Organization of Corollary Discharge Neurons in Monkey Medial Dorsal Thalamus.

A corollary discharge (CD) is a copy of a neuronal command for movement sent to other brain regions to inform them of the impending movement. In monkeys, a circuit from superior colliculus (SC) through medial-dorsal nucleus of the thalamus (MD) to frontal eye field (FEF) carries such a CD for saccadic eye movements. This circuit provides the clearest example of such internal monitoring reaching cerebral cortex. In this report we first investigated the functional organization of the critical MD relay by systematically recording neurons within a grid of penetrations. In two male rhesus macaque monkeys (Macaca mulatta), we found that lateral MD neurons carrying CD signals discharged before saccades to ipsilateral as well as contralateral visual fields instead of just contralateral fields, often had activity over large movement fields, and had activity from both central and peripheral visual fields. Each of these characteristics has been found in FEF, but these findings indicate that these characteristics are already present in the thalamus. These characteristics show that the MD thalamic relay is not passive but instead assembles inputs from the SC before transmission to cortex. We next determined the exact location of the saccade-related CD neurons using the grid of penetrations. The neurons occupy an anterior-posterior band at the lateral edge of MD, and we established this band in stereotaxic coordinates to facilitate future study of CD neurons. These observations reveal both the organizational features of the internal CD signals within the thalamus, and the location of the thalamic relay for those signals.SIGNIFICANCE STATEMENT A corollary discharge (CD) circuit within the brain keeps an internal record of physical movements. In monkeys and humans, one such CD keeps track of rapid eye movements, and in monkeys, a circuit carrying this CD extends from midbrain to cerebral cortex through a relay in the thalamus. This circuit provides guidance for eye movements, contributes to stable visual perception, and when defective, might be related to difficulties that schizophrenic patients have in recognizing their own movements. This report facilitates the comparison of the circuit in monkeys and humans, particularly for comparison of the location of the thalamic relay in monkeys and in humans.

Visual Responses in FEF, Unlike V1, Primarily Reflect When the Visual Context Renders a Receptive Field Salient.

When light falls within a neuronal visual receptive field (RF) the resulting activity is referred to as the visual response. Recent work suggests this activity is in response to both the visual stimulation and the abrupt appearance, or salience, of the presentation. Here we present a novel method for distinguishing the two, based on the timing of random and nonrandom presentations. We examined these contributions in frontal eye field (FEF; N = 51) and as a comparison, an early stage in the primary visual cortex (V1; N = 15) of male monkeys (Macaca mulatta). An array of identical stimuli was presented within and outside the neuronal RF while we manipulated salience by varying the time between stimulus presentations. We hypothesized that the rapid presentation would reduce salience (the sudden appearance within the visual field) of a stimulus at any one location, and thus decrease responses driven by salience in the RF. We found that when the interstimulus interval decreased from 500 to 16 ms there was an approximate 79% reduction in the FEF response compared with an estimated 17% decrease in V1. This reduction in FEF response for rapid presentation was evident even when the random sequence preceding a stimulus did not stimulate the RF for 500 ms. The time course of these response changes in FEF suggest that salience is represented much earlier (<100 ms following stimulus onset) than previously estimated. Our results suggest that the contribution of salience dominates at higher levels of the visual system.SIGNIFICANCE STATEMENT The neuronal responses in early visual processing [e.g., primary visual cortex (V1)] reflect primarily the retinal stimulus. Processing in higher visual areas is modulated by a combination of the visual stimulation and contextual factors, such as salience, but identifying these components separately has been difficult. Here we quantified these contributions at a late stage of visual processing [frontal eye field (FEF)] and as a comparison, an early stage in V1. Our results suggest that as visual information continues through higher levels of processing the neural responses are no longer driven primarily by the visual stimulus in the receptive field, but by the broader context that stimulus defines-very different from current views about visual signals in FEF.

Saccadic Corollary Discharge Underlies Stable Visual Perception.

Saccadic eye movements direct the high-resolution foveae of our retinas toward objects of interest. With each saccade, the image jumps on the retina, causing a discontinuity in visual input. Our visual perception, however, remains stable. Philosophers and scientists over centuries have proposed that visual stability depends upon an internal neuronal signal that is a copy of the neuronal signal driving the eye movement, now referred to as a corollary discharge (CD) or efference copy. In the old world monkey, such a CD circuit for saccades has been identified extending from superior colliculus through MD thalamus to frontal cortex, but there is little evidence that this circuit actually contributes to visual perception. We tested the influence of this CD circuit on visual perception by first training macaque monkeys to report their perceived eye direction, and then reversibly inactivating the CD as it passes through the thalamus. We found that the monkey's perception changed; during CD inactivation, there was a difference between where the monkey perceived its eyes to be directed and where they were actually directed. Perception and saccade were decoupled. We established that the perceived eye direction at the end of the saccade was not derived from proprioceptive input from eye muscles, and was not altered by contextual visual information. We conclude that the CD provides internal information contributing to the brain's creation of perceived visual stability. More specifically, the CD might provide the internal saccade vector used to unite separate retinal images into a stable visual scene. SIGNIFICANCE STATEMENT: Visual stability is one of the most remarkable aspects of human vision. The eyes move rapidly several times per second, displacing the retinal image each time. The brain compensates for this disruption, keeping our visual perception stable. A major hypothesis explaining this stability invokes a signal within the brain, a corollary discharge, that informs visual regions of the brain when and where the eyes are about to move. Such a corollary discharge circuit for eye movements has been identified in macaque monkey. We now show that selectively inactivating this brain circuit alters the monkey's visual perception. We conclude that this corollary discharge provides a critical signal that can be used to unite jumping retinal images into a consistent visual scene.

A circuit for saccadic suppression in the primate brain.

Saccades should cause us to see a blur as the eyes sweep across a visual scene. Specific brain mechanisms prevent this by producing suppression during saccades. Neuronal correlates of such suppression were first established in the visual superficial layers of the superior colliculus (SC) and subsequently have been observed in cortical visual areas, including the middle temporal visual area (MT). In this study, we investigated suppression in a recently identified circuit linking visual SC (SCs) to MT through the inferior pulvinar (PI). We examined responses to visual stimuli presented just before saccades to reveal a neuronal correlate of suppression driven by a copy of the saccade command, referred to as a corollary discharge. We found that visual responses were similarly suppressed in SCs, PI, and MT. Within each region, suppression of visual responses occurred with saccades into both visual hemifields, but only in the contralateral hemifield did this suppression consistently begin before the saccade (~100 ms). The consistency of the signal along the circuit led us to hypothesize that the suppression in MT was influenced by input from the SC. We tested this hypothesis in one monkey by inactivating neurons within the SC and found evidence that suppression in MT depends on corollary discharge signals from motor SC (SCi). Combining these results with recent findings in rodents, we propose a complete circuit originating with corollary discharge signals in SCi that produces suppression in visual SCs, PI, and ultimately, MT cortex.NEW & NOTEWORTHY A fundamental puzzle in visual neuroscience is that we frequently make rapid eye movements (saccades) but seldom perceive the visual blur accompanying each movement. We investigated neuronal correlates of this saccadic suppression by recording from and perturbing a recently identified circuit from brainstem to cortex. We found suppression at each stage, with evidence that it was driven by an internally generated signal. We conclude that this circuit contributes to neuronal suppression of visual signals during eye movements.

Compression and suppression of shifting receptive field activity in frontal eye field neurons.

Before each saccade, neurons in frontal eye field anticipate the impending eye movement by showing sensitivity to stimuli appearing where the neuron's receptive field will be at the end of the saccade, referred to as the future field (FF) of the neuron. We explored the time course of this anticipatory activity in monkeys by briefly flashing stimuli in the FF at different times before saccades. Different neurons showed substantial variation in FF time course, but two salient observations emerged. First, when we compared the time span of stimulus probes before the saccade to the time span of FF activity, we found a striking temporal compression of FF activity, similar to compression seen for perisaccadic stimuli in human psychophysics. Second, neurons with distinct FF activity also showed suppression at the time of the saccade. The increase in FF activity and the decrease with suppression were temporally independent, making the patterns of activity difficult to separate. We resolved this by constructing a simple model with values for the start, peak, and duration of FF activity and suppression for each neuron. The model revealed the different time courses of FF sensitivity and suppression, suggesting that information about the impending saccade triggering suppression reaches the frontal eye field through a different pathway, or a different mechanism, than that triggering FF activity. Recognition of the variations in the time course of anticipatory FF activity provides critical information on its function and its relation to human visual perception at the time of the saccade.

Guarding the gateway to cortex with attention in visual thalamus.

The massive visual input from the eye to the brain requires selective processing of some visual information at the expense of other information, a process referred to as visual attention. Increases in the responses of visual neurons with attention have been extensively studied along the visual processing streams in monkey cerebral cortex, from primary visual areas to parietal and frontal cortex. Here we show, by recording neurons in attending macaque monkeys (Macaca mulatta), that attention modulates visual signals before they even reach cortex by increasing responses of both magnocellular and parvocellular neurons in the first relay between retina and cortex, the lateral geniculate nucleus (LGN). At the same time, attention decreases neuronal responses in the adjacent thalamic reticular nucleus (TRN). Crick argued for such modulation of the LGN by observing that it is inhibited by the TRN, and suggested that "if the thalamus is the gateway to the cortex, the reticular complex might be described as the guardian of the gateway", a reciprocal relationship we now show to be more than just hypothesis. The reciprocal modulation in LGN and TRN appears only during the initial visual response, but the modulation of LGN reappears later in the response, suggesting separate early and late sources of attentional modulation in LGN.

Suppressive surrounds of receptive fields in monkey frontal eye field.

A critical step in determining how a neuron contributes to visual processing is determining its visual receptive field (RF). While recording from neurons in frontal eye field (FEF) of awake monkeys (Macaca mulatta), we probed the visual field with small spots of light and found excitatory RFs that decreased in strength from RF center to periphery. However, presenting stimuli with different diameters centered on the RF revealed suppressive surrounds that overlapped the previously determined excitatory RF and reduced responses by 84%, on average. Consequently, in that overlap area, stimulation produced excitation or suppression, depending on the stimulus. Strong stimulation of the RF periphery with annular stimuli allowed us to quantify this effect. A modified difference of gaussians model that independently varied center and surround activation accounted for the nonlinear activity in the overlap area. Our results suggest that (1) the suppressive surrounds found in FEF are fundamentally the same as those in V1 except for the size and strength of excitatory and suppressive mechanisms, (2) methodically assaying suppressive surrounds in FEF is essential for correctly interpreting responses to large and/or peripheral stimuli and therefore understanding the effects of stimulus context, and (3) regulating the relative strength of the surround clearly changes neuronal responses and may therefore play a significant part in the neuronal changes resulting from visual attention and stimulus salience.

Corollary discharge contributes to perceived eye location in monkeys.

Despite saccades changing the image on the retina several times per second, we still perceive a stable visual world. A possible mechanism underlying this stability is that an internal retinotopic map is updated with each saccade, with the location of objects being compared before and after the saccade. Psychophysical experiments have shown that humans derive such location information from a corollary discharge (CD) accompanying saccades. Such a CD has been identified in the monkey brain in a circuit extending from superior colliculus to frontal cortex. There is a missing piece, however. Perceptual localization is established only in humans and the CD circuit only in monkeys. We therefore extended measurement of perceptual localization to the monkey by adapting the target displacement detection task developed in humans. During saccades to targets, the target disappeared and then reappeared, sometimes at a different location. The monkeys reported the displacement direction. Detections of displacement were similar in monkeys and humans, but enhanced detection of displacement from blanking the target at the end of the saccade was observed only in humans, not in monkeys. Saccade amplitude varied across trials, but the monkey's estimates of target location did not follow that variation, indicating that eye location depended on an internal CD rather than external visual information. We conclude that monkeys use a CD to determine their new eye location after each saccade, just as humans do.

Signals conveyed in the pulvinar pathway from superior colliculus to cortical area MT.

We previously established a functional pathway extending from the superficial layers of the superior colliculus (SC) through the inferior pulvinar (PI) to cortical area MT in the primate (Macaca mulatta). Here, we characterized the signals that this pathway conveys to cortex by recording from pulvinar neurons that we identified by microstimulation as receiving input from SC and/or projecting to MT. The basic properties of these ascending-path PI neurons resembled those of SC visual neurons. Namely, they had brisk responses to spots of light, inhibitory surrounds, and relatively large receptive fields that increased with eccentricity, as well as minimal presaccadic activity. Beyond these basic properties, there were two salient results regarding the modulatory and motion signals conveyed by this ascending pathway. First, the PI neurons appeared to convey only a subset of the modulations found in the SC: they exhibited saccadic suppression, the inhibition of activity at the time of the saccade, but did not clearly show the attentional enhancement of the visual response seen in SC. Second, directional selectivity was minimal in PI neurons belonging to the ascending path but was significantly more prominent in PI neurons receiving input from MT. This finding casts doubt on earlier assumptions that PI provides directionally selective signals to MT and instead suggests that PI derives its selectivity from MT. The identification of this pathway and its transmitted activity establishes the first functional pathway from brainstem to cortex through pulvinar and makes it possible to examine its contribution to cortical visual processing, perception, and action.

Influence of the thalamus on spatial visual processing in frontal cortex.

Each of our movements activates our own sensory receptors, and therefore keeping track of self-movement is a necessary part of analysing sensory input. One way in which the brain keeps track of self-movement is by monitoring an internal copy, or corollary discharge, of motor commands. This concept could explain why we perceive a stable visual world despite our frequent quick, or saccadic, eye movements: corollary discharge about each saccade would permit the visual system to ignore saccade-induced visual changes. The critical missing link has been the connection between corollary discharge and visual processing. Here we show that such a link is formed by a corollary discharge from the thalamus that targets the frontal cortex. In the thalamus, neurons in the mediodorsal nucleus relay a corollary discharge of saccades from the midbrain superior colliculus to the cortical frontal eye field. In the frontal eye field, neurons use corollary discharge to shift their visual receptive fields spatially before saccades. We tested the hypothesis that these two components-a pathway for corollary discharge and neurons with shifting receptive fields-form a circuit in which the corollary discharge drives the shift. First we showed that the known spatial and temporal properties of the corollary discharge predict the dynamic changes in spatial visual processing of cortical neurons when saccades are made. Then we moved from this correlation to causation by isolating single cortical neurons and showing that their spatial visual processing is impaired when corollary discharge from the thalamus is interrupted. Thus the visual processing of frontal neurons is spatiotemporally matched with, and functionally dependent on, corollary discharge input from the thalamus. These experiments establish the first link between corollary discharge and visual processing, delineate a brain circuit that is well suited for mediating visual stability, and provide a framework for studying corollary discharge in other sensory systems.

Functional identification of a pulvinar path from superior colliculus to cortical area MT.

The idea of a second visual pathway, in which visual signals travel from brainstem to cortex via the pulvinar thalamus, has had considerable influence as an alternative to the primary geniculo-striate pathway. Existence of this second pathway in primates, however, is not well established. A major question centers on whether the pulvinar acts as a relay, particularly in the path from the superior colliculus (SC) to the motion area in middle temporal cortex (MT). We used physiological microstimulation to identify pulvinar neurons belonging to the path from SC to MT in the macaque. We made three salient observations. First, we identified many neurons in the visual pulvinar that received input from SC or projected to MT, as well as a largely separate set of neurons that received input from MT. Second, and more importantly, we identified a subset of neurons as relay neurons that both received SC input and projected to MT. The identification of these relay neurons demonstrates a continuous functional path from SC to MT through the pulvinar in primates. Third, we histologically localized a subset of SC-MT relay neurons to the subdivision of inferior pulvinar known to project densely to MT but also localized SC-MT relay neurons to an adjacent subdivision. This pattern indicates that the pulvinar pathway is not limited to a single anatomically defined region. These findings bring new perspective to the functional organization of the pulvinar and its role in conveying signals to the cerebral cortex.

Modulation of shifting receptive field activity in frontal eye field by visual salience.

In the monkey frontal eye field (FEF), the sensitivity of some neurons to visual stimulation changes just before a saccade. Sensitivity shifts from the spatial location of its current receptive field (RF) to the location of that field after the saccade is completed (the future field, FF). These shifting RFs are thought to contribute to the stability of visual perception across saccades, and in this study we investigated whether the salience of the FF stimulus alters the magnitude of FF activity. We reduced the salience of the usually single flashed stimulus by adding other visual stimuli. We isolated 171 neurons in the FEF of 2 monkeys and did experiments on 50 that had FF activity. In 30% of these, that activity was higher before salience was reduced by adding stimuli. The mean magnitude reduction was 16%. We then determined whether the shifting RFs were more frequent in the central visual field, which would be expected if vision across saccades were only stabilized for the visual field near the fovea. We found no evidence of any skewing of the frequency of shifting receptive fields (or the effects of salience) toward the central visual field. We conclude that the salience of the FF stimulus makes a substantial contribution to the magnitude of FF activity in FEF. In so far as FF activity contributes to visual stability, the salience of the stimulus is probably more important than the region of the visual field in which it falls for determining which objects remain perceptually stable across saccades.

Amplitudes and directions of individual saccades can be adjusted by corollary discharge.

There is strong evidence that the brain can use an internally generated copy of motor commands, a corollary discharge, to guide rapid sequential saccades. Much of this evidence comes from the double-step paradigm: after two briefly flashed visual targets have disappeared, the subject makes two sequential saccades to the targets. Recent studies on the monkey revealed that amplitude variations of the first saccade led to compensation by the second saccade, mediated by a corollary discharge. Here, we investigated whether such saccade-by-saccade compensation occurs in humans, and we made three new observations. First, we replicated previous findings from the monkey: following first saccade amplitude variations, the direction of the second saccade compensated for the error. Second, the change in direction of the second saccade followed variations in vertical as well as horizontal first saccades although the compensation following horizontal saccades was significantly more accurate. Third, by examining oblique saccades, we are able to show that first saccade variations are compensated by adjustment in saccade amplitude in addition to direction. Together, our results demonstrate that it is likely that a corollary discharge in humans can be used to adjust both saccade direction and amplitude following variations in individual saccades.

Recounting the impact of Hubel and Wiesel.

David Hubel and Torsten Wiesel provided a quantum step in our understanding of the visual system. In this commemoration of the 50th year of their initial publication, I would like to examine two aspects of the impact of their work. First, from the viewpoint of those interested in the relation of brain to behaviour, I recount why their initial experiments produced such an immediate impact. Hubel and Wiesel's work appeared against a background of substantial behavioural knowledge about visual perception, a growing desire to know the underlying brain mechanisms for this perception, and an abysmal lack of physiological information about the neurons in visual cortex that might underlie these mechanisms. Their initial results showed both the transformations that occur from one level of processing to the next and how a sequence of these transformations might lead to at least the elements of pattern perception. Their experiments immediately provided a structure for conceptualizing how cortical neurons could be organized to produce perception. A second impact of Hubel and Wiesel's work has been the multiple paths of research they blazed. I comment here on just one of these paths, the analysis of visual cortex in the monkey, particularly in the awake monkey. This direction has led to an explosion in the number of investigations of cortical areas beyond striate cortex and has addressed more complex behavioural questions, but it has evolved from the approach to neuronal processing pioneered by Hubel and Wiesel.

Target selection and saccade generation in monkey superior colliculus.

The superior colliculus (SC) of the monkey has been shown to be involved in not only initiation of saccades but in the selection of the target to which the saccade can be directed. The present experiments examine whether SC neuronal activity related to target selection is also related to saccade generation. In an asynchronous target task, the monkey was required to make a saccade to the first of two spots of light to appear. Using choice probability analysis over multiple trials, we determined the earliest time at which neurons in the SC intermediate layers indicated target selection. We then determined how closely the neuronal selection was correlated to saccade onset by using our asynchronous reaction time task, which allowed the monkey to make a saccade to the target as soon as the selection had been made. We found that the selection became evident at widely differing times for different neurons. Some neurons indicated target selection just before the saccade (close to the pre-saccadic burst of activity), others did so at the time of the visual response, and some showed an increase in their activity even before the target appeared. A fraction of this pre-stimulus bias resulted from a priming effect of the previous trial; a saccade to the target in the movement field on the previous trial produced both a higher level of neuronal activity and a higher probability for a saccade to that target on the current trial. We found that most of the neurons (76%) showed a correlation between selection time and reaction time. Furthermore, within this 76% of neurons, many indicated a selection very early during the visual response. There was no evidence of a sequence from target selection first and saccade selection later, but rather that target selection and saccade initiation are intertwined and are probably inseparable.

Corollary Discharge Contributions to Perceptual Continuity Across Saccades.

Our vision depends upon shifting our high-resolution fovea to objects of interest in the visual field. Each saccade displaces the image on the retina, which should produce a chaotic scene with jerks occurring several times per second. It does not. This review examines how an internal signal in the primate brain (a corollary discharge) contributes to visual continuity across saccades. The article begins with a review of evidence for a corollary discharge in the monkey and evidence from inactivation experiments that it contributes to perception. The next section examines a specific neuronal mechanism for visual continuity, based on corollary discharge that is referred to as visual remapping. Both the basic characteristics of this anticipatory remapping and the factors that control it are enumerated. The last section considers hypotheses relating remapping to the perceived visual continuity across saccades, including remapping's contribution to perceived visual stability across saccades.

Attentional modulation of thalamic reticular neurons.

The major pathway for visual information reaching cerebral cortex is through the lateral geniculate nucleus (LGN) of the thalamus. Acting on this vital relay is another thalamic nucleus, the thalamic reticular nucleus (TRN). This nucleus receives topographically organized collaterals from both thalamus and cortex and sends similarly organized projections back to thalamus. The inputs to the TRN are excitatory, but the output back to the thalamic relay is inhibitory, providing an ideal organization for modulating visual activity during early processing. This functional architecture led Crick in 1984 to hypothesize that TRN serves to direct a searchlight of attention to different regions of the topographic map; however, despite the substantial influence of this hypothesis, the activity of TRN neurons has never been determined during an attention task. We have determined the nature of the response of visual TRN neurons in awake monkeys, and the modulation of that response as the monkeys shifted attention between visual and auditory stimuli. Visual TRN neurons had a strong (194 spikes/s) and fast (25 ms latency) transient increase of activity to spots of light falling in their receptive fields, as well as high background firing rate (45 spikes/s). When attention shifted to the spots of light, the amplitude of the transient visual response typically increased, whereas other neuronal response characteristics remained unchanged. Thus, as predicted previously, TRN activity is modified by shifts of visual attention, and these attentional changes could influence visual processing in LGN via the inhibitory connections back to the thalamus.

Enhanced performance with brain stimulation: attentional shift or visual cue?

The premotor theory of visual spatial attention proposes that the same brain activity that prepares for saccades to one part of the visual field also facilitates visual processing at that same region of the visual field. Strong support comes from improvements in performance by electrical stimulation of presaccadic areas, including the frontal eye field and superior colliculus (SC). Interpretations of these stimulation experiments are hampered by the possibility that stimulation might be producing an internal visual flash or phosphene that attracts attention as a real flash would. We tested this phosphene hypothesis in the SC by comparing the effect of interchanging real visual stimuli and electrical stimulation. We first presented a veridical visual cue at the time SC stimulation improved performance; if a phosphene improved performance at this time, a real cue should do so in the same manner, but it did not. We then changed the time of SC visual-motor stimulation to when we ordinarily presented the veridical visual cue, and failed to improve performance. Last, we shifted the site of SC stimulation from the visual-motor neurons of the SC intermediate layers to the visual neurons of the superficial layers to determine whether stimulating visual neurons produced a larger improvement in performance, but it did not. Our experiments provide evidence that a phosphene is not responsible for the shift of attention that follows SC stimulation. This added evidence of a direct shift of attention is consistent with a key role of the SC in the premotor theory of attention.

Neuronal activity in substantia nigra pars reticulata during target selection.

Complex visual scenes require that a target for an impending saccadic eye movement be selected from a number of possible targets. We investigated whether changing the number of stimuli from which a target would be identified altered the activity of substantia nigra pars reticulata (SNr) neurons of the basal ganglia (BG) and how such changes might contribute to changes we observed previously in the superior colliculus (SC). One, two, four, or eight visual stimuli appeared on random trials while monkeys fixated a centrally located spot. After a delay, one of the stimuli in the array changed luminance, indicating that it was the saccade target. We found that SNr neurons that had a pause in tonic activity after target onset and when the saccade was made to the target showed a modulation of activity during the multitarget task. Because the number of stimuli in the array increased from one to eight, the initial pause after the onset of the visual stimulus decreased. Activity during the preselection delay was reduced but was independent of the number of possible targets present. When one of the stimuli was identified as the saccade target, but before the saccade was made, we found a sharp decline in activity. This decline was related to the monkey's selecting the target rather than the luminance change identifying the target, because on error trials, when the luminance changed but a saccade was not made to the target, the activity did not decline. The decline for the preferred target location was also accompanied by a lesser decline for adjacent locations. Our findings indicate that SNr activity changes with target selection as it does with saccade initiation and that the SNr could make substantial, direct contributions to the SC at both times. The pause in SNr activity with target selection is consistent with the hypothesis that BG provide a disinhibition for the selection of desired movements.