Functional heterogeneity of macaque lateral intraparietal neurons

Visual, delay, and oculomotor timing and tuning in macaque dorsal pulvinar during instructed and free choice memory saccades

Causal perturbations suggest that primate dorsal pulvinar plays a crucial role in target selection and saccade planning, though its basic neuronal properties remain unclear. Some functional aspects of dorsal pulvinar and interconnected frontoparietal areas-e.g. ipsilesional choice bias after inactivation-are similar. But it is unknown if dorsal pulvinar shares oculomotor properties of cortical circuitry, in particular delay and choice-related activity. We investigated such properties in macaque dorsal pulvinar during instructed and free-choice memory saccades. Most recorded units showed visual (12%), saccade-related (30%), or both types of responses (22%). Visual responses were primarily contralateral; diverse saccade-related responses were predominantly post-saccadic with a weak contralateral bias. Memory delay and pre-saccadic enhancement was infrequent (11-9%)-instead, activity was often suppressed during saccade planning (25%) and further during execution (15%). Surprisingly, only few units exhibited classical visuomotor patterns combining cue and continuous delay activity or pre-saccadic ramping; moreover, most spatially-selective neurons did not encode the upcoming decision during free-choice delay. Thus, in absence of a visible goal, the dorsal pulvinar has a limited role in prospective saccade planning, with patterns partially complementing its frontoparietal partners. Conversely, prevalent visual and post-saccadic responses imply its participation in integrating spatial goals with processing across saccades.

New insights on single-neuron selectivity in the era of population-level approaches

In the past, neuroscience was focused on individual neurons seen as the functional units of the nervous system, but this approach fell short over time to account for new experimental evidence, especially for what concerns associative and motor cortices. For this reason and thanks to great technological advances, a part of modern research has shifted the focus from the responses of single neurons to the activity of neural ensembles, now considered the real functional units of the system. However, on a microscale, individual neurons remain the computational components of these networks, thus the study of population dynamics cannot prescind from studying also individual neurons which represent their natural substrate. In this new framework, ideas such as the capability of single cells to encode a specific stimulus (neural selectivity) may become obsolete and need to be profoundly revised. One step in this direction was made by introducing the concept of “mixed selectivity,” the capacity of single cells to integrate multiple variables in a flexible way, allowing individual neurons to participate in different networks. In this review, we outline the most important features of mixed selectivity and we also present recent works demonstrating its presence in the associative areas of the posterior parietal cortex. Finally, in discussing these findings, we present some open questions that could be addressed by future studies.

Recurrent circuit dynamics underlie persistent activity in the macaque frontoparietal network

During delayed oculomotor response tasks, neurons in the lateral intraparietal area (LIP) and the frontal eye fields (FEF) exhibit persistent activity that reflects the active maintenance of behaviorally relevant information. Despite many computational models of the mechanisms of persistent activity, there is a lack of circuit-level data from the primate to inform the theories. To fill this gap, we simultaneously recorded ensembles of neurons in both LIP and FEF while macaques performed a memory-guided saccade task. A population encoding model revealed strong and symmetric long-timescale recurrent excitation between LIP and FEF. Unexpectedly, LIP exhibited stronger local functional connectivity than FEF, and many neurons in LIP had longer network and intrinsic timescales. The differences in connectivity could be explained by the strength of recurrent dynamics in attractor networks. These findings reveal reciprocal multi-area circuit dynamics in the frontoparietal network during persistent activity and lay the groundwork for quantitative comparisons to theoretical models.

Localization of movable electrodes in a multi-electrode microdrive in nonhuman primates

BACKGROUND

Recently, large-scale semi-chronic recording systems have been developed, unique in their capability to record simultaneously from multiple individually moveable electrodes. As these recording systems can cover a large area, knowledge of the exact location of each individual electrode is crucial. Currently, the only method of keeping track of electrode depth and thus location is through detailed notebook keeping on neural activity.

NEW METHOD

We have improved the electrode localization by combining pre- and postoperative anatomical magnetic resonance imaging (MRI) scans with high resolution computed tomography (CT) scans throughout the experiment, and validated our method by comparing the resulting location estimates with traditional notebook-keeping. Finally, the actual location of a selection of electrodes was marked at the end of the experiment by creating small metallic depositions using electrical stimulation, and thereby made visible on MRI.

RESULTS

Combining CT scans with a high resolution, artefact reducing sequence during the experiment with a preoperative MRI scan provides crucial information about the exact electrode location of multielectrode arrays with individually moveable electrodes.

COMPARISON WITH EXISTING METHODS

The information obtained from the hybrid CT-MR image and the notes on spiking activity showed a similar pattern, with the clear advantage of the visualization of the exact position of the electrodes using our method.

CONCLUSIONS

The described technique allows for a precise anatomical identification of the recorded brain areas and thus to draw strong conclusions about the role of each targeted cortical area in the behavior under study.

Distinct roles of prefrontal and parietal areas in the encoding of attentional priority

When searching for an object in a crowded scene, information about the similarity of stimuli to the target object is thought to be encoded in spatial priority maps, which are subsequently used to guide shifts of attention and gaze to likely targets. Two key cortical areas that have been described as holding priority maps are the frontal eye field (FEF) and the lateral intraparietal area (LIP). However, little is known about their distinct contributions in priority encoding. Here, we compared neuronal responses in FEF and LIP during free-viewing visual search. Although saccade selection signals emerged earlier in FEF, information about the target emerged at similar latencies in distinct populations within the two areas. Notably, however, effects in FEF were more pronounced. Moreover, LIP neurons encoded the similarity of stimuli to the target independent of saccade selection, whereas in FEF, encoding of target similarity was strongly modulated by saccade selection. Taken together, our findings suggest hierarchical processing of saccade selection signals and parallel processing of feature-based attention signals within the parietofrontal network with FEF having a more prominent role in priority encoding. Furthermore, they suggest discrete roles of FEF and LIP in the construction of priority maps.

Coding of spatial attention priorities and object features in the macaque lateral intraparietal cortex

Primate posterior parietal cortex (PPC) is known to be involved in controlling spatial attention. Neurons in one part of the PPC, the lateral intraparietal area (LIP), show enhanced responses to objects at attended locations. Although many are selective for object features, such as the orientation of a visual stimulus, it is not clear how LIP circuits integrate feature-selective information when providing attentional feedback about behaviorally relevant locations to the visual cortex. We studied the relationship between object feature and spatial attention properties of LIP cells in two macaques by measuring the cells' orientation selectivity and the degree of attentional enhancement while performing a delayed match-to-sample task. Monkeys had to match both the location and orientation of two visual gratings presented separately in time. We found a wide range in orientation selectivity and degree of attentional enhancement among LIP neurons. However, cells with significant attentional enhancement had much less orientation selectivity in their response than cells which showed no significant modulation by attention. Additionally, orientation-selective cells showed working memory activity for their preferred orientation, whereas cells showing attentional enhancement also synchronized with local neuronal activity. These results are consistent with models of selective attention incorporating two stages, where an initial feature-selective process guides a second stage of focal spatial attention. We suggest that LIP contributes to both stages, where the first stage involves orientation-selective LIP cells that support working memory of the relevant feature, and the second stage involves attention-enhanced LIP cells that synchronize to provide feedback on spatial priorities.

Functional interactions between the macaque dorsal and ventral visual pathways during three-dimensional object vision

The division of labor between the dorsal and the ventral visual stream in the primate brain has inspired numerous studies on the visual system in humans and in nonhuman primates. However, how and under which circumstances the two visual streams interact is still poorly understood. Here we review evidence from anatomy, modelling, electrophysiology, electrical microstimulation (EM), reversible inactivation and functional imaging in the macaque monkey aimed at clarifying at which levels in the hierarchy of visual areas the two streams interact, and what type of information might be exchanged between the two streams during three-dimensional (3D) object viewing. Neurons in both streams encode 3D structure from binocular disparity, synchronized activity between parietal and inferotemporal areas is present during 3D structure categorization, and clusters of 3D structure-selective neurons in parietal cortex are anatomically connected to ventral stream areas. In addition, caudal intraparietal cortex exerts a causal influence on 3D-structure related activations in more anterior parietal cortex and in inferotemporal cortex. Thus, both anatomical and functional evidence indicates that the dorsal and the ventral visual stream interact during 3D object viewing.

Autocorrelation structure at rest predicts value correlates of single neurons during reward-guided choice

Correlates of value are routinely observed in the prefrontal cortex (PFC) during reward-guided decision making. In previous work (Hunt et al., 2015), we argued that PFC correlates of chosen value are a consequence of varying rates of a dynamical evidence accumulation process. Yet within PFC, there is substantial variability in chosen value correlates across individual neurons. Here we show that this variability is explained by neurons having different temporal receptive fields of integration, indexed by examining neuronal spike rate autocorrelation structure whilst at rest. We find that neurons with protracted resting temporal receptive fields exhibit stronger chosen value correlates during choice. Within orbitofrontal cortex, these neurons also sustain coding of chosen value from choice through the delivery of reward, providing a potential neural mechanism for maintaining predictions and updating stored values during learning. These findings reveal that within PFC, variability in temporal specialisation across neurons predicts involvement in specific decision-making computations.

DOI:http://dx.doi.org/10.7554/eLife.18937.001

Failed cooperative, but not competitive, interaction between large-scale brain networks impairs working memory in schizophrenia

BACKGROUND

A large-scale network named the default mode network (DMN) dynamically cooperates and competes with an external attention system (EAS) to facilitate various cognitive functioning that is prominently impaired in schizophrenia. However, it is unclear whether the cognitive deficit in schizophrenia is related to the disrupted competition and/or cooperation between these two networks.

METHOD

A total of 35 schizophrenia patients and 30 healthy controls were scanned using gradient-echo echo-planar imaging during n-back working memory (WM) processing. Brain activities of the DMN and EAS were measured using general linear modelling of the functional magnetic resonance imaging data. Dynamic interaction between the DMN and EAS was decomposed into two directions using Granger causality analysis.

RESULTS

We observed a significant failure of DMN suppression in patients with schizophrenia, which was significantly related to WM/attentional deficit. Granger causality modelling showed that in healthy controls, while the EAS inhibitorily influenced the DMN, the DMN exerted an 'excitatory' or cooperative influence back on the EAS, especially in those with lower WM accuracy. In schizophrenia, this 'excitatory' DMN→EAS influence within the reciprocal EAS-DMN loop was significantly reduced, especially in patients with WM/attentional deficit.

CONCLUSIONS

The dynamic interaction between the DMN and EAS is likely to be comprised of both competitive and cooperative influences. In healthy controls, both the 'inhibitory' EAS→DMN interaction and 'excitatory' DMN→EAS interaction are correlated with WM performance. In schizophrenia, reduced 'cooperative' influence from the DMN to dorsal nodes of the EAS occurs in the context of non-suppression of the DMN and may form a possible pathophysiological substrate of WM deficit and attention disorder.

Effector specificity in macaque frontal and parietal cortex

Single neurons in the frontal eye fields (FEFs) and lateral intraparietal area (LIP) of macaques are preferentially activated by saccade- versus reach-related processes. fMRI studies focusing on saccade- and reach-specific activity in human cortex, however, provided conflicting evidence for effector specificity. To gain further insights into effector preferences throughout monkey cortex using the same technique as in humans, we performed a mixed block/event-related fMRI experiment in macaques. Within single fMRI runs, monkeys alternated between a visually guided saccade task, a visually guided arm movement task, and a fixation-only task requiring no saccades or arm movements. The detection of a peripheral pop-out go cue initiating the required operant behavior and the identification of a target among distractors was identical in the arm and saccade tasks. We found saccade-related activity in parietal areas V6, V6A, LIP, and caudal intraparietal area and frontal areas FEF, 45a, 45b, and 46. Areas 45 and FEF even showed markedly decreased fMRI activity during arm movements relative to fixation only. Conversely, medial and anterior intraparietal areas (MIP and AIP), and parietal area PEip; somatosensory areas S1 and S2; and (pre)motor areas F1, F3, F5, and F6 showed increased arm movement-related activity. F1, F5, PEip, and somatosensory cortex also showed deactivations during saccades relative to fixation only. Control experiments showed that such deactivations in both operant-specific functional networks did not depend on training history or rapid task switching requiring active suppression of the unpreferred operant behavior. Therefore, although both tasks required divided attention to detect a pop-out go cue and target, two largely segregated and mainly effector-driven cortical networks were activated.

Differential dynamics of spatial attention, position, and color coding within the parietofrontal network

Despite an ever growing knowledge on how parietal and prefrontal neurons encode low-level spatial and color information or higher-level information, such as spatial attention, an understanding of how these cortical regions process neuronal information at the population level is still missing. A simple assumption would be that the function and temporal response profiles of these neuronal populations match that of its constituting individual cells. However, several recent studies suggest that this is not necessarily the case and that the single-cell approach overlooks dynamic changes in how information is distributed over the neuronal population. Here, we use a time-resolved population pattern analysis to explore how spatial position, spatial attention and color information are differentially encoded and maintained in the macaque monkey prefrontal (frontal eye fields) and parietal cortex (lateral intraparietal area). Overall, our work brings about three novel observations. First, we show that parietal and prefrontal populations operate in two distinct population regimens for the encoding of sensory and cognitive information: a stationary mode and a dynamic mode. Second, we show that the temporal dynamics of a heterogeneous neuronal population brings about complementary information to that of its functional subpopulations. Thus, both need to be investigated in parallel. Last, we show that identifying the neuronal configuration in which a neuronal population encodes given information can serve to reveal this same information in a different context. All together, this work challenges common views on neural coding in the parietofrontal network.

Effective connectivity of depth-structure-selective patches in the lateral bank of the macaque intraparietal sulcus

Extrastriate cortical areas are frequently composed of subpopulations of neurons encoding specific features or stimuli, such as color, disparity, or faces, and patches of neurons encoding similar stimulus properties are typically embedded in interconnected networks, such as the attention or face-processing network. The goal of the current study was to examine the effective connectivity of subsectors of neurons in the same cortical area with highly similar neuronal response properties. We first recorded single- and multi-unit activity to identify two neuronal patches in the anterior part of the macaque intraparietal sulcus (IPS) showing the same depth structure selectivity and then employed electrical microstimulation during functional magnetic resonance imaging in these patches to determine the effective connectivity of these patches. The two IPS subsectors we identified-with the same neuronal response properties and in some cases separated by only 3 mm-were effectively connected to remarkably distinct cortical networks in both dorsal and ventral stream in three macaques. Conversely, the differences in effective connectivity could account for the known visual-to-motor gradient within the anterior IPS. These results clarify the role of the anterior IPS as a pivotal brain region where dorsal and ventral visual stream interact during object analysis. Thus, in addition to the anatomical connectivity of cortical areas and the properties of individual neurons in these areas, the effective connectivity provides novel key insights into the widespread functional networks that support behavior.

Dopamine neurons encode errors in predicting movement trigger occurrence

The capacity to anticipate the timing of events in a dynamic environment allows us to optimize the processes necessary for perceiving, attending to, and responding to them. Such anticipation requires neuronal mechanisms that track the passage of time and use this representation, combined with prior experience, to estimate the likelihood that an event will occur (i.e., the event's "hazard rate"). Although hazard-like ramps in activity have been observed in several cortical areas in preparation for movement, it remains unclear how such time-dependent probabilities are estimated to optimize response performance. We studied the spiking activity of dopamine neurons in the substantia nigra pars compacta of monkeys during an arm-reaching task for which the foreperiod preceding the "go" signal varied randomly along a uniform distribution. After extended training, the monkeys' reaction times correlated inversely with foreperiod duration, reflecting a progressive anticipation of the go signal according to its hazard rate. Many dopamine neurons modulated their firing rates as predicted by a succession of hazard-related prediction errors. First, as time passed during the foreperiod, slowly decreasing anticipatory activity tracked the elapsed time as if encoding negative prediction errors. Then, when the go signal appeared, a phasic response encoded the temporal unpredictability of the event, consistent with a positive prediction error. Neither the anticipatory nor the phasic signals were affected by the anticipated magnitudes of future reward or effort, or by parameters of the subsequent movement. These results are consistent with the notion that dopamine neurons encode hazard-related prediction errors independently of other information.

Encoding and decoding in parietal cortex during sensorimotor decision-making

The lateral intraparietal area (LIP) of macaques has been asserted to play a fundamental role in sensorimotor decision-making. Here we dissect the neural code in LIP at the level of individual trial spike trains using a statistical approach based on generalized linear models. We show that LIP responses reflect a combination of temporally-overlapping task and decision-related signals. Our model accounts for the detailed statistics of LIP spike trains, and accurately predicts spike trains from task events on single trials. Moreover, we derive an optimal decoder for heterogeneous, multiplexed LIP responses that could be implemented in biologically plausible circuits. In contrast to interpretations of LIP as providing an instantaneous code for decision variables, we show that optimal decoding requires integrating LIP spikes over two timescales. These analyses provide a detailed understanding of the neural code in LIP, and a framework for studying the coding of multiplexed signals in higher brain areas.

Topographic organization in the brain: searching for general principles

The neurons comprising many cortical areas have long been known to be arranged topographically such that nearby neurons have receptive fields at nearby locations in the world. Although this type of organization may be universal in primary sensory and motor cortex, in this review we demonstrate that associative cortical areas may not represent the external world in a complete and continuous fashion. After reviewing evidence for novel principles of topographic organization in macaque lateral intraparietal area (LIP) - one of the most-studied associative areas in the parietal cortex - we explore the implications of these new principles for brain function.

Transition of target-location signaling in activity of macaque lateral intraparietal neurons during delayed-response visual search

Neurons in the lateral intraparietal area (LIP) are involved in signaling the location of behaviorally relevant objects during visual discrimination and working memory maintenance. Although previous studies have examined these cognitive processes separately, they often appear as inseparable sequential processes in real-life situations. Little is known about how the neural representation of the target location is altered when both cognitive processes are continuously required for executing a task. We investigated this issue by recording single-unit activity from LIP of monkeys performing a delayed-response visual search task in which they were required to discriminate the target from distractors in the stimulus period, remember the location at which the extinguished target had been presented in the delay period, and make a saccade to that location in the response period. Target-location signaling was assessed using response modulations contingent on whether the target location was inside or opposite the receptive field. Although the population-averaged response modulation was consistent and changed only slightly during a trial, the across-neuron pattern of response modulations showed a marked and abrupt change around 170 ms after stimulus offset due to concurrent changes in the response modulations of a subset of LIP neurons, which manifested heterogeneous patterns of activity changes during the task. Our findings suggest that target-location signaling by the across-neuron pattern of LIP activity discretely changes after the stimulus disappearance under conditions that continuously require visual discrimination and working memory to perform a single behavioral task.

Predictive saccade target selection in superior colliculus during visual search

Searching for a visual object naturally involves sequences of gaze fixations, during which the current foveal image is analyzed and the next object to inspect is selected as a saccade target. Fixation durations during such sequences are short, suggesting that saccades may be concurrently processed. Therefore, the selection of the next saccade target may occur before the current saccade target is acquired. To test this hypothesis, we trained four female rhesus monkeys (Macaca mulatta) to perform a multiple-fixation visual conjunction search task. We simultaneously recorded the activity of sensorimotor neurons in the midbrain superior colliculus (SC) in two monkeys. In this task, monkeys made multiple fixations before foveating the target. Fixation durations were significantly shorter than the latency of the initial responses to the search display, with approximately one-quarter being shorter than the shortest response latencies. The time at which SC sensorimotor activity discriminated the target from distracters occurred significantly earlier for the selection of subsequent fixations than for the selection of the first fixation. Target selection during subsequent fixations occurred even before the visual afferent delay in more than half of the neuronal sample, suggesting that the process of selection can encompass at least two future saccade targets. This predictive selection was present even when differences in saccade latencies were taken into account. Altogether, these findings demonstrate how neural representations on the visual salience map are processed in parallel, thus facilitating visual search.

Posterior parietal cortex activity reflects the significance of others' actions during natural viewing

The posterior parietal cortex (PPC) has been associated with multiple stimulus-driven (e.g., processing stimulus movements, providing visual signals for the motor system), goal-directed (e.g., directing visual attention to a target, processing behavioral priority of intentions), and action-related functions in previous studies with non-naturalistic paradigms. Here, we examined how these functions reflect PPC activity during natural viewing. Fourteen healthy volunteers watched a re-edited movie during functional magnetic resonance imaging (fMRI). Participants separately annotated behavioral priority (accounting for percepts, thoughts, and emotions) they had experienced during movie episodes. Movements in the movie were quantified with computer vision and eye movements were recorded from a separate group of subjects. Our results show that while overlapping dorsomedial PPC areas respond to episodes with multiple types of stimulus content, ventrolateral PPC areas exhibit enhanced activity when viewing goal-directed human hand actions. Furthermore, PPC activity related to viewing goal-directed human hand actions was more accurately explained by behavioral priority than by movements of the stimulus or eye movements. Taken together, our results suggest that PPC participates in perception of goal-directed human hand actions, supporting the view that PPC has a special role in providing visual signals for the motor system ("how"), in addition to processing visual spatial movements ("where").

The effect of FEF microstimulation on the responses of neurons in the lateral intraparietal area

The macaque FEFs and the lateral intraparietal area (LIP) are high-level cortical areas involved in both spatial attention and oculomotor behavior. Stimulating FEF at a level below the threshold for evoking saccades increases fMRI activity and gamma power in area LIP, but the precise effect exerted by the FEF on LIP neurons is unknown. In our study, we recorded LIP single-unit activity during a visually guided saccade task with a peripherally presented go signal during microstimulation of FEF. We found that FEF microstimulation increased the LIP spike rate immediately after the highly salient go signal inside the LIP receptive field when both target and go signal were presented inside the receptive field, and no other possible go cues were present on the screen. The effect of FEF microstimulation on the LIP response was positive until at least 800 msec after microstimulation had ceased, but reversed for longer trial durations. Therefore, FEF microstimulation can modulate the LIP spike rate only when attention is selectively directed toward the stimulated location. These results provide the first direct evidence for LIP spike rate modulations caused by FEF microstimulation, thus showing that FEF activity can be the source of top-down control of area LIP.

A functional hierarchy within the parietofrontal network in stimulus selection and attention control

Although we are confronted with an ever-changing environment, we do not have the capacity to analyze all incoming sensory information. Perception is selective and is guided both by salient events occurring in our visual field and by cognitive premises about what needs our attention. Although the lateral intraparietal area (LIP) and frontal eye field (FEF) are known to represent the position of visual attention, their respective contributions to its control are still unclear. Here, we report LIP and FEF neuronal activities recorded while monkeys performed a voluntary attention-orientation target-detection task. We show that both encode behaviorally significant events, but that the FEF plays a specific role in mapping abstract cue instructions onto a spatial priority map to voluntarily guide attention. On the basis of a latency analysis, we show that the coding of stimulus identity and position precedes the emergence of an explicit attentional signal within the FEF. We also describe dynamic temporal hierarchies between LIP and FEF: stimuli carrying the highest intrinsic saliency are signaled by LIP before FEF, whereas stimuli carrying the highest extrinsic saliency are signaled in FEF before LIP. This suggests that whereas the parietofrontal attentional network most probably processes visual information in a recurrent way, exogenous processing predominates in the parietal cortex and the endogenous control of attention takes place in the FEF.

Activity patterns in the category-selective occipitotemporal cortex predict upcoming motor actions

Converging lines of evidence point to the occipitotemporal cortex (OTC) as a critical structure in visual perception. For instance, human functional magnetic resonance imaging (fMRI) has revealed a modular organisation of object-selective, face-selective, body-selective and scene-selective visual areas in the OTC, and disruptions to the processing within these regions, either in neuropsychological patients or through transcranial magnetic stimulation, can produce category-specific deficits in visual recognition. Here we show, using fMRI and pattern classification methods, that the activity in the OTC also represents how stimuli will be interacted with by the body--a level of processing more traditionally associated with the preparatory activity in sensorimotor circuits of the brain. Combining functional mapping of different OTC areas with a real object-directed delayed movement task, we found that the pre-movement spatial activity patterns across the OTC could be used to predict both the action of an upcoming hand movement (grasping vs. reaching) and the effector (left hand vs. right hand) to be used. Interestingly, we were able to extract this wide range of predictive movement information even though nearly all OTC areas showed either baseline-level or below baseline-level activity prior to action onset. Our characterisation of different OTC areas according to the features of upcoming movements that they could predict also revealed a general gradient of effector-to-action-dependent movement representations along the posterior-anterior OTC axis. These findings suggest that the ventral visual pathway, which is well known to be involved in object recognition and perceptual processing, plays a larger than previously expected role in preparing object-directed hand actions.

How embodied is perceptual decision making? Evidence for separate processing of perceptual and motor decisions

The extent to which different cognitive processes are "embodied" is widely debated. Previous studies have implicated sensorimotor regions such as lateral intraparietal (LIP) area in perceptual decision making. This has led to the view that perceptual decisions are embodied in the same sensorimotor networks that guide body movements. We use event-related fMRI and effective connectivity analysis to investigate whether the human sensorimotor system implements perceptual decisions. We show that when eye and hand motor preparation is disentangled from perceptual decisions, sensorimotor areas are not involved in accumulating sensory evidence toward a perceptual decision. Instead, inferior frontal cortex increases its effective connectivity with sensory regions representing the evidence, is modulated by the amount of evidence, and shows greater task-positive BOLD responses during the perceptual decision stage. Once eye movement planning can begin, however, an intraparietal sulcus (IPS) area, putative LIP, participates in motor decisions. Moreover, sensory evidence levels modulate decision and motor preparation stages differently in different IPS regions, suggesting functional heterogeneity of the IPS. This suggests that different systems implement perceptual versus motor decisions, using different neural signatures.

Signal multiplexing and single-neuron computations in lateral intraparietal area during decision-making

Previous work has revealed a remarkably direct neural correlate of decisions in the lateral intraparietal area (LIP). Specifically, firing rate has been observed to ramp up or down in a manner resembling the accumulation of evidence for a perceptual decision reported by making a saccade into (or away from) the neuron's response field (RF). However, this link between LIP response and decision formation emerged from studies where a saccadic target was always stimulating the RF during decisions, and where the neural correlate was the averaged activity of a restricted sample of neurons. Because LIP cells are (1) highly responsive to the presence of a visual stimulus in the RF, (2) heterogeneous, and (3) not clearly anatomically segregated from large numbers of neurons that fail selection criteria, the underlying neuronal computations are potentially obscured. To address this, we recorded single neuron spiking activity in LIP during a well-studied moving-dot direction-discrimination task and manipulated whether a saccade target was present in the RF during decision-making. We also recorded from a broad sample of LIP neurons, including ones conventionally excluded in prior studies. Our results show that cells multiplex decision signals with decision-irrelevant visual signals. We also observed disparate, repeating response "motifs" across neurons that, when averaged together, resemble traditional ramping decision signals. In sum, neural responses in LIP simultaneously carry decision signals and decision-irrelevant sensory signals while exhibiting diverse dynamics that reveal a broader range of neural computations than previously entertained.

Thinking in spatial terms: decoupling spatial representation from sensorimotor control in monkey posterior parietal areas 7a and LIP

Perhaps the simplest and most complete description of the cerebral cortex is that it is a sensorimotor controller whose primary purpose is to represent stimuli and movements, and adaptively control the mapping between them. However, in order to think, the cerebral cortex has to generate patterns of neuronal activity that encode abstract, generalized information independently of ongoing sensorimotor events. A critical question confronting cognitive systems neuroscience at present therefore is how neural signals encoding abstract information emerge within the sensorimotor control networks of the brain. In this review, we approach that question in the context of the neural representation of space in posterior parietal cortex of non-human primates. We describe evidence indicating that parietal cortex generates a hierarchy of spatial representations with three basic levels: including (1) sensorimotor signals that are tightly coupled to stimuli or movements, (2) sensorimotor signals modified in strength or timing to mediate cognition (examples include attention, working memory, and decision-processing), as well as (3) signals that encode frankly abstract spatial information (such as spatial relationships or categories) generalizing across a wide diversity of specific stimulus conditions. Here we summarize the evidence for this hierarchy, and consider data showing that signals at higher levels derive from signals at lower levels. That in turn could help characterize neural mechanisms that derive a capacity for abstraction from sensorimotor experience.

Cortical circuits for the control of attention

How are some thoughts favored over others? A wealth of data at the level of single neurons has yielded candidate brain areas and mechanisms for our best-understood model: visual attention. Recent work has naturally evolved toward efforts at a more integrative, network, understanding. It suggests that focusing attention arises from interactions between widespread cortical and subcortical networks that may be regulated via their rhythmic synchronization.

Distinct neural mechanisms of distractor suppression in the frontal and parietal lobe

The posterior parietal cortex and the prefrontal cortex are associated with eye movements and visual attention, but their specific contributions are poorly understood. We compared the dorsolateral prefrontal cortex (dlPFC) and the lateral intraparietal area (LIP) in monkeys using a memory saccade task in which a salient distractor flashed at a variable timing and location during the memory delay. We found that the two areas had similar responses to target selection, but made distinct contributions to distractor suppression. Distractor responses were more strongly suppressed and more closely correlated with performance in the dlPFC relative to LIP. Moreover, reversible inactivation of the dlPFC produced much larger increases in distractibility than inactivation of LIP. These findings suggest that LIP and dlPFC mediate different aspects of selective attention. Although both areas can contribute to the perceptual selection of salient information, the dlPFC has a decisive influence on whether and how attended stimulus is linked with actions.

Selectivity for three-dimensional shape and grasping-related activity in the macaque ventral premotor cortex

Anatomical studies indicate that area F5 in the macaque ventral premotor cortex consists of three different sectors. One of these is F5a in the posterior bank of the inferior arcuate sulcus, but no functional characterization of F5a at the single-cell level exists. We investigated the neuronal selectivity for three-dimensional (3D) shape and grasping activity in F5a. In contrast to neighboring regions F5p and 45B, the great majority of F5a neurons showed selectivity for disparity-defined curved surfaces, and most neurons preserved this selectivity across positions in depth, indicating higher-order disparity selectivity. Thus, as predicted by monkey fMRI data, F5a neurons showed robust 3D-shape selectivity in the absence of a motor response. To investigate the relationship between disparity selectivity and grasping activity, we recorded from 3D-shape-selective F5a neurons during a visually guided grasping task and during grasping in the dark. F5a neurons encoding the depth profile of curved surfaces frequently responded during grasping of real-world objects in the light, but not in the dark, whereas nearby neurons were also active in the dark. The presence of 3D-shape-selective and "visual-dominant" neurons demonstrates that the F5a sector is distinct from neighboring regions of ventral premotor cortex, in line with recent anatomical connectivity studies.

Temporal production signals in parietal cortex

We often perform movements and actions on the basis of internal motivations and without any explicit instructions or cues. One common example of such behaviors is our ability to initiate movements solely on the basis of an internally generated sense of the passage of time. In order to isolate the neuronal signals responsible for such timed behaviors, we devised a task that requires nonhuman primates to move their eyes consistently at regular time intervals in the absence of any external stimulus events and without an immediate expectation of reward. Despite the lack of sensory information, we found that animals were remarkably precise and consistent in timed behaviors, with standard deviations on the order of 100 ms. To examine the potential neural basis of this precision, we recorded from single neurons in the lateral intraparietal area (LIP), which has been implicated in the planning and execution of eye movements. In contrast to previous studies that observed a build-up of activity associated with the passage of time, we found that LIP activity decreased at a constant rate between timed movements. Moreover, the magnitude of activity was predictive of the timing of the impending movement. Interestingly, this relationship depended on eye movement direction: activity was negatively correlated with timing when the upcoming saccade was toward the neuron's response field and positively correlated when the upcoming saccade was directed away from the response field. This suggests that LIP activity encodes timed movements in a push-pull manner by signaling for both saccade initiation towards one target and prolonged fixation for the other target. Thus timed movements in this task appear to reflect the competition between local populations of task relevant neurons rather than a global timing signal.

Neural correlates of prior expectations of motion in the lateral intraparietal and middle temporal areas

Successful decision making involves combining observations of the external world with prior knowledge. Recent studies suggest that neural activity in macaque lateral intraparietal area (LIP) provides a useful window into this process. This study examines how rapidly changing prior knowledge about an upcoming sensory stimulus influences the computations that convert sensory signals into plans for action. Two monkeys performed a cued direction discrimination task, in which an arrow cue presented at the start of each trial communicated the prior probability of the direction of stimulus motion. We hypothesized that the cue would either shift the initial level of LIP activity before sensory evidence arrived, or it would scale sensory responses according to the prior probability of each stimulus, manifesting as a change in slope of LIP firing rates. Neural recordings demonstrated a clear shift in the activity level of LIP neurons following the arrow cue, which persisted into the presentation of the motion stimulus. No significant change in slope of responses was observed, suggesting that sensory gain was not strongly modulated. To confirm the latter observation, middle temporal area (MT) neurons were recorded during a version of the cued direction discrimination task, and we found no change in MT responses resulting from the presentation of the directional cue. These results suggest that information about an immediately upcoming stimulus does not scale the sensory response, but rather changes the amount of evidence that must be accumulated to reach a decision in areas that are involved in planning action.

Neural correlates and neural computations in posterior parietal cortex during perceptual decision-making

A recent line of work has found remarkable success in relating perceptual decision-making and the spiking activity in the macaque lateral intraparietal area (LIP). In this review, we focus on questions about the neural computations in LIP that are not answered by demonstrations of neural correlates of psychological processes. We highlight three areas of limitations in our current understanding of the precise neural computations that might underlie neural correlates of decisions: (1) empirical questions not yet answered by existing data; (2) implementation issues related to how neural circuits could actually implement the mechanisms suggested by both extracellular neurophysiology and psychophysics; and (3) ecological constraints related to the use of well-controlled laboratory tasks and whether they provide an accurate window on sensorimotor computation. These issues motivate the adoption of a more general "encoding-decoding framework" that will be fruitful for more detailed contemplation of how neural computations in LIP relate to the formation of perceptual decisions.

Optogenetically induced behavioral and functional network changes in primates

Optogenetics is currently the state-of-the-art method for causal-oriented brain research. Despite an increasingly large number of invertebrate and rodent studies showing profound electrophysiological and behavioral effects induced by optogenetics, only two primate studies have reported modulation of local single-cell activity but with no behavioral effects. Here, we show that optogenetic stimulation of cortical neurons within rhesus monkey arcuate sulcus, during the execution of a visually guided saccade task, evoked significant and reproducible changes in saccade latencies as a function of target position. Moreover, using concurrent optogenetic stimulation and opto-fMRI), we observed optogenetically induced changes in fMRI activity in specific functional cortical networks throughout the monkey brain. This is critical information for the advancement of optogenetic primate research models and for initiating the development of optogenetically based cell-specific therapies with which to treat neurological diseases in humans.

Early involvement of prefrontal cortex in visual bottom-up attention

Visual attention is guided to stimuli based either on their intrinsic saliency against their background (bottom-up factors) or through willful search of known targets (top-down factors). Posterior parietal cortex is thought to play a critical role in the guidance of visual bottom-up attention, whereas prefrontal cortex is thought to represent top-down factors. Contrary to this established view, we found that when monkeys were tested in a task requiring detection of a salient stimulus defined purely by bottom-up factors and whose identity was unknown prior to the presentation of a visual display, prefrontal neurons represented the salient stimulus no later than those in the posterior parietal cortex. This was true even though visual response latency was shorter in parietal than in prefrontal cortex. These results suggest an early involvement of the prefrontal cortex in the bottom-up guidance of visual attention.

Frontal eye field microstimulation induces task-dependent gamma oscillations in the lateral intraparietal area

Macaque frontal eye fields (FEF) and the lateral intraparietal area (LIP) are high-level oculomotor control centers that have been implicated in the allocation of spatial attention. Electrical microstimulation of macaque FEF elicits functional magnetic resonance imaging (fMRI) activations in area LIP, but no study has yet investigated the effect of FEF microstimulation on LIP at the single-cell or local field potential (LFP) level. We recorded spiking and LFP activity in area LIP during weak, subthreshold microstimulation of the FEF in a delayed-saccade task. FEF microstimulation caused a highly time- and frequency-specific, task-dependent increase in gamma power in retinotopically corresponding sites in LIP: FEF microstimulation produced a significant increase in LIP gamma power when a saccade target appeared and remained present in the LIP receptive field (RF), whereas less specific increases in alpha power were evoked by FEF microstimulation for saccades directed away from the RF. Stimulating FEF with weak currents had no effect on LIP spike rates or on the gamma power during memory saccades or passive fixation. These results provide the first evidence for task-dependent modulations of LFPs in LIP caused by top-down stimulation of FEF. Since the allocation and disengagement of spatial attention in visual cortex have been associated with increases in gamma and alpha power, respectively, the effects of FEF microstimulation on LIP are consistent with the known effects of spatial attention.

Local Field Potential Activity Associated with Temporal Expectations in the Macaque Lateral Intraparietal Area

Oscillatory brain activity is attracting increasing interest in cognitive neuroscience. Numerous EEG (magnetoencephalography) and local field potential (LFP) measurements have related cognitive functions to different types of brain oscillations, but the functional significance of these rhythms remains poorly understood. Despite its proven value, LFP activity has not been extensively tested in the macaque lateral intraparietal area (LIP), which has been implicated in a wide variety of cognitive control processes. We recorded action potentials and LFPs in area LIP during delayed eye movement tasks and during a passive fixation task, in which the time schedule was fixed so that temporal expectations about task-relevant cues could be formed. LFP responses in the gamma band discriminated reliably between saccade targets and distractors inside the receptive field (RF). Alpha and beta responses were much less strongly affected by the presence of a saccade target, however, but rose sharply in the waiting period before the go signal. Surprisingly, conditions without visual stimulation of the LIP-RF-evoked robust LFP responses in every frequency band—most prominently in those below 50 Hz—precisely time-locked to the expected time of stimulus onset in the RF. These results indicate that in area LIP, oscillations in the LFP, which reflect synaptic input and local network activity, are tightly coupled to the temporal expectation of task-relevant cues.

Unique and shared roles of the posterior parietal and dorsolateral prefrontal cortex in cognitive functions

The dorsolateral prefrontal cortex (PFC) and posterior parietal cortex (PPC) are two parts of a broader brain network involved in the control of cognitive functions such as working-memory, spatial attention, and decision-making. The two areas share many functional properties and exhibit similar patterns of activation during the execution of mental operations. However, neurophysiological experiments in non-human primates have also documented subtle differences, revealing functional specialization within the fronto-parietal network. These differences include the ability of the PFC to influence memory performance, attention allocation, and motor responses to a greater extent, and to resist interference by distracting stimuli. In recent years, distinct cellular and anatomical differences have been identified, offering insights into how functional specialization is achieved. This article reviews the common functions and functional differences between the PFC and PPC, and their underlying mechanisms.