticity may underlie recognition memory. LTD relies on internalization of AMPA receptors (AMPARs) through interaction between their GluR2 subunits and AP2, the clathrin adaptor protein required for endocytosis. We demonstrate that a peptide that blocks interactions between GluR2 and AP2 blocks LTD in perirhinal cortex in vitro. Viral transduction of this peptide in perirhinal cortex produced striking deficits in visual recognition memory. Furthermore, there was a deficit of LTD in perirhinal cortex slices from virally transduced, recognition memory-deficient animals. These results suggest that internalization of AMPA receptors, a process critical for the expression of LTD in perirhinal cortex, underlies visual recognition memory.