Neural coding of behavioral relevance in parietal cortex

The influence of cortical activity on perception depends on behavioral state and sensory context

The mechanistic link between neural circuit activity and behavior remains unclear. While manipulating cortical activity can bias certain behaviors and elicit artificial percepts, some tasks can still be solved when cortex is silenced or removed. Here, mice were trained to perform a visual detection task during which we selectively targeted groups of visually responsive and co-tuned neurons in L2/3 of primary visual cortex (V1) for two-photon photostimulation. The influence of photostimulation was conditional on two key factors: the behavioral state of the animal and the contrast of the visual stimulus. The detection of low-contrast stimuli was enhanced by photostimulation, while the detection of high-contrast stimuli was suppressed, but crucially, only when mice were highly engaged in the task. When mice were less engaged, our manipulations of cortical activity had no effect on behavior. The behavioral changes were linked to specific changes in neuronal activity. The responses of non-photostimulated neurons in the local network were also conditional on two factors: their functional similarity to the photostimulated neurons and the contrast of the visual stimulus. Functionally similar neurons were increasingly suppressed by photostimulation with increasing visual stimulus contrast, correlating with the change in behavior. Our results show that the influence of cortical activity on perception is not fixed, but dynamically and contextually modulated by behavioral state, ongoing activity and the routing of information through specific circuits.

Experimental Studies of the Attention Processing Model in Multiple Object Tracking Task

(1) Background: Attention is an important cognitive process in daily life. However, limited cognitive resources have been allocated to attention, especially for multiple objects and its mechanism is still unclear. Most of the previous studies have been based on the static attention paradigms with relatively lower ecological validity. Thus, we aimed to explore the attention processing mechanism in a multiple object tracking (MOT) task by using a dynamic attention paradigm. Two experiments were conducted to assess whether there was a multi-focus attention processing model, and whether the processing model changes with the number of target balls. (2) Methods: During the experiments, 33 university students completed MOT combined with the simultaneous-sequential paradigm, with tracking accuracy and reaction time of correct reaction as indicators. (3) Results: (i) When there were two target balls, an obvious bilateral field advantage was apparent. (ii) When there were four target balls, participants' performance was significantly better when stimuli were presented simultaneously than when they were presented sequentially, showing a multi-focus attention processing model. (4) Conclusion: Attention processing is characterized by flexibility, providing strong evidence to support the multi-focus theory.

Dopamine increases risky choice while D2 blockade shortens decision time

Dopamine is crucially involved in decision-making and overstimulation within dopaminergic pathways can lead to impulsive behaviour, including a desire to take risks and reduced deliberation before acting. These behavioural changes are side effects of treatment with dopaminergic drugs in Parkinson disease, but their likelihood of occurrence is difficult to predict and may be influenced by the individual's baseline endogenous dopamine state, and indeed correlate with sensation-seeking personality traits. We here collected data on a standard gambling task in healthy volunteers given either placebo, 2.5 mg of the dopamine antagonist haloperidol or 100/25 mg of the dopamine precursor levodopa in a within-subject design. We found an increase in risky choices on levodopa. Choices were, however, made faster on haloperidol with no effect of levodopa on deliberation time. Shortened deliberation times on haloperidol occurred in low sensation-seekers only, suggesting a correlation between sensation-seeking personality trait and baseline dopamine levels. We hypothesise that levodopa increases risk-taking behaviour via overstimulation at both D1 and D2 receptor level, while a single low dose of haloperidol, as previously reported (Frank and O'Reilly 2006), may block D2 receptors pre- and post-synaptically and may paradoxically lead to higher striatal dopamine acting on remaining striatal D1 receptors, causing speedier decision without influencing risk tolerance. These effects could also fit with a recently proposed computational model of the basal ganglia (Moeller and Bogacz 2019; Moeller et al. 2021). Furthermore, our data suggest that the actual dopaminergic drug effect may be dependent on the individual's baseline dopamine state, which may influence our therapeutic decision as clinicians in the future.

Cannabinoid Modulation of Dopamine Release During Motivation, Periodic Reinforcement, Exploratory Behavior, Habit Formation, and Attention

Motivational and attentional processes energize action sequences to facilitate evolutionary competition and promote behavioral fitness. Decades of neuropharmacology, electrophysiology and electrochemistry research indicate that the mesocorticolimbic DA pathway modulates both motivation and attention. More recently, it was realized that mesocorticolimbic DA function is tightly regulated by the brain's endocannabinoid system and greatly influenced by exogenous cannabinoids-which have been harnessed by humanity for medicinal, ritualistic, and recreational uses for 12,000 years. Exogenous cannabinoids, like the primary psychoactive component of cannabis, delta-9-tetrahydrocannabinol, produce their effects by acting at binding sites for naturally occurring endocannabinoids. The brain's endocannabinoid system consists of two G-protein coupled receptors, endogenous lipid ligands for these receptor targets, and several synthetic and metabolic enzymes involved in their production and degradation. Emerging evidence indicates that the endocannabinoid 2-arachidonoylglycerol is necessary to observe concurrent increases in DA release and motivated behavior. And the historical pharmacology literature indicates a role for cannabinoid signaling in both motivational and attentional processes. While both types of behaviors have been scrutinized under manipulation by either DA or cannabinoid agents, there is considerably less insight into prospective interactions between these two important signaling systems. This review attempts to summate the relevance of cannabinoid modulation of DA release during operant tasks designed to investigate either motivational or attentional control of behavior. We first describe how cannabinoids influence DA release and goal-directed action under a variety of reinforcement contingencies. Then we consider the role that endocannabinoids might play in switching an animal's motivation from a goal-directed action to the search for an alternative outcome, in addition to the formation of long-term habits. Finally, dissociable features of attentional behavior using both the 5-choice serial reaction time task and the attentional set-shifting task are discussed along with their distinct influences by DA and cannabinoids. We end with discussing potential targets for further research regarding DA-cannabinoid interactions within key substrates involved in motivation and attention.

Functional Interplay Between Posterior Parietal Cortex and Hippocampus During Detection of Memory Targets and Non-targets

Posterior parietal cortex is frequently activated during episodic memory retrieval but its role during retrieval and its interactions with the hippocampus are not yet clear. In this fMRI study, we investigated the neural bases of recognition memory when study repetitions and retrieval goals were manipulated. During encoding participants studied words either once or three times, and during retrieval they were rewarded more to detect either studied words or new words. We found that (1) dorsal parietal cortex (DPC) was more engaged during detection of items studied once compared to three times, whereas regions in the ventral parietal cortex (VPC) responded more to items studied multiple times; (2) DPC, within a network of brain regions functionally connected to the anterior hippocampus, responded more to items consistent with retrieval goals (associated with high reward); (3) VPC, within a network of brain regions functionally connected to the posterior hippocampus, responded more to items not aligned with retrieval goals (i.e., unexpected). These findings support the hypothesis that DPC and VPC regions contribute differentially to top-down vs. bottom-up attention to memory. Moreover, they reveal a dissociation in the functional profile of the anterior and posterior hippocampi.

Differential brain mechanisms for processing distracting information in task-relevant and -irrelevant dimensions in visual search

A crucial function of our goal-directed behavior is to select task-relevant targets among distractor stimuli, some of which may share properties with the target and thus compete for attentional selection. Here, by applying functional magnetic resonance imaging (fMRI) to a visual search task in which a target was embedded in an array of distractors that were homogeneous or heterogeneous along the task-relevant (orientation or form) and/or task-irrelevant (color) dimensions, we demonstrate that for both (orientation) feature search and (form) conjunction search, the fusiform gyrus is involved in processing the task-irrelevant color information, while the bilateral frontal eye fields (FEF), the cortex along the left intraparietal sulcus (IPS), and the left junction of intraparietal and transverse occipital sulci (IPTO) are involved in processing task-relevant distracting information, especially for target-absent trials. Moreover, in conjunction (but not in feature) search, activity in these frontoparietal regions is affected by stimulus heterogeneity along the task-irrelevant dimension: heterogeneity of the task-irrelevant information increases the activity in these regions only when the task-relevant information is homogeneous, not when it is heterogeneous. These findings suggest that differential neural mechanisms are involved in processing task-relevant and task-irrelevant dimensions of the searched-for objects. In addition, they show that the top-down task set plays a dominant role in determining whether or not task-irrelevant information can affect the processing of the task-relevant dimension in the frontoparietal regions.

Auditory Processing in the Posterior Parietal Cortex

Goal-directed behavior can be characterized as a dynamic link between a sensory stimulus and a motor act. Neural correlates of many of the intermediate events of goal-directed behavior are found in the posterior parietal cortex. Although the parietal cortex’s role in guiding visual behaviors has received considerable attention, relatively little is known about its role in mediating auditory behaviors. Here, the authors review recent studies that have focused on how neurons in the lateral intraparietal area (area LIP) differentially process auditory and visual stimuli. These studies suggest that area LIP contains a modality-dependent representation that is highly dependent on behavioral context.

Dynamic integration of task-relevant visual features in posterior parietal cortex

The primate visual system consists of multiple hierarchically organized cortical areas, each specialized for processing distinct aspects of the visual scene. For example, color and form are encoded in ventral pathway areas such as V4 and inferior temporal cortex, while motion is preferentially processed in dorsal pathway areas such as the middle temporal area. Such representations often need to be integrated perceptually to solve tasks that depend on multiple features. We tested the hypothesis that the lateral intraparietal area (LIP) integrates disparate task-relevant visual features by recording from LIP neurons in monkeys trained to identify target stimuli composed of conjunctions of color and motion features. We show that LIP neurons exhibit integrative representations of both color and motion features when they are task relevant and task-dependent shifts of both direction and color tuning. This suggests that LIP plays a role in flexibly integrating task-relevant sensory signals.

Dissociable neural mechanisms for goal-directed versus incidental memory reactivation

Remembering a past event involves reactivation of distributed patterns of neural activity that represent the features of that event-a process that depends on associative mechanisms supported by medial temporal lobe structures. Although efficient use of memory requires prioritizing those features of a memory that are relevant to current behavioral goals (target features) over features that may be goal-irrelevant (incidental features), there remains ambiguity concerning how this is achieved. We tested the hypothesis that although medial temporal lobe structures may support reactivation of both target and incidental event features, frontoparietal cortex preferentially reactivates those features that match current goals. Here, human participants were cued to remember either the category (face/scene) to which a picture belonged (category trials) or the location (left/right) in which a picture appeared (location trials). Multivoxel pattern analysis of fMRI data were used to measure reactivation of category information as a function of its behavioral relevance (target vs incidental reactivation). In ventral/medial temporal lobe (VMTL) structures, incidental reactivation was as robust as target reactivation. In contrast, frontoparietal cortex exhibited stronger target than incidental reactivation; that is, goal-modulated reactivation. Reactivation was also associated with later memory. Frontoparietal biases toward target reactivation predicted subsequent memory for target features, whereas incidental reactivation in VMTL predicted subsequent memory for nontested features. These findings reveal a striking dissociation between goal-modulated reactivation in frontoparietal cortex and incidental reactivation in VMTL.

Robust Visual Tracking Using an Effective Appearance Model Based on Sparse Coding

Intelligent video surveillance is currently one of the most active research topics in computer vision, especially when facing the explosion of video data captured by a large number of surveillance cameras. As a key step of an intelligent surveillance system, robust visual tracking is very challenging for computer vision. However, it is a basic functionality of the human visual system (HVS). Psychophysical findings have shown that the receptive fields of simple cells in the visual cortex can be characterized as being spatially localized, oriented, and bandpass, and it forms a sparse, distributed representation of natural images. In this article, motivated by these findings, we propose an effective appearance model based on sparse coding and apply it in visual tracking. Specifically, we consider the responses of general basis functions extracted by independent component analysis on a large set of natural image patches as features and model the appearance of the tracked target as the probability distribution of these features. In order to make the tracker more robust to partial occlusion, camouflage environments, pose changes, and illumination changes, we further select features that are related to the target based on an entropy-gain criterion and ignore those that are not. The target is finally represented by the probability distribution of those related features. The target search is performed by minimizing the Matusita distance between the distributions of the target model and a candidate using Newton-style iterations. The experimental results validate that the proposed method is more robust and effective than three state-of-the-art methods.

When attention wanders: how uncontrolled fluctuations in attention affect performance

No matter how hard subjects concentrate on a task, their minds wander (Raichle et al., 2001; Buckner et al., 2008; Christoff et al., 2009; Killingsworth and Gilbert, 2010). Internal fluctuations cannot be measured behaviorally or from conventional neurophysiological measures, so their effects on performance have been difficult to study. Previously, we measured fluctuations in visual attention using the responses of populations of simultaneously recorded neurons in macaque visual cortex (Cohen and Maunsell, 2010). Here, we use this ability to investigate how attentional fluctuations affect performance. We found that attentional fluctuations have large and complex effects on performance, the sign of which depends on the difficulty of the perceptual judgment. As expected, attention greatly improves the detection of subtle changes in a stimulus. Surprisingly, we found that attending too strongly to a particular stimulus impairs the ability to notice when that stimulus changes dramatically. Our results suggest that all previously reported measures of behavioral performance should be viewed as amalgamations of different attentional states, whether or not those studies specifically addressed attention.

Using neuronal populations to study the mechanisms underlying spatial and feature attention

Visual attention affects both perception and neuronal responses. Whether the same neuronal mechanisms mediate spatial attention, which improves perception of attended locations, and nonspatial forms of attention has been a subject of considerable debate. Spatial and feature attention have similar effects on individual neurons. Because visual cortex is retinotopically organized, however, spatial attention can comodulate local neuronal populations, whereas feature attention generally requires more selective modulation. We compared the effects of feature and spatial attention on local and spatially separated populations by recording simultaneously from dozens of neurons in both hemispheres of V4. Feature and spatial attention affect the activity of local populations similarly, modulating both firing rates and correlations between pairs of nearby neurons. However, whereas spatial attention appears to act on local populations, feature attention is coordinated across hemispheres. Our results are consistent with a unified attentional mechanism that can modulate the responses of arbitrary subgroups of neurons.

Compositionality of rule representations in human prefrontal cortex

Rules are widely used in everyday life to organize actions and thoughts in accordance with our internal goals. At the simplest level, single rules can be used to link individual sensory stimuli to their appropriate responses. However, most tasks are more complex and require the concurrent application of multiple rules. Experiments on humans and monkeys have shown the involvement of a frontoparietal network in rule representation. Yet, a fundamental issue still needs to be clarified: Is the neural representation of multiple rules compositional, that is, built on the neural representation of their simple constituent rules? Subjects were asked to remember and apply either simple or compound rules. Multivariate decoding analyses were applied to functional magnetic resonance imaging data. Both ventrolateral frontal and lateral parietal cortex were involved in compound representation. Most importantly, we were able to decode the compound rules by training classifiers only on the simple rules they were composed of. This shows that the code used to store rule information in prefrontal cortex is compositional. Compositional coding in rule representation suggests that it might be possible to decode other complex action plans by learning the neural patterns of the known composing elements.

A neuronal population measure of attention predicts behavioral performance on individual trials

Visual attention improves perception for an attended location or feature and also modulates the responses of sensory neurons. In laboratory studies, the sensory stimuli and task instructions are held constant within an attentional condition, but despite experimenters' best efforts, attention likely varies from moment to moment. Because most previous studies have focused on single neurons, it has been impossible to use neuronal responses to identify attentional fluctuations and determine whether these are associated with changes in behavior. We show that an instantaneous measure of attention based on the responses of a modest number of neurons in area V4 of the rhesus monkey (Macaca mulatta) can reliably predict large changes in an animal's ability to perform a difficult psychophysical task. Unexpectedly, this measure shows that the amount of attention allocated at any moment to locations in opposite hemifields is uncorrelated, suggesting that animals allocate attention to each stimulus independently rather than moving their attentional focus from one location to another.

Multimodal activity in the parietal cortex

Goal-directed behavior can be thought of as dynamic links between sensory stimuli and motor acts. Neural correlates of many of the intermediate events of both auditory and visual goal-directed behaviors are found in the posterior parietal cortex. Here, we review studies that have focused on how neurons in the lateral intraparietal area (area LIP) differentially process auditory and visual stimuli. Together, these studies suggest that area LIP contains a modality-dependent representation that is highly dependent on behavioral context.

Attention improves performance primarily by reducing interneuronal correlations

Visual attention can dramatically improve behavioural performance by allowing observers to focus on the important information in a complex scene. Attention also typically increases the firing rates of cortical sensory neurons. Rate increases improve the signal-to-noise ratio of individual neurons, and this improvement has been assumed to underlie attention-related improvements in behaviour. We recorded dozens of neurons simultaneously in visual area V4 and found that changes in single neurons accounted for only a small fraction of the improvement in the sensitivity of the population. Instead, over 80% of the attentional improvement in the population signal was caused by decreases in the correlations between the trial-to-trial fluctuations in the responses of pairs of neurons. These results suggest that the representation of sensory information in populations of neurons and the way attention affects the sensitivity of the population may only be understood by considering the interactions between neurons.

Neural substrates of contingency learning and executive control: dissociating physical, valuative, and behavioral changes

Contingency learning is fundamental to cognition. Knowledge about environmental contingencies allows behavioral flexibility, as executive control processes accommodate the demands of novel or changing environments. Studies of experiential learning have focused on the relationship between actions and the values of associated outcomes. However, outcome values have often been confounded with the physical changes in the outcomes themselves. Here, we dissociated contingency learning into valuative and non-valuative forms, using a novel version of the two-alternative choice task, while measuring the neural effects of contingency changes using functional magnetic resonance imaging (fMRI). Changes in value-relevant contingencies evoked activation in the lateral prefrontal cortex (LPFC), posterior parietal cortex (PPC), and dorsomedial prefrontal cortex (DMPFC) consistent with prior results (e.g., reversal-learning paradigms). Changes in physical contingencies unrelated to value or to action produced similar activations within the LPFC, indicating that LPFC may engage in generalized contingency learning that is not specific to valuation. In contrast, contingency changes that required behavioral shifts evoked activation localized to the DMPFC, supplementary motor, and precentral cortices, suggesting that these regions play more specific roles within the executive control of behavior.

Cognitive set reconfiguration signaled by macaque posterior parietal neurons

When faced with problems, we can flexibly change our ways of thinking or our point of view. Our cognitive flexibility arises from this ability of shifting cognitive sets. To elucidate how this dynamic process is implemented in the primate brain, single-unit activity was recorded from the posterior parietal cortex (PPC) of two monkeys performing analogs of the Wisconsin Card Sorting Test, which is most commonly used to test cognitive flexibility in humans. We successfully trained the monkeys to promptly perform set shifting, mostly within a single trial, and found shift-related activity: PPC neurons were transiently activated when the monkeys shifted from one cognitive set to another, but not when they shifted in the opposite direction. This shift-related activity emerged about 4 s before the actual behavioral responses, and it well predicted whether the cognitive set would be successfully shifted. These results provide insights into single-unit level mechanisms of cognitive flexibility.

Separate representations of target and timing cue locations in the supplementary eye fields

When different stimuli indicate where and when to make an eye movement, the brain areas involved in oculomotor control must selectively plan an eye movement to the stimulus that encodes the target position and also encode the information available from the timing cue. This could pose a challenge to the oculomotor system since the representation of the timing stimulus location in one brain area might be interpreted by downstream neurons as a competing motor plan. Evidence from diverse sources has suggested that the supplementary eye fields (SEF) play an important role in behavioral timing, so we recorded single-unit activity from SEF to characterize how target and timing cues are encoded in this region. Two monkeys performed a variant of the memory-guided saccade task, in which a timing stimulus was presented at a randomly chosen eccentric location. Many spatially tuned SEF neurons encoded only the location of the target and not the timing stimulus, whereas several other SEF neurons encoded the location of the timing stimulus and not the target. The SEF population therefore encoded the location of each stimulus with largely distinct neuronal subpopulations. For comparison, we recorded a small population of lateral intraparietal (LIP) neurons in the same task. We found that most LIP neurons that encoded the location of the target also encoded the location of the timing stimulus after its presentation, but selectively encoded the intended eye movement plan in advance of saccade initiation. These results suggest that SEF, by conditionally encoding the location of instructional stimuli depending on their meaning, can help identify which movement plan represented in other oculomotor structures, such as LIP, should be selected for the next eye movement.

Emergence of novel representations in primary motor cortex and premotor neurons during associative learning

Neurons in the motor areas of cortex play a key role in associating sensory instructions with movements. However, their ability to acquire and maintain representations of novel stimulus features, especially when these features are behaviorally relevant, remains unknown. We investigated neuronal changes in these areas during and after associative learning, by training monkeys on a novel reaching task that required associating target colors with movement directions. Before and after learning, the monkeys performed a well known center-out task. We found that during learning, up to 48% of the neurons developed learning-related responses, differentiating between the associative task and the center-out task, although movement kinematics were the same. After learning, on returning to the center-out task in which color was irrelevant, many of these neurons maintained their response to the associative task; they displayed novel sensitivity to the color of the target that was relevant during learning. These neuronal responses prevailed in both the primary motor cortex and the ventral and dorsal premotor cortices, without degrading the information that the neurons firing carried about movement direction. Our results show that motor cortical neurons can rapidly develop and maintain sensitivities to novel arbitrary sensory features such as color, when such features are behaviorally relevant.

Cholinergic deafferentation of prefrontal cortex increases sensitivity to cross-modal distractors during a sustained attention task

The effects of restricted cholinergic deafferentation of prefrontal cortex in rats on sustained attention were assessed. Attentional demands were increased by presentation of distractor stimuli in a different modality (auditory) or the same modality (visual) as target stimuli. Additionally, the effects of the regularity of the distractor on rats' ability to disregard this stimulus were assessed by testing different frequencies of stimuli for each modality. Cholinergically lesioned rats were more sensitive to the effects of auditory distractors than nonlesioned rats, whereas visual distractors of any frequency potently impaired the performance of all subjects. The effects of the auditory stimuli on attentional performance varied depending on the frequency of the tone. A tone with a predictable pattern enhanced signal detection in all rats. An irregular tone selectively impaired performance of rats with cholinergic lesions. Additional tests suggest that rats use the regular tone to time when to attend. Lesioned rats were impaired when the regular tone was presented with a more variable intertrial interval in a subsequent testing session, suggesting impairments in top-down control. In addition to changes in top-down control of attention, differential effects on performance based on the regularity of the tone suggest that stimulus properties encoded by bottom-up processes are also altered after lesioning. The current data suggest that cholinergic deafferentation of prefrontal cortex alters top-down and bottom-up processing of stimuli.

Where vision meets memory: prefrontal-posterior networks for visual object constancy during categorization and recognition

Objects seen from unusual relative to more canonical views require more time to categorize and recognize, and, according to object model verification theories, additionally recruit prefrontal processes for cognitive control that interact with parietal processes for mental rotation. To test this using functional magnetic resonance imaging, people categorized and recognized known objects from unusual and canonical views. Canonical views activated some components of a default network more on categorization than recognition. Activation to unusual views showed that both ventral and dorsal visual pathways, and prefrontal cortex, have key roles in visual object constancy. Unusual views activated object-sensitive and mental rotation (and not saccade) regions in ventrocaudal intraparietal, transverse occipital, and inferotemporal sulci, and ventral premotor cortex for verification processes of model testing on any task. A collateral-lingual sulci "place" area activated for mental rotation, working memory, and unusual views on correct recognition and categorization trials to accomplish detailed spatial matching. Ventrolateral prefrontal cortex and object-sensitive lateral occipital sulcus activated for mental rotation and unusual views on categorization more than recognition, supporting verification processes of model prediction. This visual knowledge framework integrates vision and memory theories to explain how distinct prefrontal-posterior networks enable meaningful interactions with objects in diverse situations.

Neural mechanisms of visual categorization: insights from neurophysiology

How does the brain recognize the meaning of sensory stimuli? Through experience, we easily learn to group stimuli into meaningful categories such as "chair", "table" and "vehicle". Although much is known about how the brain processes and encodes basic visual features (e.g. color, orientation, and motion direction), much less is known about how the brain learns and represents the behavioral relevance, or category, of stimuli. This article will review a number of recent experiments which suggest that neuronal activity in primate prefrontal, temporal and parietal cortical areas likely plays significant, though complementary, roles in visual categorization and category learning.

Functional neural circuits for mental timekeeping

Theories of mental timekeeping suggest frontostriatal networks may mediate performance of tasks requiring precise timing. We assessed whether frontostriatal networks are functionally integrated during the performance of timing tasks. Functional magnetic resonance imaging (fMRI) data from 31 healthy adults were collected during performance of several different types of discrete interval timing tasks. Independent component analysis (ICA) was used to examine functional connectivity within frontostriatal circuits. ICA identifies groups of spatially discrete brain regions sharing similar patterns of hemodynamic signal change over time. The results confirm the existence of a frontostriatal neural timing circuit that includes anterior cingulate gyrus, supplementary motor area, bilateral anterior insula, bilateral putamen/globus pallidus, bilateral thalamus, and right superior temporal gyrus and supramarginal gyrus. Several other distinct neural circuits were identified that may represent the neurobiological substrates of different information processing stages of mental timekeeping. Small areas of right cerebellum were engaged in several of these circuits, suggesting that cerebellar function may be important in, but not the primary substrate of, the mental timing tasks used in this experiment. These findings are discussed within the context of current biological and information processing models of neural timekeeping.

Dynamic circuitry for updating spatial representations. III. From neurons to behavior

Each time the eyes move, the visual system must adjust internal representations to account for the accompanying shift in the retinal image. In the lateral intraparietal cortex (LIP), neurons update the spatial representations of salient stimuli when the eyes move. In previous experiments, we found that split-brain monkeys were impaired on double-step saccade sequences that required updating across visual hemifields, as compared to within hemifield. Here we describe a subsequent experiment to characterize the relationship between behavioral performance and neural activity in LIP in the split-brain monkey. We recorded from single LIP neurons while split-brain and intact monkeys performed two conditions of the double-step saccade task: one required across-hemifield updating and the other required within-hemifield updating. We found that, despite extensive experience with the task, the split-brain monkeys were significantly more accurate for within-hemifield than for across-hemifield sequences. In parallel, we found that population activity in LIP of the split-brain monkeys was significantly stronger for the within-hemifield than for the across-hemifield condition of the double-step task. In contrast, in the normal monkey, both the average behavioral performance and population activity showed no bias toward the within-hemifield condition. Finally, we found that the difference between within-hemifield and across-hemifield performance in the split-brain monkeys was reflected at the level of single-neuron activity in LIP. These findings indicate that remapping activity in area LIP is present in the split-brain monkey for the double-step task and covaries with spatial behavior on within-hemifield compared to across-hemifield sequences.

Responses of neurons in the lateral intraparietal area to central visual cues

Goal-directed behavior is characterized by flexible stimulus-action mappings. The lateral intraparietal area (area LIP) contains a representation of extra-personal space that is used to guide goal-directed behavior. To examine further how area LIP contributes to these flexible stimulus-action mappings, we recorded LIP activity while rhesus monkeys participated in two different cueing tasks. In the first task, the color of a central light indicated the location of a monkey's saccadic endpoint in the absence of any other visual stimuli. In the second task, the color of a central light indicated which of two visual targets was the saccadic goal. In both tasks, LIP activity was modulated by these non-spatial cues. These observations further suggest a role for area LIP in mediating endogenous associations that link stimuli with actions.

Parietal disruption impairs reflexive spatial attention within and between sensory modalities

Reflexive spatial attention is critical for controlling perception and action. An established body of evidence suggests that mechanisms of spatial attention operate both within and between sensory modalities; however the attentional mechanisms that link modalities in the human brain are unclear. Here we used transcranial magnetic stimulation (TMS) to explore the role of the parietal cortex in coordinating reflexive shifts of spatial attention between vision and touch. In two experiments, healthy participants localised visual and somatosensory targets that were preceded by non-informative visual or somatosensory spatial cues. To determine the role of parietal cortex in spatial orienting, TMS was delivered synchronously with cue onset for 100 ms. Results revealed a critical role of the right angular gyrus and supramarginal gyrus in reflexive orienting to visual and somatosensory targets that followed a somatosensory cue. In contrast, the same TMS protocol was ineffective in modulating reflexive orienting based on visual cues. This dependence on cue modality may reflect subcortical redundancy of visual orienting mechanisms. Overall, our results indicate a critical role of the inferior parietal cortex in mediating reflexive shifts of attention within and between sensory modalities.

Correlates of Stimulus-Response Congruence in the Posterior Parietal Cortex

Primate behavior is flexible: The response to a stimulus often depends on the task in which it occurs. Here we study how single neurons in the posterior parietal cortex (PPC) respond to stimuli which are associated with different responses in different tasks. Two rhesus monkeys performed a task-switching paradigm. Each trial started with a task cue instructing which of two tasks to perform, followed by a stimulus requiring a left or right button press. For half the stimuli, the associated responses were different in the two tasks, meaning that the task context was necessary to disambiguate the incongruent stimuli. The other half of stimuli required the same response irrespective of task context (congruent). Using this paradigm, we previously showed that behavioral responses to incongruent stimuli are significantly slower than to congruent stimuli. We now demonstrate a neural correlate in the PPC of the additional processing time required for incongruent stimuli. Furthermore, we previously found that 29% of parietal neurons encode the task being performed (task-selective cells). We now report differences in neuronal timing related to congruency in task-selective versus task nonselective cells. These differences in timing suggest that the activity in task nonselective cells reflects a motor command, whereas activity in task-selective cells reflects a decision process.

Mental rotation and object categorization share a common network of prefrontal and dorsal and ventral regions of posterior cortex

The multiple-views-plus-transformation variant of object model verification theories predicts that parietal regions that are critical for mental rotation contribute to visual object cognition. Some neuroimaging studies have shown that the intraparietal sulcus region is critically involved in mental rotation. Other studies indicate that both ventral and dorsal posterior regions are object-sensitive and involved in object perception and categorization tasks. However, it is unknown whether dorsal object-sensitive areas overlap with regions recruited for object mental rotation. Functional magnetic resonance imaging was used to test this directly. Participants performed standard tasks of object categorization, mental rotation, and eye movements. Results provided clear support for the prediction, demonstrating overlap between dorsal object-sensitive regions in ventral-caudal intraparietal sulcus (vcIPS) and an adjacent dorsal occipital area and the regions that are activated during mental rotation but not during saccades. In addition, object mental rotation (but not saccades) activated object-sensitive areas in lateral dorsal occipitotemporal cortex (DOT), and both mental rotation and object categorization recruited ventrolateral prefrontal cortex areas implicated in attention, working memory, and cognitive control. These findings provide clear evidence that a prefrontal-posterior cortical system implicated in mental rotation, including the occipitoparietal regions critical for this spatial task, is recruited during visual object categorization. Altogether, the findings provide a key link in understanding the role of dorsal and ventral visual areas in spatial and object perception and cognition: Regions in occipitoparietal cortex, as well as DOT cortex, have a general role in visual object cognition, supporting not only mental rotation but also categorization.

Feature-based attention in visual cortex

Although most studies of visual attention have examined the effects of shifting attention between different locations in the visual field, attention can also be directed to particular visual features, such as a color, orientation or a direction of motion. Single-unit studies have shown that attention to a feature modulates neuronal signals in a range of areas in monkey visual cortex. The location-independent property of feature-based attention makes it particularly well suited to modify selectively the neural representations of stimuli or parts within complex visual scenes that match the currently attended feature. This review is part of the TINS special issue on The Neural Substrates of Cognition.

Mapping the parietal cortex of human and non-human primates

The present essay reviews a series of functional magnetic resonance imaging (fMRI) studies conducted in parallel in humans and awake monkeys, concentrating on the intraparietal sulcus (IPS). MR responses to a range of visual stimuli indicate that the human IPS contains more functional regions along its anterior-posterior extent than are known in the monkey. Human IPS includes four motion sensitive regions, ventral IPS (VIPS), parieto-occipital IPS (POIPS), dorsal IPS medial (DIPSM) and dorsal IPS anterior (DIPSA), which are also sensitive to three-dimensional structure from motion (3D SFM). On the other hand, the monkey IPS contains only one motion sensitive area (VIP), which is not particularly sensitive to 3D SFM. The human IPS includes four regions sensitive to two-dimensional shape and three representations of central vision, while monkey IPS appears to contain only two shape sensitive regions and one central representation. These data support the hypothesis that monkey LIP corresponds to the region of human IPS between DIPSM and POIPS and that a portion of the anterior part of human IPS is evolutionarily new. This additional cortical tissue may provide the capacity for an enhanced visual analysis of moving images necessary for sophisticated control of manipulation and tool handling.

Goal-driven selective attention in patients with right hemisphere lesions: how intact is the ipsilesional field?

Patients with right hemisphere (RH) lesions often display a spatial bias in attention towards the ipsilesional hemifield. The behavioural manifestations of this spatial bias are typically interpreted as reflecting increased or enhanced attention for stimuli within the 'intact' ipsilesional field, and impaired attentional functioning within the contralesional field. In the healthy brain, goal-driven and stimulus-driven attentional processes interact to determine which stimuli should be prioritized for selection. Although unilateral brain damage increases the relative attentional salience of stimuli within the ipsilesional field, it might also cause problems in filtering or attenuating task-irrelevant information. We examined whether goal-driven attention modulates the processing of ipsilesional and contralesional information in 6 patients with unilateral brain damage following RH stroke (5 male, 1 female; mean age 60.8 years) and a group of age and sex-matched controls. We used a flanker task in which participants made speeded judgements on a central target item (a coloured letter). On each trial the target was flanked by a coloured letter in the left and right hemifields. In separate blocks, participants were instructed to judge either the identity or the colour of the central target and to ignore the flankers. The flanker on one side could be congruent, incongruent or neutral with respect to the target, on either the letter or the colour dimension, whereas the flanker on the other side was always neutral on both dimensions. Healthy controls showed significant interference from incongruent flankers on either side. Crucially, however, this effect only occurred for the task-relevant dimension [F(2,10) = 24.60; P < 0.001]. For patients, however, both the task-relevant and task-irrelevant dimensions of ipsilesional flankers interfered with response times [task-relevant: F(2,10) = 7.50, P < 0.05; task-irrelevant: F(1,5) = 6.20, P < 0.05]. Conversely, contralesional flankers influenced response times only when the target and distractor were incongruent on the task-relevant dimension [F(2,10) = 4.85; P < 0.05]. Our findings demonstrate that following RH damage, goal-driven biases cannot constrain the processing of task-irrelevant features of ipsilesional stimuli. We speculate that a lateralized bias in spatial attention leads to unselective prioritization of all feature-based attributes of stimuli appearing within the ipsilesional hemifield, whether or not they are relevant to performance. Attentional selection for ipsilesional stimuli in disorders such as spatial neglect and extinction may not therefore be entirely normal, as previously assumed.

Disrupted tonotopy of the auditory cortex in mice lacking M1 muscarinic acetylcholine receptor

Sensory cortices have multiple and distinct functional maps that systematically represent environmental information. Development of these maps is precisely controlled by a number of intrinsic and extrinsic factors. Cortical cholinergic regulation is a crucial factor for normal cortical morphogenesis. In this study, we test the role of the M1 muscarinic acetylcholine receptor, the main muscarinic receptor subtype in the neocortex in the development of tonotopic maps in the auditory cortex. Mice lacking M1 receptors have normal hearing sensitivity but exhibit disrupted tonotopic organization and frequency tuning in the auditory cortex. In contrast, tonotopic organization and frequency tuning remain normal in the auditory midbrain. In addition, cortical layer IV neurons of M1 mutants exhibit significantly shorter or sparser dendrites compared to neurons of wildtype mice. In summary, our data suggest that the M1 receptor appears to be critical for the refinement or normal maturation of cortical tonotopy that is guided by thalamocortical inputs during early development.

The impact of irrelevant dimensional variation on rule-based category learning in patients with Parkinson's disease

This study examined the impact of irrelevant dimensional variation on rule-based category learning in patients with Parkinson's disease (PD), older controls (OC), and younger controls (YC). Participants were presented with 4-dimensional, binary-valued stimuli and were asked to categorize each into 1 of 2 categories. Category membership was based on the value of a single dimension. Four experimental conditions were administered in which there were zero, 1, 2, or 3 randomly varying irrelevant dimensions. Results indicated that patients with PD were impacted to a greater extent than both the OC and YC participants when the number of randomly varying irrelevant dimensions increased. These results suggest that the degree of working memory and selective attention requirements of a categorization task will impact whether PD patients are impaired in rule-based category learning, and help to clarify recent discrepancies in the literature.

Single neurons in posterior parietal cortex of monkeys encode cognitive set

The primate posterior parietal cortex (PPC), part of the dorsal visual pathway, is best known for its role in encoding salient spatial information. Yet there are indications that neural activity in the PPC can also be modulated by nonspatial task-related information. In this study, we tested whether neurons in the PPC encode signals related to cognitive set, that is, the preparation to perform a particular task. Cognitive set has previously been associated with the frontal cortex but not the PPC. In this study, monkeys performed a cognitive set shifting paradigm in which they were cued in advance to apply one of two different task rules to the subsequent stimulus on every trial. Here we show that a subset of neurons in the PPC, concentrated in the lateral bank of the intraparietal sulcus and on the angular gyrus, responds selectively to cues for different task rules.

Effect of a Central Fixation Light on Auditory Spatial Responses in Area LIP

A recent report demonstrated that, while fixating a central light, lateral intraparietal area (LIP) neurons are not modulated by the location of auditory stimuli until monkeys learn to saccade to the location of an auditory stimulus. This finding suggests that auditory spatial responses in area LIP are dependent on auditory-saccadic training. We found that, in monkeys that had not been trained to make behavioral responses to auditory stimuli, LIP neurons are modulated by auditory-stimulus location when a central light is not present in the environment. These results indicate that LIP auditory responses are not wholly dependent on behavioral training with auditory stimuli.

Visual attention: the where, what, how and why of saliency

Attention influences the processing of visual information even in the earliest areas of primate visual cortex. There is converging evidence that the interaction of bottom-up sensory information and top-down attentional influences creates an integrated saliency map, that is, a topographic representation of relative stimulus strength and behavioral relevance across visual space. This map appears to be distributed across areas of the visual cortex, and is closely linked to the oculomotor system that controls eye movements and orients the gaze to locations in the visual scene characterized by a high salience.