Comparison of adaptation characteristics between visually and memory-guided saccades

Saccade adaptation plays a crucial role in maintaining saccade accuracy. The behavioral characteristics and neural mechanisms of saccade adaptation for an externally cued movement, such as visually guided saccades (VGS), are well studied in nonhuman primates. In contrast, little is known about the saccade adaptation of an internally driven movement, such as memory-guided saccades (MGS), which are guided by visuospatial working memory. As the oculomotor plant changes because of growth, aging, or skeletomuscular problems, both types of saccades need to be adapted. Do both saccade types engage a common adaptation mechanism? In this study, we compared the characteristics of amplitude decrease adaptation in MGS with VGS in nonhuman primates. We found that the adaptation speed was faster for MGS than for VGS. Saccade duration changed during MGS adaptation, whereas saccade peak velocity changed during VGS adaptation. We also compared the adaptation field, that is, the gain change for saccade amplitudes other than the adapted. The gain change for MGS declines on both smaller and larger sides of adapted amplitude, more rapidly for larger than smaller amplitudes, whereas the decline in VGS was reversed. Thus, the differences between VGS and MGS adaptation characteristics support the previously suggested hypothesis that the adaptation mechanisms of VGS and MGS are distinct. Furthermore, the result suggests that the MGS adaptation site is a brain structure that influences saccade duration, whereas the VGS adaptation site influences saccade peak velocity. These results should be beneficial for future neurophysiological experiments.NEW & NOTEWORTHY Plasticity helps to overcome persistent motor errors. Such motor plasticity or adaptation can be investigated with saccades. Thus far our knowledge is primarily about visually guided saccades, an externally cued movement, which we can make only when the object is visible at the time of saccade. However, as the world is complex, we can make saccades even when the object is not visible. Here, we investigate the adaptation of an internally driven movement: the memory-guided saccade.

Temporal Information Processing in the Cerebellum and Basal Ganglia

Temporal information processing in the range of a few hundred milliseconds to seconds involves the cerebellum and basal ganglia. In this chapter, we present recent studies on nonhuman primates. In the studies presented in the first half of the chapter, monkeys were trained to make eye movements when a certain amount of time had elapsed since the onset of the visual cue (time production task). The animals had to report time lapses ranging from several hundred milliseconds to a few seconds based on the color of the fixation point. In this task, the saccade latency varied with the time length to be measured and showed stochastic variability from one trial to the other. Trial-to-trial variability under the same conditions correlated well with pupil diameter and the preparatory activity in the deep cerebellar nuclei and the motor thalamus. Inactivation of these brain regions delayed saccades when asked to report subsecond intervals. These results suggest that the internal state, which changes with each trial, may cause fluctuations in cerebellar neuronal activity, thereby producing variations in self-timing. When measuring different time intervals, the preparatory activity in the cerebellum always begins approximately 500 ms before movements, regardless of the length of the time interval being measured. However, the preparatory activity in the striatum persists throughout the mandatory delay period, which can be up to 2 s, with different rate of increasing activity. Furthermore, in the striatum, the visual response and low-frequency oscillatory activity immediately before time measurement were altered by the length of the intended time interval. These results indicate that the state of the network, including the striatum, changes with the intended timing, which lead to different time courses of preparatory activity. Thus, the basal ganglia appear to be responsible for measuring time in the range of several hundred milliseconds to seconds, whereas the cerebellum is responsible for regulating self-timing variability in the subsecond range. The second half of this chapter presents studies related to periodic timing. During eye movements synchronized with alternating targets at regular intervals, different neurons in the cerebellar nuclei exhibit activity related to movement timing, predicted stimulus timing, and the temporal error of synchronization. Among these, the activity associated with target appearance is particularly enhanced during synchronized movements and may represent an internal model of the temporal structure of stimulus sequence. We also considered neural mechanism underlying the perception of periodic timing in the absence of movement. During perception of rhythm, we predict the timing of the next stimulus and focus our attention on that moment. In the missing oddball paradigm, the subjects had to detect the omission of a regularly repeated stimulus. When employed in humans, the results show that the fastest temporal limit for predicting each stimulus timing is about 0.25 s (4 Hz). In monkeys performing this task, neurons in the cerebellar nuclei, striatum, and motor thalamus exhibit periodic activity, with different time courses depending on the brain region. Since electrical stimulation or inactivation of recording sites changes the reaction time to stimulus omission, these neuronal activities must be involved in periodic temporal processing. Future research is needed to elucidate the mechanism of rhythm perception, which appears to be processed by both cortico-cerebellar and cortico-basal ganglia pathways.

Corresponding anatomical of the macaque superior parietal lobule areas 5 (PE) subdivision reveal similar connectivity patterns with humans

Using the animal brain as a cross-species tool for human brain research based on imaging features can provide more potential to reveal comprehensive human brain analysis. Previous studies have shown that human Brodmann area 5 (BA5) and macaque PE are homologous regions. They are both involved in processes depth and direction information during the touch process in the arm movement. However, recent studies show that both BA5 and PE are not homogeneous. According to the cytoarchitecture, BA5 is subdivided into three different subregions, and PE can be subdivided into PEl, PEla, and PEm. The species homologous relationship among the subregions is not clear between BA5 and PE. At the same time, the subdivision of PE based on the anatomical connection of white matter fiber bundles needs more verification. This research subdivided the PE of macaques based on the anatomical connection of white matter fiber bundles. Two PE subregions are defined based on probabilistic fiber tracking, one on the anterior side and the other on the dorsal side. Finally, the research draws connectivity fingerprints with predefined homologous target areas for the BA5 and PE subregions to reveal the characteristics of structure and functions and gives the homologous correspondence identified.

Time encoding migrates from prefrontal cortex to dorsal striatum during learning of a self-timed response duration task

Although time is a fundamental dimension of life, we do not know how brain areas cooperate to keep track and process time intervals. Notably, analyses of neural activity during learning are rare, mainly because timing tasks usually require training over many days. We investigated how the time encoding evolves when animals learn to time a 1.5 s interval. We designed a novel training protocol where rats go from naive- to proficient-level timing performance within a single session, allowing us to investigate neuronal activity from very early learning stages. We used pharmacological experiments and machine-learning algorithms to evaluate the level of time encoding in the medial prefrontal cortex and the dorsal striatum. Our results show a double dissociation between the medial prefrontal cortex and the dorsal striatum during temporal learning, where the former commits to early learning stages while the latter engages as animals become proficient in the task.

A precise and adaptive neural mechanism for predictive temporal processing in the frontal cortex

The theory of predictive processing posits that the brain computes expectations to process information predictively. Empirical evidence in support of this theory, however, is scarce and largely limited to sensory areas. Here, we report a precise and adaptive mechanism in the frontal cortex of non-human primates consistent with predictive processing of temporal events. We found that the speed of neural dynamics is precisely adjusted according to the average time of an expected stimulus. This speed adjustment, in turn, enables neurons to encode stimuli in terms of deviations from expectation. This lawful relationship was evident across multiple experiments and held true during learning: when temporal statistics underwent covert changes, neural responses underwent predictable changes that reflected the new mean. Together, these results highlight a precise mathematical relationship between temporal statistics in the environment and neural activity in the frontal cortex that may serve as a mechanism for predictive temporal processing.

Effects of Optogenetic Suppression of Cortical Input on Primate Thalamic Neuronal Activity during Goal-Directed Behavior

The motor thalamus relays signals from subcortical structures to the motor cortical areas. Previous studies in songbirds and rodents suggest that cortical feedback inputs crucially contribute to the generation of movement-related activity in the motor thalamus. In primates, however, it remains uncertain whether the corticothalamic projections may play a role in shaping neuronal activity in the motor thalamus. Here, using an optogenetic inactivation technique with the viral vector system expressing halorhodopsin, we investigated the role of cortical input in modulating thalamic neuronal activity during goal-directed behavior. In particular, we assessed whether the suppression of signals originating from the supplementary eye field at the corticothalamic terminals could change the task-related neuronal modulation in the oculomotor thalamus in monkeys performing a self-initiated saccade task. We found that many thalamic neurons exhibited changes in their firing rates depending on saccade direction or task event, indicating that optical stimulation exerted task-specific effects on neuronal activity beyond the global changes in baseline activity. These results suggest that the corticothalamic projections might be actively involved in the signal processing necessary for goal-directed behavior. However, we also found that some thalamic neurons exhibited overall, non-task-specific changes in the firing rate during optical stimulation, even in control animals without vector injections. The stimulation effects in these animals started with longer latency, implying a possible thermal effect on neuronal activity. Thus, our results not only reveal the importance of direct cortical input in neuronal activity in the primate motor thalamus, but also provide useful information for future optogenetic studies.

Internal models of sensorimotor integration regulate cortical dynamics

Sensorimotor control during overt movements is characterized in terms of three building blocks: a controller, a simulator, and a state estimator. We asked whether the same framework could explain the control of internal states in the absence of movements. Recently, it was shown that the brain controls the timing of future movements by adjusting an internal speed command. We trained monkeys in a novel task in which the speed command had to be controlled dynamically based on the timing of a sequence of flashes. Recordings from the frontal cortex provided evidence that the brain updates the internal speed command after each flash based on the error between the timing of the flash and the anticipated timing of the flash derived from a simulated motor plan. These findings suggest that cognitive control of internal states may be understood in terms of the same computational principles as motor control.

Saccadic intrusions in amyotrophic lateral sclerosis (ALS)

The attempt to quietly fixate at a small visual object is continuously interrupted by a variety of fixational eye movements comprising, among others, a continuum of saccadic intrusions (SI) which range in size from microsaccades with amplitudes ≤0.25° to larger refixation saccades of up to about 2°. The size and frequency of SI varies considerably among individuals and is known to increase in neurodegenerative diseases such as progressive supranuclear palsy (PSP), and amyotrophic lateral sclerosis (ALS). However, studies of ALS disagree whether also the frequency of SI increases. We undertook an analysis of SI in 119 ALS patients and 47 age-matched healthy controls whose eye movements during fixation and tests of executive functions (e.g antisaccades) had been recorded by video-oculography according to standardised procedures. SI were categorised according to their spatio-temporal patterns as stair case, back-and-forth and square wave jerks (a subcategory of back-and-forth). The SI of patients and controls were qualitatively similar (same direction preferences, similar differences between patterns), but were enlarged in ALS. Notably however, no increase of SI frequency could be demonstrated. Yet, there were clear correlations with parameters such as eye blink rate or errors in a delayed saccade task that suggest an impairment of inhibitory mechanisms, in keeping with the notion of a frontal dysfunction in ALS. However, it remains unclear how the impairment of inhibitory mechanisms in ALS could selectively increase the amplitude of intrusions without changing their frequency of occurrence.

Entrained neuronal activity to periodic visual stimuli in the primate striatum compared with the cerebellum

Rhythmic events recruit neuronal activity in the basal ganglia and cerebellum, but their roles remain elusive. In monkeys attempting to detect a single omission of isochronous visual stimulus, we found that neurons in the caudate nucleus showed increased activity for each stimulus in sequence, while those in the cerebellar dentate nucleus showed decreased activity. Firing modulation in the majority of caudate neurons and all cerebellar neurons was proportional to the stimulus interval, but a quarter of caudate neurons displayed a clear duration tuning. Furthermore, the time course of population activity in the cerebellum well predicted stimulus timing, whereas that in the caudate reflected stochastic variation of response latency. Electrical stimulation to the respective recording sites confirmed a causal role in the detection of stimulus omission. These results suggest that striatal neurons might represent periodic response preparation while cerebellar nuclear neurons may play a role in temporal prediction of periodic events.

Flexible Sensorimotor Computations through Rapid Reconfiguration of Cortical Dynamics

Neural mechanisms that support flexible sensorimotor computations are not well understood. In a dynamical system whose state is determined by interactions among neurons, computations can be rapidly reconfigured by controlling the system's inputs and initial conditions. To investigate whether the brain employs such control mechanisms, we recorded from the dorsomedial frontal cortex of monkeys trained to measure and produce time intervals in two sensorimotor contexts. The geometry of neural trajectories during the production epoch was consistent with a mechanism wherein the measured interval and sensorimotor context exerted control over cortical dynamics by adjusting the system's initial condition and input, respectively. These adjustments, in turn, set the speed at which activity evolved in the production epoch, allowing the animal to flexibly produce different time intervals. These results provide evidence that the language of dynamical systems can be used to parsimoniously link brain activity to sensorimotor computations.

A Dynamical Systems Perspective on Flexible Motor Timing

A hallmark of higher brain function is the ability to rapidly and flexibly adjust behavioral responses based on internal and external cues. Here, we examine the computational principles that allow decisions and actions to unfold flexibly in time. We adopt a dynamical systems perspective and outline how temporal flexibility in such a system can be achieved through manipulations of inputs and initial conditions. We then review evidence from experiments in nonhuman primates that support this interpretation. Finally, we explore the broader utility and limitations of the dynamical systems perspective as a general framework for addressing open questions related to the temporal control of movements, as well as in the domains of learning and sequence generation.

Different contributions of preparatory activity in the basal ganglia and cerebellum for self-timing

The ability to flexibly adjust movement timing is important for everyday life. Although the basal ganglia and cerebellum have been implicated in monitoring of supra- and sub-second intervals, respectively, the underlying neuronal mechanism remains unclear. Here, we show that in monkeys trained to generate a self-initiated saccade at instructed timing following a visual cue, neurons in the caudate nucleus kept track of passage of time throughout the delay period, while those in the cerebellar dentate nucleus were recruited only during the last part of the delay period. Conversely, neuronal correlates of trial-by-trial variation of self-timing emerged earlier in the cerebellum than the striatum. Local inactivation of respective recording sites confirmed the difference in their relative contributions to supra- and sub-second intervals. These results suggest that the basal ganglia may measure elapsed time relative to the intended interval, while the cerebellum might be responsible for the fine adjustment of self-timing.

Integration of speed and time for estimating time to contact

Existing theories suggest that reacting to dynamic stimuli is made possible by relying on internal estimates of kinematic variables. For example, to catch a bouncing ball the brain relies on the position and speed of the ball. However, when kinematic information is unreliable one may additionally rely on temporal cues. In the bouncing ball example, when visibility is low one may benefit from the temporal information provided by the sound of the bounces. Our work provides evidence that humans rely on such temporal cues and automatically integrate them with kinematic information to optimize their performance. This finding reveals a hitherto unappreciated role of the brain’s timing mechanisms in sensorimotor function.

To coordinate movements with events in a dynamic environment the brain has to anticipate when those events occur. A classic example is the estimation of time to contact (TTC), that is, when an object reaches a target. It is thought that TTC is estimated from kinematic variables. For example, a tennis player might use an estimate of distance (d) and speed (v) to estimate TTC (TTC = d/v). However, the tennis player may instead estimate TTC as twice the time it takes for the ball to move from the serve line to the net line. This latter strategy does not rely on kinematics and instead computes TTC solely from temporal cues. Which of these two strategies do humans use to estimate TTC? Considering that both speed and time estimates are inherently uncertain and the ability of the human brain to combine different sources of information, we hypothesized that humans estimate TTC by integrating speed information with temporal cues. We evaluated this hypothesis systematically using psychophysics and Bayesian modeling. Results indicated that humans rely on both speed information and temporal cues and integrate them to optimize their TTC estimates when both cues are present. These findings suggest that the brain’s timing mechanisms are actively engaged when interacting with dynamic stimuli.

Flexible timing by temporal scaling of cortical responses

Musicians can perform at different tempos, speakers can control the cadence of their speech, and children can flexibly vary their temporal expectations of events. To understand the neural basis of such flexibility, we recorded from the medial frontal cortex of nonhuman primates trained to produce different time intervals with different effectors. Neural responses were heterogeneous, nonlinear and complex, and exhibited a remarkable form of temporal invariance: firing rate profiles were temporally scaled to match the produced intervals. Recording from downstream neurons in the caudate and thalamic neurons projecting to the medial frontal cortex indicated that this phenomenon originates within cortical networks. Recurrent neural network models trained to perform the task revealed that temporal scaling emerges from nonlinearities in the network and degree of scaling is controlled by the strength of external input. These findings demonstrate a simple and general mechanism for conferring temporal flexibility upon sensorimotor and cognitive functions.

Predictive and tempo-flexible synchronization to a visual metronome in monkeys

Predictive and tempo-flexible synchronization to an auditory beat is a fundamental component of human music. To date, only certain vocal learning species show this behaviour spontaneously. Prior research training macaques (vocal non-learners) to tap to an auditory or visual metronome found their movements to be largely reactive, not predictive. Does this reflect the lack of capacity for predictive synchronization in monkeys, or lack of motivation to exhibit this behaviour? To discriminate these possibilities, we trained monkeys to make synchronized eye movements to a visual metronome. We found that monkeys could generate predictive saccades synchronized to periodic visual stimuli when an immediate reward was given for every predictive movement. This behaviour generalized to novel tempi, and the monkeys could maintain the tempo internally. Furthermore, monkeys could flexibly switch from predictive to reactive saccades when a reward was given for each reactive response. In contrast, when humans were asked to make a sequence of reactive saccades to a visual metronome, they often unintentionally generated predictive movements. These results suggest that even vocal non-learners may have the capacity for predictive and tempo-flexible synchronization to a beat, but that only certain vocal learning species are intrinsically motivated to do it.

Cerebellar Roles in Self-Timing for Sub- and Supra-Second Intervals

Previous studies suggest that the cerebellum and basal ganglia are involved in sub-second and supra-second timing, respectively. To test this hypothesis at the cellular level, we examined the activity of single neurons in the cerebellar dentate nucleus in monkeys performing the oculomotor version of the self-timing task. Animals were trained to report the passage of time of 400, 600, 1200, or 2400 ms following a visual cue by making self-initiated memory-guided saccades. We found a sizeable preparatory neuronal activity before self-timed saccades across delay intervals, while the time course of activity correlated with the trial-by-trial variation of saccade latency in different ways depending on the length of the delay intervals. For the shorter delay intervals, the ramping up of neuronal firing rate started just after the visual cue and the rate of rise of neuronal activity correlated with saccade timing. In contrast, for the longest delay (2400 ms), the preparatory activity started late during the delay period, and its onset time correlated with self-timed saccade latency. Because electrical microstimulation applied to the recording sites during saccade preparation advanced self-timed but not reactive saccades, regardless of their directions, the signals in the cerebellum may have a causal role in self-timing. We suggest that the cerebellum may regulate timing in both sub-second and supra-second ranges, although its relative contribution might be greater for sub-second than for supra-second time intervals.SIGNIFICANCE STATEMENT How we decide the timing of self-initiated movement is a fundamental question. According to the prevailing hypothesis, the cerebellum plays a role in monitoring sub-second timing, whereas the basal ganglia are important for supra-second timing. To verify this, we explored neuronal signals in the monkey cerebellum while animals reported the passage of time in the range 400-2400 ms by making eye movements. Contrary to our expectations, we found that neurons in the cerebellar dentate nucleus exhibited a similar preparatory activity for both sub-second and supra-second intervals, and that electrical simulation advanced self-timed saccades in both conditions. We suggest that the cerebellum plays a causal role in the fine adjustment of self-timing in a larger time range than previously thought.

Correlation between Pupil Size and Subjective Passage of Time in Non-Human Primates

Our daily experience of time is strongly influenced by internal states, such as arousal, attention, and mood. However, the underlying neuronal mechanism remains largely unknown. To investigate this, we recorded pupil diameter, which is closely linked to internal factors and neuromodulatory signaling, in monkeys performing the oculomotor version of the time production paradigm. In the self-timed saccade task, animals were required to make a memory-guided saccade during a predetermined time interval following a visual cue. We found that pupil diameter was negatively correlated with trial-by-trial latency of self-timed saccades. Because no significant correlation was found for visually guided saccades, correlation of self-timed saccades could not be explained solely by the facilitation of saccade execution. As the reward amount was manipulated, pupil diameter and saccade latency altered in opposite directions and the magnitudes of modulation correlated strongly across sessions, further supporting the close link between pupil diameter and the subjective passage of time. When the animals were trained to produce two different intervals depending on the instruction, the pupil size again correlated with the trial-by-trial variation of saccade latency in each condition; however, pupil diameter differed significantly for saccades with similar latencies generated under different conditions. Our results indicate that internal brain states indexed by pupil diameter, which parallel noradrenergic neuronal activity (Aston-Jones and Cohen, 2005), may bias trial-by-trial variation in the subjective passage of time.

SIGNIFICANCE STATEMENT

Daily experience of time is strongly influenced by our internal state, but the underlying neuronal mechanism remains elusive. Here we demonstrate that pupil diameter is negatively correlated with subjective elapsed time in monkeys performing an oculomotor version of the time production task. When the animals reported two different intervals depending on the instruction, pupil size was correlated with reported timing in each condition but differed for similar timing under different conditions. Given the close correlation between pupil diameter and noradrenergic signaling reported previously, our data indicate that brain states probed by pupil diameter and noradrenergic neuronal activity might modulate subjective passage of time.

Reversed Procrastination by Focal Disruption of Medial Frontal Cortex

An enduring puzzle in the neuroscience of voluntary action is the origin of the remarkably wide dispersion of the reaction time distribution, an interval far greater than is explained by synaptic or signal transductive noise [1, 2]. That we are able to change our planned actions—a key criterion of volition [3]—so close to the time of their onset implies decision-making must reach deep into the execution of action itself [4, 5, 6]. It has been influentially suggested the reaction time distribution therefore reflects deliberate neural procrastination [7], giving alternative response tendencies sufficient time for fair competition in pursuing a decision threshold that determines which one is behaviorally manifest: a race model, where action selection and execution are closely interrelated [8, 9, 10, 11]. Although the medial frontal cortex exhibits a sensitivity to reaction time on functional imaging that is consistent with such a mechanism [12, 13, 14], direct evidence from disruptive studies has hitherto been lacking. If movement-generating and movement-delaying neural substrates are closely co-localized here, a large-scale lesion will inevitably mask any acceleration, for the movement itself could be disrupted. Circumventing this problem, here we observed focal intracranial electrical disruption of the medial frontal wall in the context of the pre-surgical evaluation of two patients with epilepsy temporarily reversing such hypothesized procrastination. Effector-specific behavioral acceleration, time-locked to the period of electrical disruption, occurred exclusively at a specific locus at the ventral border of the pre-supplementary motor area. A cardinal prediction of race models of voluntary action is thereby substantiated in the human brain.

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Voluntary reaction times are slower and more variable than neural noise explains

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Such procrastination is theorized to reflect a neural race selecting each action

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Electrically disrupting medial frontal cortex reverses procrastination

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A cardinal prediction of race models of action in the brain is thereby confirmed

Application of radiosurgical techniques to produce a primate model of brain lesions

Behavioral analysis of subjects with discrete brain lesions provides important information about the mechanisms of various brain functions. However, it is generally difficult to experimentally produce discrete lesions in deep brain structures. Here we show that a radiosurgical technique, which is used as an alternative treatment for brain tumors and vascular malformations, is applicable to create non-invasive lesions in experimental animals for the research in systems neuroscience. We delivered highly focused radiation (130-150 Gy at ISO center) to the frontal eye field (FEF) of macaque monkeys using a clinical linear accelerator (LINAC). The effects of irradiation were assessed by analyzing oculomotor performance along with magnetic resonance (MR) images before and up to 8 months following irradiation. In parallel with tissue edema indicated by MR images, deficits in saccadic and smooth pursuit eye movements were observed during several days following irradiation. Although initial signs of oculomotor deficits disappeared within a month, damage to the tissue and impaired eye movements gradually developed during the course of the subsequent 6 months. Postmortem histological examinations showed necrosis and hemorrhages within a large area of the white matter and, to a lesser extent, in the adjacent gray matter, which was centered at the irradiated target. These results indicated that the LINAC system was useful for making brain lesions in experimental animals, while the suitable radiation parameters to generate more focused lesions need to be further explored. We propose the use of a radiosurgical technique for establishing animal models of brain lesions, and discuss the possible uses of this technique for functional neurosurgical treatments in humans.

Short-duration stimulation of the supplementary eye fields perturbs anti-saccade performance while potentiating contralateral head orienting

Many forms of brain stimulation utilize the notion of state dependency, whereby greater influences are observed when a given area is more engaged at the time of stimulation. Here, by delivering intracortical microstimulation (ICMS) to the supplementary eye fields (SEF) of monkeys performing interleaved pro- and anti-saccades, we show a surprising diversity of state-dependent effects of ICMS-SEF. Short-duration ICMS-SEF passed around cue presentation selectively disrupted anti-saccades by increasing reaction times and error rates bilaterally, and also recruited neck muscles, favoring contralateral head turning to a greater degree on anti-saccade trials. These results are consistent with the functional relevance of the SEF for anti-saccades. The multiplicity of stimulation-evoked effects, with ICMS-SEF simultaneously disrupting anti-saccade performance and facilitating contralateral head orienting, probably reflects both the diversity of cortical and subcortical targets of SEF projections, and the response of this oculomotor network to stimulation. We speculate that the bilateral disruption of anti-saccades arises via feedback loops that may include the thalamus, whereas neck muscle recruitment arises via feedforward polysynaptic pathways to the motor periphery. Consideration of both sets of results reveals a more complete picture of the highly complex and multiphasic response to ICMS-SEF that can play out differently in different effector systems.

Intracortical microstimulation of supplementary eye field impairs ability of monkeys to make serially ordered saccades

Neurons in the supplementary eye field (SEF) of the macaque monkey exhibit rank selectivity, firing differentially as a function of the phase attained during the performance of a task requiring the execution of saccades to a series of objects in fixed order. The activity of these neurons is commonly thought to represent ordinal position in the service of serial-order performance. However, there is little evidence causally linking neuronal activity in the SEF to sequential behavior. To explore the role of the SEF in serial-order performance, we delivered intracortical microstimulation while monkeys performed a task requiring them to make saccades to three objects in a fixed order on each trial. Microstimulation, considered on average across all SEF sites and all phases of the trial, affected saccadic kinematics. In particular, it prolonged the reaction time, increased the peak velocity, and slightly increased the amplitude of saccades. In addition, it interfered with the monkeys' ability to select the target appropriate to a given phase of the trial. The pattern of the errors was such as would be expected if microstimulation shifted the neural representation of ordinal position toward a later phase of the trial.

Temporally specific sensory signals for the detection of stimulus omission in the primate deep cerebellar nuclei

The cerebellum is implicated in sensory prediction in the subsecond range. To explore how neurons in the cerebellum encode temporal information for the prediction of sensory events, we trained monkeys to make a saccade in response to either a single omission or deviation of isochronous repetitive stimuli. We found that neurons in the cerebellar dentate nucleus exhibited a gradual elevation of the baseline firing rate as the repetition progressed. Most neurons showed a transient suppression for each stimulus, and this firing modulation also increased gradually, opposed to the sensory adaptation. The magnitude of the enhanced sensory response positively correlated with interstimulus interval. Furthermore, when stimuli appeared unexpectedly earlier than the regular timing, the neuronal modulation became smaller, suggesting that the sensory response depended on the time elapsed since the previous stimulus. The enhancement of neuronal modulation was context dependent and was reduced or even absent when monkeys were unmotivated to detect stimulus omission. A significant negative correlation between neuronal activity at stimulus omission and saccade latency suggested that the timing of each stimulus was predicted by the amount of recovery from the transient response. Because inactivation of the recording sites delayed the detection of stimulus omission but only slightly altered the detection of stimulus deviation, these signals might be necessary for the prediction of stimulus timing but may not be involved only in the generation of saccades. Our results demonstrate a novel mechanism for temporal prediction of upcoming stimuli that accompanies the time-dependent modification of sensory gain in the cerebellum.