Presaccadic discrimination of receptive field stimuli by area V4 neurons

Dissociable components of attention exhibit distinct neuronal signatures in primate visual cortex

Attention can be deployed in multiple forms and facilitates behavior by influencing perceptual sensitivity and choice bias. Attention is also associated with a myriad of changes in sensory neural activity. Yet, the relationship between the behavioral components of attention and the accompanying changes in neural activity remains largely unresolved. We examined this relationship by quantifying sensitivity and bias in monkeys performing a task that dissociated eye movement responses from the focus of covert attention. Unexpectedly, bias, not sensitivity, increased at the focus of covert attention, whereas sensitivity increased at the location of planned eye movements. Furthermore, neuronal activity within visual area V4 varied robustly with bias, but not sensitivity, at the focus of covert attention. In contrast, correlated variability between neuronal pairs was lowest at the location of planned eye movements, and varied with sensitivity, but not bias. Thus, dissociable behavioral components of attention exhibit distinct neuronal signatures within the visual cortex.

Time-varying generalized linear models: characterizing and decoding neuronal dynamics in higher visual areas

To create a behaviorally relevant representation of the visual world, neurons in higher visual areas exhibit dynamic response changes to account for the time-varying interactions between external (e.g., visual input) and internal (e.g., reward value) factors. The resulting high-dimensional representational space poses challenges for precisely quantifying individual factors' contributions to the representation and readout of sensory information during a behavior. The widely used point process generalized linear model (GLM) approach provides a powerful framework for a quantitative description of neuronal processing as a function of various sensory and non-sensory inputs (encoding) as well as linking particular response components to particular behaviors (decoding), at the level of single trials and individual neurons. However, most existing variations of GLMs assume the neural systems to be time-invariant, making them inadequate for modeling nonstationary characteristics of neuronal sensitivity in higher visual areas. In this review, we summarize some of the existing GLM variations, with a focus on time-varying extensions. We highlight their applications to understanding neural representations in higher visual areas and decoding transient neuronal sensitivity as well as linking physiology to behavior through manipulation of model components. This time-varying class of statistical models provide valuable insights into the neural basis of various visual behaviors in higher visual areas and hold significant potential for uncovering the fundamental computational principles that govern neuronal processing underlying various behaviors in different regions of the brain.

Pre-saccadic Neural Enhancements in Marmoset Area MT

Each time we make an eye movement, attention moves before the eyes, resulting in a perceptual enhancement at the target. Recent psychophysical studies suggest that this pre-saccadic attention enhances the visual features at the saccade target, whereas covert attention causes only spatially selective enhancements. While previous nonhuman primate studies have found that pre-saccadic attention does enhance neural responses spatially, no studies have tested whether changes in neural tuning reflect an automatic feature enhancement. Here we examined pre-saccadic attention using a saccade foraging task developed for marmoset monkeys (one male and one female). We recorded from neurons in the middle temporal area with peripheral receptive fields that contained a motion stimulus, which would either be the target of a saccade or a distracter as a saccade was made to another location. We established that marmosets, like macaques, show enhanced pre-saccadic neural responses for saccades toward the receptive field, including increases in firing rate and motion information. We then examined if the specific changes in neural tuning might support feature enhancements for the target. Neurons exhibited diverse changes in tuning but predominantly showed additive and multiplicative increases that were uniformly applied across motion directions. These findings confirm that marmoset monkeys, like macaques, exhibit pre-saccadic neural enhancements during saccade foraging tasks with minimal training requirements. However, at the level of individual neurons, the lack of feature-tuned enhancements is similar to neural effects reported during covert spatial attention.

Presaccadic attention does not facilitate the detection of changes in the visual field

Planning a rapid eye movement (saccade) changes how we perceive our visual world. Even before we move the eyes visual discrimination sensitivity improves at the impending target of eye movements, a phenomenon termed "presaccadic attention." Yet, it is unknown if such presaccadic selection merely affects perceptual sensitivity, or also affects downstream decisional processes, such as choice bias. We report a surprising lack of presaccadic perceptual benefits in a common, everyday setting-detection of changes in the visual field. Despite the lack of sensitivity benefits, choice bias for reporting changes increased reliably for the saccade target. With independent follow-up experiments, we show that presaccadic change detection is rendered more challenging because percepts at the saccade target location are biased toward, and more precise for, only the most recent of two successive stimuli. With a Bayesian model, we show how such perceptual and choice biases are crucial to explain the effects of saccade plans on change detection performance. In sum, visual change detection sensitivity does not improve presaccadically, a result that is readily explained by teasing apart distinct components of presaccadic selection. The findings may have critical implications for real-world scenarios, like driving, that require rapid gaze shifts in dynamically changing environments.

Neural correlates of perisaccadic visual mislocalization in extrastriate cortex

When interacting with the visual world using saccadic eye movements (saccades), the perceived location of visual stimuli becomes biased, a phenomenon called perisaccadic mislocalization, which is indeed an exemplar of the brain's dynamic representation of the visual world. However, the neural mechanism underlying this altered visuospatial perception and its potential link to other perisaccadic perceptual phenomena have not been established. Using a combined experimental and computational approach, we were able to quantify spatial bias around the saccade target (ST) based on the perisaccadic dynamics of extrastriate spatiotemporal sensitivity captured by statistical models. This approach could predict the perisaccadic spatial bias around the ST, consistent with the psychophysical studies, and revealed the precise neuronal response components underlying representational bias. These findings also established the crucial role of response remapping toward ST representation for neurons with receptive fields far from the ST in driving the ST spatial bias. Moreover, we showed that, by allocating more resources for visual target representation, visual areas enhance their representation of the ST location, even at the expense of transient distortions in spatial representation. This potential neural basis for perisaccadic ST representation, also supports a general role for extrastriate neurons in creating the perception of stimulus location.

Sensory tuning in neuronal movement commands

Movement control is critical for successful interaction with our environment. However, movement does not occur in complete isolation of sensation, and this is particularly true of eye movements. Here, we show that the neuronal eye movement commands emitted by the superior colliculus (SC), a structure classically associated with oculomotor control, encompass a robust visual sensory representation of eye movement targets. Thus, similar saccades toward different images are associated with different saccade-related "motor" bursts. Such sensory tuning in SC saccade motor commands appeared for all image manipulations that we tested, from simple visual features to real-life object images, and it was also strongest in the most motor neurons in the deeper collicular layers. Visual-feature discrimination performance in the motor commands was also stronger than in visual responses. Comparing SC motor command feature discrimination performance to that in the primary visual cortex during steady-state gaze fixation revealed that collicular motor bursts possess a reliable perisaccadic sensory representation of the peripheral saccade target's visual appearance, exactly when retinal input is expected to be most uncertain. Our results demonstrate that SC neuronal movement commands likely serve a fundamentally sensory function.

To look or not to look: dissociating presaccadic and covert spatial attention

Attention is a central neural process that enables selective and efficient processing of visual information. Individuals can attend to specific visual information either overtly, by making an eye movement to an object of interest, or covertly, without moving their eyes. We review behavioral, neuropsychological, neurophysiological, and computational evidence of presaccadic attentional modulations that occur while preparing saccadic eye movements, and highlight their differences from those of covert spatial endogenous (voluntary) and exogenous (involuntary) attention. We discuss recent studies and experimental procedures on how these different types of attention impact visual performance, alter appearance, differentially modulate the featural representation of basic visual dimensions (orientation and spatial frequency), engage different neural computations, and recruit partially distinct neural substrates. We conclude that presaccadic attention and covert attention are dissociable.

Different computations underlie overt presaccadic and covert spatial attention

Perception and action are tightly coupled: visual responses at the saccade target are enhanced right before saccade onset. This phenomenon, presaccadic attention, is a form of overt attention-deployment of visual attention with concurrent eye movements. Presaccadic attention is well-documented, but its underlying computational process remains unknown. This is in stark contrast to covert attention-deployment of visual attention without concurrent eye movements-for which the computational processes are well characterized by a normalization model. Here, a series of psychophysical experiments reveal that presaccadic attention modulates visual performance only via response gain changes. A response gain change was observed even when attention field size increased, violating the predictions of a normalization model of attention. Our empirical results and model comparisons reveal that the perceptual modulations by overt presaccadic and covert spatial attention are mediated through different computations.

Frontotemporal coordination predicts working memory performance and its local neural signatures

Neurons in some sensory areas reflect the content of working memory (WM) in their spiking activity. However, this spiking activity is seldom related to behavioral performance. We studied the responses of inferotemporal (IT) neurons, which exhibit object-selective activity, along with Frontal Eye Field (FEF) neurons, which exhibit spatially selective activity, during the delay period of an object WM task. Unlike the spiking activity and local field potentials (LFPs) within these areas, which were poor predictors of behavioral performance, the phase-locking of IT spikes and LFPs with the beta band of FEF LFPs robustly predicted successful WM maintenance. In addition, IT neurons exhibited greater object-selective persistent activity when their spikes were locked to the phase of FEF LFPs. These results reveal that the coordination between prefrontal and temporal cortex predicts the successful maintenance of visual information during WM.

Saccadic momentum and attentive control in V4 neurons during visual search

Saccadic momentum refers to the increased probability of making a saccade in a forward direction relative to the previous saccade. During visual search and free viewing conditions saccadic probability falls in a gradient from forward to backward directions. It has been considered to reflect an oculomotor bias for a continuing motor plan. Here we report that a saccadic momentum gradient is observed in nonhuman primate behavior and in the visual responses of cortical area V4 neurons during a conjunction style visual search task. This result suggests that saccadic momentum arises in part from a biased spatial distribution of visual responses to stimuli. The effect is independent of feature-based selective attention and overridden by directed spatial attention. The implications of saccadic momentum for search guidance are much broader and robust than the inhibition-of-return's presumed role in preventing refixation of recent locations.

Spatial working memory alters the efficacy of input to visual cortex

Prefrontal cortex modulates sensory signals in extrastriate visual cortex, in part via its direct projections from the frontal eye field (FEF), an area involved in selective attention. We find that working memory-related activity is a dominant signal within FEF input to visual cortex. Although this signal alone does not evoke spiking responses in areas V4 and MT during memory, the gain of visual responses in these areas increases, and neuronal receptive fields expand and shift towards the remembered location, improving the stimulus representation by neuronal populations. These results provide a basis for enhancing the representation of working memory targets and implicate persistent FEF activity as a basis for the interdependence of working memory and selective attention.

Frontal eye field (FEF) is a visual prefrontal area involved in top-down attention. Here the authors report that FEF neurons projecting to V4/MT are persistently active during spatial working memory, and V4/MT neurons show changes in receptive field and gain at the location held in working memory.

Neural Mechanisms of Selective Visual Attention

Selective visual attention describes the tendency of visual processing to be confined largely to stimuli that are relevant to behavior. It is among the most fundamental of cognitive functions, particularly in humans and other primates for whom vision is the dominant sense. We review recent progress in identifying the neural mechanisms of selective visual attention. We discuss evidence from studies of different varieties of selective attention and examine how these varieties alter the processing of stimuli by neurons within the visual system, current knowledge of their causal basis, and methods for assessing attentional dysfunctions. In addition, we identify some key questions that remain in identifying the neural mechanisms that give rise to the selective processing of visual information.

Saccade Preparation Reshapes Sensory Tuning

Human observers make large rapid eye movements-saccades-to bring behaviorally relevant information into the fovea, where spatial resolution is high. In some visual tasks [1-4], performance at the location of a saccade target improves before the eyes move. Although these findings provide evidence that extra-retinal signals evoked by saccades can enhance visual perception, it remains unknown whether and how presaccadic modulations change the processing of feature information and thus modulate visual representations. To answer this question, one must go beyond the use of methods that only probe performance accuracy (d') in different tasks. Here, using a psychophysical reverse correlation approach [5-8], we investigated how saccade preparation influences the processing of orientation and spatial frequency-two building blocks of early vision. We found that saccade preparation selectively enhanced the gain of high spatial frequency information and narrowed orientation tuning at the upcoming saccade landing position. These modulations were time locked to saccade onset, peaking right before the eyes moved (-50-0 ms). Moreover, merely deploying covert attention within the same temporal interval without preparing a saccade did not alter performance. The observed presaccadic tuning changes may correspond to the presaccadic enhancement [9-11] and receptive field shifts reported in neurophysiological studies [12-14]. Saccade preparation may support transaccadic integration by reshaping the representation of the saccade target to be more fovea-like just before the eyes move. The presaccadic modulations on spatial frequency and orientation processing illustrate a strong perception-action coupling by revealing that the visual system dynamically reshapes feature selectivity contingent upon eye movements.

Perisaccadic Updating of Visual Representations and Attentional States: Linking Behavior and Neurophysiology

During natural vision, saccadic eye movements lead to frequent retinal image changes that result in different neuronal subpopulations representing the same visual feature across fixations. Despite these potentially disruptive changes to the neural representation, our visual percept is remarkably stable. Visual receptive field remapping, characterized as an anticipatory shift in the position of a neuron's spatial receptive field immediately before saccades, has been proposed as one possible neural substrate for visual stability. Many of the specific properties of remapping, e.g., the exact direction of remapping relative to the saccade vector and the precise mechanisms by which remapping could instantiate stability, remain a matter of debate. Recent studies have also shown that visual attention, like perception itself, can be sustained across saccades, suggesting that the attentional control system can also compensate for eye movements. Classical remapping could have an attentional component, or there could be a distinct attentional analog of visual remapping. At this time we do not yet fully understand how the stability of attentional representations relates to perisaccadic receptive field shifts. In this review, we develop a vocabulary for discussing perisaccadic shifts in receptive field location and perisaccadic shifts of attentional focus, review and synthesize behavioral and neurophysiological studies of perisaccadic perception and perisaccadic attention, and identify open questions that remain to be experimentally addressed.

Neuroscience: Tiny Eye Movements Link Vision and Attention

Eye movements are essential to human vision. A new study shows that the tiny eye movements we make while holding our gaze on a point of interest are associated with brief, attention-like changes in the sensitivity of visual neurons.

Visual attention: Linking prefrontal sources to neuronal and behavioral correlates

Attention is a means of flexibly selecting and enhancing a subset of sensory input based on the current behavioral goals. Numerous signatures of attention have been identified throughout the brain, and now experimenters are seeking to determine which of these signatures are causally related to the behavioral benefits of attention, and the source of these modulations within the brain. Here, we review the neural signatures of attention throughout the brain, their theoretical benefits for visual processing, and their experimental correlations with behavioral performance. We discuss the importance of measuring cue benefits as a way to distinguish between impairments on an attention task, which may instead be visual or motor impairments, and true attentional deficits. We examine evidence for various areas proposed as sources of attentional modulation within the brain, with a focus on the prefrontal cortex. Lastly, we look at studies that aim to link sources of attention to its neuronal signatures elsewhere in the brain.

Population coding in area V4 during rapid shape detections

While previous studies have suggested that neuronal correlations are common in visual cortex over a range of timescales, the effect of correlations on rapid visually based decisions has received little attention. We trained Macaca mulatta to saccade to a peripherally presented shape embedded in dynamic noise as soon as the shape appeared. While the monkeys performed the task, we recorded from neuronal populations (5-29 cells) using a microelectrode array implanted in area V4, a visual area thought to be involved in form perception. While modest correlations were present between cells during visual stimulation, their magnitude did not change significantly subsequent to the appearance of a shape. We quantified the reliability and temporal precision with which neuronal populations signaled the appearance of the shape and predicted the animals' choices using mutual information analyses. To study the impact of correlations, we shuffled the activity from each cell across observations while retaining stimulus-dependent modulations in firing rate. We found that removing correlations by shuffling across trials minimally affected the reliability or timing with which pairs, or larger groups of cells, signaled the presence of a shape. To assess the downstream impact of correlations, we also studied how shuffling affected the ability of V4 populations to predict behavioral choices. Surprisingly, shuffling created a modest increase in the accuracy of such predictions, suggesting that the reliability of downstream neurons is slightly compromised by activity correlations. Our findings are consistent with neuronal correlations having a minimal effect on the reliability and timing of rapid perceptual decisions.

Saccades and shifting receptive fields: anticipating consequences or selecting targets?

Saccadic eye movements cause frequent and substantial displacements of the retinal image, but those displacements go unnoticed. It has been widely assumed that this perceived stability emerges from the shifting of visual receptive fields from their current, presaccadic locations to their future, postsaccadic locations in anticipation of the retinal consequences of saccades. Although evidence consistent with this anticipatory remapping has accumulated over the years, more recent work suggests an alternative view. In this opinion article, we examine the evidence of presaccadic receptive field shifts and their relationship to the perceptual changes that accompany saccades. We argue that both reflect the selection of targets for saccades rather than the anticipation of a displaced retinal image.

Eye movement preparation modulates neuronal responses in area V4 when dissociated from attentional demands

We examined whether the preparation of saccadic eye movements, when behaviorally dissociated from covert attention, modulates activity within visual cortex. We measured single-neuron and local field potential (LFP) responses to visual stimuli in area V4 while monkeys covertly attended a stimulus at one location and prepared saccades to a potential target at another. In spite of the irrelevance of visual information at the saccade target, visual activity at that location was modulated at least as much as, and often more than, activity at the covertly attended location. Modulations of activity at the attended and saccade target locations were qualitatively similar and included increased response magnitude, stimulus selectivity, and spiking reliability, as well as increased gamma and decreased low-frequency power of LFPs. These results demonstrate that saccade preparation is sufficient to modulate visual cortical representations and suggest that the interrelationship of oculomotor and attention-related mechanisms extends to posterior visual cortex.

Global selection of saccadic target features by neurons in area v4

Psychophysical and neurophysiological studies indicate that during the preparation of saccades, visual processing at the target location is facilitated automatically by the deployment of attention. It has been assumed that the neural mechanisms involved in presaccadic shifts of attention are purely spatial in nature. Saccade preparation modulates the visual responses of neurons within extrastriate area V4, where the responses to targets are enhanced and responses to nontargets are suppressed. We tested whether this effect also engages a nonspatial form of modulation. We measured the responses of area V4 neurons to oriented gratings in two monkeys (Macaca mulatta) making delayed saccades to targets distant from the neuronal receptive field (RF). We varied the orientation of both the RF stimulus and the saccadic target. We found that, in addition to the spatial modulation, saccade preparation involves a feature-dependent modulation of V4 neuronal responses. Specifically, we found that the suppression of area V4 responses to nontarget stimuli during the preparation of saccades depends on the features of the saccadic target. Presaccadic suppression was absent when the features of the saccadic target matched the features preferred by individual V4 neurons. This feature-dependent modulation occurred in the absence of any feature-attention task. We show that our observations are consistent with a computational framework in which feature-based effects automatically emerge from saccade-related feedback signals that are spatial in nature.

A distinct contribution of the frontal eye field to the visual representation of saccadic targets

The responses of neurons within posterior visual cortex are enhanced when response field (RF) stimuli are targeted with saccadic eye movements. Although the motor-related activity within oculomotor structures seems a likely source of the enhancement, the origin of the modulation is unknown. We tested the role of the frontal eye field (FEF) in driving presaccadic modulation in area V4 by inactivating FEF neurons at retinotopically corresponding sites within the macaque monkey (Macaca mulatta) brain. As previously observed, FEF inactivation produced profound, and spatially specific, deficits in memory-guided saccades, and increased the latency, scatter, and duration of visually guided saccades. Despite the clear behavioral deficits, we found that rather than being eliminated or reduced by FEF inactivation, presaccadic enhancement of V4 activity was increased. FEF inactivation nonetheless diminished the stimulus discriminability of V4 visual responses both during fixation and in the presaccadic period. Thus, without input from the FEF, V4 neurons signaled more about the direction of saccades and less about the features of the saccadic target. In addition, FEF inactivation significantly increased the suppressive effects of non-RF stimuli on V4 activity. These results reveal multiple sources of presaccadic modulation in V4 and suggest that the FEF contributes uniquely to the presaccadic specification of visual target features.

Eye movements and attention: the role of pre-saccadic shifts of attention in perception, memory and the control of saccades

Saccadic eye movements and perceptual attention work in a coordinated fashion to allow selection of the objects, features or regions with the greatest momentary need for limited visual processing resources. This study investigates perceptual characteristics of pre-saccadic shifts of attention during a sequence of saccades using the visual manipulations employed to study mechanisms of attention during maintained fixation. The first part of this paper reviews studies of the connections between saccades and attention, and their significance for both saccadic control and perception. The second part presents three experiments that examine the effects of pre-saccadic shifts of attention on vision during sequences of saccades. Perceptual enhancements at the saccadic goal location relative to non-goal locations were found across a range of stimulus contrasts, with either perceptual discrimination or detection tasks, with either single or multiple perceptual targets, and regardless of the presence of external noise. The results show that the preparation of saccades can evoke a variety of attentional effects, including attentionally-mediated changes in the strength of perceptual representations, selection of targets for encoding in visual memory, exclusion of external noise, or changes in the levels of internal visual noise. The visual changes evoked by saccadic planning make it possible for the visual system to effectively use saccadic eye movements to explore the visual environment.

Lumping and splitting the neural circuitry of visual attention

Shifts of gaze and of covert attention rely on tightly linked yet divergent neural mechanisms. In this issue of Neuron, Gregoriou et al. (2012) provide interesting evidence that different functional classes of neurons within the frontal eye field contribute uniquely to these two functions.

Electrophysiological recordings in humans reveal reduced location-specific attentional-shift activity prior to recentering saccades

Being able to effectively explore the visual world is of fundamental importance, and it has been suggested that the straight-ahead gaze position within the egocentric reference frame ("primary position") might play a special role in this context. In the present study we employed human electroencephalography (EEG) to examine neural activity related to the spatial guidance of saccadic eye movements. Moreover, we sought to investigate whether such activity would be modulated by the spatial relation of saccade direction to the primary gaze position (recentering saccades). Participants executed endogenously cued saccades between five equidistant locations along the horizontal meridian. This design allowed for the comparison of isoamplitude saccades from the same starting position that were oriented either toward the primary position (centripetal) or further away from it (centrifugal). By back-averaging time-locked to the saccade onset on each trial, we identified a parietally distributed, negative-polarity EEG deflection contralateral to the direction of the upcoming saccade. Importantly, this contralateral presaccadic negativity, which appeared to reflect the location-specific attentional guidance of the eye movement, was attenuated for recentering saccades relative to isoamplitude centrifugal saccades. This differential electrophysiological signature was paralleled by faster saccadic reaction times and was substantially more apparent when time-locking the data to the onset of the saccade rather than to the onset of the cue, suggesting a tight temporal association with saccade initiation. The diminished level of this presaccadic component for recentering saccades may reflect the preferential coding of the straight-ahead gaze position, in which both the eye-centered and head-centered reference frames are perfectly aligned and from which the visual world can be effectively explored.

Visual perception and saccadic eye movements

We use saccades several times per second to move the fovea between points of interest and build an understanding of our visual environment. Recent behavioral experiments show evidence for the integration of pre- and postsaccadic information (even subliminally), the modulation of visual sensitivity, and the rapid reallocation of attention. The recent physiological literature has identified a characteristic modulation of neural responsiveness-perisaccadic reduction followed by a postsaccadic increase-that is found in many visual areas, but whose source is as yet unknown. This modulation seems optimal for reducing sensitivity during and boosting sensitivity between saccades, but no study has yet established a direct causal link between neural and behavioral changes.

Influence and limitations of popout in the selection of salient visual stimuli by area V4 neurons

The neural mechanism of bottom-up attention and its relationship to top-down attention are poorly understood. Visual stimuli that differ from others in their component features are salient and tend to draw attention in a bottom-up manner. "Popout" stimuli differ uniformly from surrounding items and are more easily detected than stimuli composed of a conjunction of surrounding features. We compared the responses of single area V4 neurons to popout and conjunction stimuli appearing within the classical receptive field (CRF) and found that their responses are modulated by popout. This selectivity was more robust when larger numbers of surrounding items and multiple features were included in the display, and it was absent when only a few items were presented immediately outside the CRF. In addition, the popout modulation of V4 activity was eliminated when top-down attention was directed to locations outside of the CRFs during saccade preparation, indicating that the salience of popout stimuli is not sufficient to drive selection by V4 neurons. These results demonstrate that neurons in feature-selective cortex are influenced by bottom-up attention, but that this influence is limited by top-down attention.

Changes in the response rate and response variability of area V4 neurons during the preparation of saccadic eye movements

The visually driven responses of macaque area V4 neurons are modulated during the preparation of saccadic eye movements, but the relationship between presaccadic modulation in area V4 and saccade preparation is poorly understood. Recent neurophysiological studies suggest that the variability across trials of spiking responses provides a more reliable signature of motor preparation than mean firing rate across trials. We compared the dynamics of the response rate and the variability in the rate across trials for area V4 neurons during the preparation of visually guided saccades. As in previous reports, we found that the mean firing rate of V4 neurons was enhanced when saccades were prepared to stimuli within a neuron's receptive field (RF) in comparison with saccades to a non-RF location. Further, we found robust decreases in response variability prior to saccades and found that these decreases predicted saccadic reaction times for saccades both to RF and non-RF stimuli. Importantly, response variability predicted reaction time whether or not there were any accompanying changes in mean firing rate. In addition to predicting saccade direction, the mean firing rate could also predict reaction time, but only for saccades directed to the RF stimuli. These results demonstrate that response variability of area V4 neurons, like mean response rate, provides a signature of saccade preparation. However, the two signatures reflect complementary aspects of that preparation.

Functional distinction between visuomovement and movement neurons in macaque frontal eye field during saccade countermanding

In the previous studies on the neural control of saccade initiation using the countermanding paradigm, movement and visuomovement neurons in the frontal eye field were grouped as movement-related neurons. The activity of both types of neurons was modulated when a saccade was inhibited in response to a stop signal, and this modulation occurred early enough to contribute to the control of the saccade initiation. We now report a functional difference between these two classes of neurons when saccades are produced. Movement neurons exhibited a progressive accumulation of discharge rate following target presentation that triggered a saccade when it reached a threshold. When saccades were inhibited with lower probability in response to a stop signal appearing at longer delays, this accumulating activity was interrupted at levels progressively closer to the threshold. In contrast, visuomovement neurons exhibited a maintained elevated discharge rate following target presentation that was followed by a further enhancement immediately before the saccade initiation. When saccades were inhibited in response to a stop signal, the late enhancement was absent and the maintained activity decayed regardless of stop-signal delay. These results demonstrate that the activity of movement neurons realizes the progressive commitment to the saccade initiation modeled by the activation of the go unit in computational models of countermanding performance. The lack of correspondence of the activity of visuomovement neurons with any elements of these models indicates that visuomovement neurons perform a function other than the saccade preparation such as a corollary discharge to update visual processing.