Adaptive modification of saccade size produces correlated changes in the discharges of fastigial nucleus neurons

How cerebellar motor learning keeps saccades accurate

The neuronal substrate underlying the learning of a sophisticated task has been difficult to study. However, the advent of a behavioral paradigm that deceives the saccadic system into thinking it is making an error has allowed the mechanisms of the adaptation that corrects this error to be revealed in a primate. The neural elements that fashion the command signal for the generation of accurate saccades involve subcortical structures in the brain stem and cerebellum. In this review we show that sites in both those structures also are involved with the gradual adaptation of saccade size, a form of motor learning. Pharmacological manipulation of the oculomotor vermis (lobules VIc and VII) impairs mechanisms that either increase or decrease saccade size during adaptation. The net saccade-related simple spike (SS) activity of its Purkinje cells is correlated with the changes in saccade characteristics that occur during adaptation. These changes in SS activity are driven by an error signal delivered over climbing fibers, which create complex spikes whose probability of occurrence reflects the motor error between the actual and desired saccade size. These climbing fibers originate in the part of the inferior olive that receives projections from the superior colliculus (SC). Disabling the SC prevents adaptation and stimulation of the SC just after a normal saccade produces a surrogate error signal that drives adaptation without an actual visual error. Therefore, the SC provides not only the initial command that generates a saccade, as shown by others, but also the error signal that ensures that saccades remain accurate.

A neuronal process for adaptive control of primate saccadic system

In 1980, Dr. Optican established the existence of an adaptive plasticity of saccades and its dependence on the cerebellum with Dr. Robinson. The advantage of saccades is that the neuronal mechanisms underlying their generation have been well established. This knowledge allows us to identify the neuronal elements that participate in saccade adaptation. Briefly, the superior colliculus (SC) produces a saccade command signal, which reaches motoneurons in the abducens nucleus via the brainstem burst generator. The SC saccade command also is sent to the oculomotor vermis (OMV), a saccade-related area of the cerebellar cortex, and finally converges on the same motoneurons via the caudal fastigial nucleus (cFN) and inhibitory burst neurons (IBN). During adaptation, the saccade-related burst of SC neurons does not change; however, the activity of the cerebellum and its downstream targets do. We demonstrate that the SC is the source of the error signal to the OMV, and the error signal increases the probability of complex spike occurrence and decreases simple spike activity in the OMV. This decrease, in turn, is delivered through the cFN and IBN neurons to decrease motoneuron activity and hence saccade amplitude.

Elimination of the error signal in the superior colliculus impairs saccade motor learning

Theories of cerebellar-dependent motor learning use the error between the desired and actual movement to correct the erroneous movement. To support this idea, several studies have tried to eliminate the error signal to the cerebellum and demonstrate an impairment of learning. However, such former approaches have not been successful because blocking the error signal also affected the movement to be learned. In this study, we selectively block an error signal for saccade adaptation, a type of cerebellar motor learning, by inactivating the source of the error signal in the superior colliculus without affecting the movement to be learned. Saccade adaptation was impaired. Thus, our study provides the first experimental evidence that an error signal is required for cerebellar motor learning.

When movements become dysmetric, the resultant motor error induces a plastic change in the cerebellum to correct the movement, i.e., motor adaptation. Current evidence suggests that the error signal to the cerebellum is delivered by complex spikes originating in the inferior olive (IO). To prove a causal link between the IO error signal and motor adaptation, several studies blocked the IO, which, unfortunately, affected not only the adaptation but also the movement itself. We avoided this confound by inactivating the source of an error signal to the IO. Several studies implicate the superior colliculus (SC) as the source of the error signal to the IO for saccade adaptation. When we inactivated the SC, the metrics of the saccade to be adapted were unchanged, but saccade adaptation was impaired. Thus, an intact rostral SC is necessary for saccade adaptation. Our data provide experimental evidence for the cerebellar learning theory that requires an error signal to drive motor adaptation.

Signals driving the adaptation of saccades that require spatial updating

Saccades adapt to persistent natural or artificially imposed dysmetrias. The characteristics and circuitry of saccade adaptation have been revealed using a visually guided task (VGT) where the vectors of the target step and the intended saccade command are the same. However, in real life, another saccade occasionally intervenes before the saccade to the target occurs. This necessitates an updating of the intended saccade to account for the intervening saccadic displacement, which dissociates the visual target signal and the intended saccade command. We determined whether the adaptation process is similar for VGT and updated saccades by studying the transfer of adaptation between them. The ultimate visual target was dissociated from the intended saccade command with double-step saccade tasks (DSTs) in which two targets are flashed sequentially at different locations while the monkey maintains fixation. The resulting saccades toward the first and second targets occur in the dark. The transfer of visually guided saccade adaptation to the second saccades of a DST and vice versa depended on the eccentricity of the second visual target, and not the second saccade command. If a target with the same eccentricity as the adapted target appears briefly during the intersaccadic interval of a DST, more adaptation transfers. Because a brief appearance of the visual target either before the first saccade or during the intersaccadic interval influences how much adaptation transfer the second saccade will express, the processing of adaptation and DST updating may overlap. NEW & NOTEWORTHY Adaptation and the spatial updating of saccades are thought to be independent processes. When we dissociate the visual target and the intended saccade command, the transfer of visually guided saccade adaptation to the saccades of the double-step saccade tasks (DST) and vice versa is driven by a visual not motor error. The visual target has an effect until the second saccade of a DST occurs. Therefore, the processing of adaptation and the spatial updating of saccades may overlap.

Characteristic Eye Movements in Ataxia-Telangiectasia-Like Disorder: An Explanatory Hypothesis

Objective

To investigate cerebellar dysfunctions and quantitatively characterize specific oculomotor changes in ataxia-telangiectasia-like disorder (ATLD), a rare autosomal recessive disease caused by mutations in the MRE11 gene. Additionally, to further elucidate the pathophysiology of cerebellar damage in the ataxia-telangiectasia (AT) spectrum disorders.

Methods

Saccade dynamics, metrics, and visual fixation deficits were investigated in two Italian adult siblings with genetically confirmed ATLD. Visually guided saccades were compared with those of 40 healthy subjects. Steady fixation was tested in primary and eccentric positions. Quantitative characterization of saccade parameters, saccadic intrusions (SI), and nystagmus was performed.

Results

Patients showed abnormally hypermetric and fast horizontal saccades to the left and greater inaccuracy than healthy subjects in all saccadic eye movements. Eye movement abnormalities included slow eye movements that preceded the initial saccade. Horizontal and vertical spontaneous jerk nystagmus, gaze-evoked, and rebound nystagmus were evident. Fixation was interrupted by large square-wave jerk SI and macrosaccadic oscillations.

Conclusion

Slow eye movements accompanying saccades, SI, and cerebellar nystagmus are frequently seen in AT patients, additionally our ATLD patients showed the presence of fast and hypermetric saccades suggesting damage of granule cell-parallel fiber-Purkinje cell synapses of the cerebellar vermis. A dual pathogenetic mechanism involving neurodevelopmental and neurodegenerative changes is hypothesized to explain the peculiar phenotype of this disease.

Ocular motor disturbances in autism spectrum disorders: Systematic review and comprehensive meta-analysis

There has been considerable focus placed on how individuals with autism spectrum disorder (ASD) visually perceive and attend to social information, such as facial expressions or social gaze. The role of eye movements is inextricable from visual perception, however this aspect is often overlooked. We performed a series of meta-analyses based on data from 28 studies of eye movements in ASD to determine whether there is evidence for ocular motor dysfunction in ASD. Tasks assessed included visually-guided saccade tasks, gap/overlap, anti-saccade, pursuit tasks and ocular fixation. These analyses revealed evidence for ocular motor dysfunction in ASD, specifically relating to saccade dysmetria, difficulty inhibiting saccades and impaired tracking of moving targets. However there was no evidence for deficits relating to initiating eye movements, or engaging and disengaging from simple visual targets. Characterizing ocular motor abnormalities in ASD may provide insight into the functional integrity of brain networks in ASD across development, and assist our understanding of visual and social attention in ASD.

Saccadic adaptation to a systematically varying disturbance

Saccadic adaptation maintains the correct mapping between eye movements and their targets, yet the dynamics of saccadic gain changes in the presence of systematically varying disturbances has not been extensively studied. Here we assessed changes in the gain of saccade amplitudes induced by continuous and periodic postsaccadic visual feedback. Observers made saccades following a sequence of target steps either along the horizontal meridian (Two-way adaptation) or with unconstrained saccade directions (Global adaptation). An intrasaccadic step-following a sinusoidal variation as a function of the trial number (with 3 different frequencies tested in separate blocks)-consistently displaced the target along its vector. The oculomotor system responded to the resulting feedback error by modifying saccade amplitudes in a periodic fashion with similar frequency of variation but lagging the disturbance by a few tens of trials. This periodic response was superimposed on a drift toward stronger hypometria with similar asymptotes and decay rates across stimulus conditions. The magnitude of the periodic response decreased with increasing frequency and was smaller and more delayed for Global than Two-way adaptation. These results suggest that-in addition to the well-characterized return-to-baseline response observed in protocols using constant visual feedback-the oculomotor system attempts to minimize the feedback error by integrating its variation across trials. This process resembles a convolution with an internal response function, whose structure would be determined by coefficients of the learning model. Our protocol reveals this fast learning process in single short experimental sessions, qualifying it for the study of sensorimotor learning in health and disease.

Individual neurons in the caudal fastigial oculomotor region convey information on both macro- and microsaccades

Recent studies have suggested that microsaccades, the small amplitude saccades made during fixation, are precisely controlled. Two lines of evidence suggest that the cerebellum plays a key role not only in improving the accuracy of macrosaccades but also of microsaccades. First, lesions of the fastigial oculomotor regions (FOR) cause horizontal dysmetria of both micro- and macrosaccades. Secondly, our previous work on Purkinje cell simple spikes in the oculomotor vermis (OV) has established qualitatively similar response preferences for these two groups of saccades. In this work, we investigated the control signals for micro- and macrosaccades in the FOR, the target of OV Purkinje cell axons. We found that the same FOR neurons discharged for micro- and macrosaccades. For both groups of saccades, FOR neurons exhibited very similar dependencies of their discharge strength on direction and amplitude and very similar burst onset time differences for ipsi- and contraversive saccades and, in both, response duration reflected saccade duration, at least at the population level. An intriguing characteristic of microsaccade-related responses is that immediate pre-saccadic firing rates decreased with distance to the target center, a pattern that strikingly parallels the eye position dependency of both microsaccade metrics and frequency, which may suggest a potential neural mechanism underlying the role of FOR in fixation. Irrespective of this specific consideration, our study supports the view that microsaccades and macrosaccades share the same cerebellar circuitry and, in general, further strengthens the notion of a microsaccade-macrosaccade continuum.

The Control of Voluntary Eye Movements: New Perspectives

Primates use two types of voluntary eye movements to track objects of interest: pursuit and saccades. Traditionally, these two eye movements have been viewed as distinct systems that are driven automatically by low-level visual inputs. However, two sets of findings argue for a new perspective on the control of voluntary eye movements. First, recent experiments have shown that pursuit and saccades are not controlled by entirely different neural pathways but are controlled by similar networks of cortical and subcortical regions and, in some cases, by the same neurons. Second, pursuit and saccades are not automatic responses to retinal inputs but are regulated by a process of target selection that involves a basic form of decision making. The selection process itself is guided by a variety of complex processes, including attention, perception, memory, and expectation. Together, these findings indicate that pursuit and saccades share a similar functional architecture. These points of similarity may hold the key for understanding how neural circuits negotiate the links between the many higher order functions that can influence behavior and the singular and coordinated motor actions that follow.

The role of the posterior cerebellum in saccadic adaptation: a transcranial direct current stimulation study

The posterior vermis of the cerebellum is considered to be critically involved in saccadic adaptation. However, recent evidence suggests that the adaptive decrease (backward adaptation) and the adaptive increase (forward adaptation) of saccade amplitude rely on partially separate neural substrates. We investigated whether the posterior cerebellum could be differentially involved in backward and forward adaptation by using transcranial direct current stimulation (TDCS). To do so, participants' saccades were adapted backward or forward while they received anodal, cathodal, or sham TDCS. In two extra groups, subjects underwent a nonadaptation session while receiving anodal or cathodal TDCS to control for the direct effects of TDCS on saccadic execution. Surprisingly, cathodal stimulation tended to increase the extent of both forward and backward adaptations, while anodal TDCS strongly impaired forward adaptation and, to a smaller extent, backward adaptation. Forward adaptation was accompanied by a greater increase in velocity with cathodal stimulation, and reduced duration of change for anodal stimulation. In contrast, the expected velocity decrease in backward adaptation was noticeably weaker with anodal stimulation. Stimulation applied during nonadaptation sessions did not affect saccadic gain, velocity, or duration, demonstrating that the reported effects are not due to direct effects of the stimulation on the generation of eye movements. Our results demonstrate that cerebellar excitability is critical for saccadic adaptation. Based on our results and the growing evidence from studies of vestibulo-ocular reflex and saccadic adaptation, we conclude that the plasticity at the level of the oculomotor vermis is more fundamentally important for forward adaptation than for backward adaptation.

Adaptation and adaptation transfer characteristics of five different saccade types in the monkey

Shifts in the direction of gaze are accomplished by different kinds of saccades, which are elicited under different circumstances. Saccade types include targeting saccades to simple jumping targets, delayed saccades to visible targets after a waiting period, memory-guided (MG) saccades to remembered target locations, scanning saccades to stationary target arrays, and express saccades after very short latencies. Studies of human cases and neurophysiological experiments in monkeys suggest that separate pathways, which converge on a common locus that provides the motor command, generate these different types of saccade. When behavioral manipulations in humans cause targeting saccades to have persistent dysmetrias as might occur naturally from growth, aging, and injury, they gradually adapt to reduce the dysmetria. Although results differ slightly between laboratories, this adaptation generalizes or transfers to all the other saccade types mentioned above. Also, when one of the other types of saccade undergoes adaptation, it often transfers to another saccade type. Similar adaptation and transfer experiments, which allow inferences to be drawn about the site(s) of adaptation for different saccade types, have yet to be done in monkeys. Here we show that simian targeting and MG saccades adapt more than express, scanning, and delayed saccades. Adaptation of targeting saccades transfers to all the other saccade types. However, the adaptation of MG saccades transfers only to delayed saccades. These data suggest that adaptation of simian targeting saccades occurs on the pathway common to all saccade types. In contrast, only the delayed saccade command passes through the adaptation site of the MG saccade.

Cerebellar transcranial direct current stimulation effects on saccade adaptation

Saccade adaptation is a cerebellar-mediated type of motor learning in which the oculomotor system is exposed to repetitive errors. Different types of saccade adaptations are thought to involve distinct underlying cerebellar mechanisms. Transcranial direct current stimulation (tDCS) induces changes in neuronal excitability in a polarity-specific manner and offers a modulatory, noninvasive, functional insight into the learning aspects of different brain regions. We aimed to modulate the cerebellar influence on saccade gains during adaptation using tDCS. Subjects performed an inward (n = 10) or outward (n = 10) saccade adaptation experiment (25% intrasaccadic target step) while receiving 1.5 mA of anodal cerebellar tDCS delivered by a small contact electrode. Compared to sham stimulation, tDCS increased learning of saccadic inward adaptation but did not affect learning of outward adaptation. This may imply that plasticity mechanisms in the cerebellum are different between inward and outward adaptation. TDCS could have influenced specific cerebellar areas that contribute to inward but not outward adaptation. We conclude that tDCS can be used as a neuromodulatory technique to alter cerebellar oculomotor output, arguably by engaging wider cerebellar areas and increasing the available resources for learning.

Fine-scale plasticity of microscopic saccades

When asked to maintain their gaze steady on a given location, humans continually perform microscopic eye movements, including fast gaze shifts known as microsaccades. It has long been speculated that these movements may contribute to the maintenance of fixation, but evidence has remained contradictory. We used a miniaturized version of saccadic adaptation, an experimental procedure by which motor control of saccades is modified through intrasaccadic displacements of the target. We found that the statistical distribution of microsaccade amplitudes changes after brief exposure to systematic shifts of the fixation point during microsaccade occurrence. Shifts in the same directions as microsaccades produce movements with larger amplitudes, whereas shifts against microsaccade directions result in smaller movements. Our findings show that microsaccades are precisely monitored during fixation and that their motor program is modified if the postsaccadic target position is not at the expected retinal location. These results demonstrate that saccadic adaptation occurs even when the stimulus is already close to the foveal center and precise execution of the movement may not be critical. They support the proposal that adaptation is necessary to maintain a consistent relationship between motor control and its visual consequences and that the representation of space is intrinsically multimodal, even during fixation.

Cerebellar fastigial nucleus influence on ipsilateral abducens activity during saccades

To characterize the cerebellar influence on neurons in the abducens (ABD) nucleus, we recorded ABD neurons before and after we inactivated the caudal part of the ipsilateral cerebellar fastigial nucleus (cFN) with muscimol injection. cFN activity influences the horizontal component of saccades. cFN inactivation increased the activity of most ipsilateral ABD neurons (19/22 in 2 monkeys) during ipsiversive (hypermetric) saccades, primarily by increasing burst duration. During contraversive (hypometric) saccades, the off-direction pause of most (10/15) ABD neurons was shorter than normal because of the early resumption of ABD activity. Early ABD firing caused the early contraction of antagonist muscles that reduced eye rotation and made contraversive saccades hypometric. Thus the cerebellum controls ipsilateral ABD activity by truncating on-direction bursts during ipsiversive saccades and extending off-direction pauses during contraversive saccades. We conclude that cFN output keeps saccades accurate by controlling when ABD on-direction bursts and off-direction pauses end.

Convergent synaptic inputs from the caudal fastigial nucleus and the superior colliculus onto pontine and pontomedullary reticulospinal neurons

The caudal fastigial nucleus (FN) is known to be related to the control of eye movements and projects mainly to the contralateral reticular nuclei where excitatory and inhibitory burst neurons for saccades exist [the caudal portion of the nucleus reticularis pontis caudalis (NRPc), and the rostral portion of the nucleus reticularis gigantocellularis (NRG) respectively]. However, the exact reticular neurons targeted by caudal fastigioreticular cells remain unknown. We tried to determine the target reticular neurons of the caudal FN and superior colliculus (SC) by recording intracellular potentials from neurons in the NRPc and NRG of anesthetized cats. Neurons in the rostral NRG received bilateral, monosynaptic excitation from the caudal FNs, with contralateral predominance. They also received strong monosynaptic excitation from the rostral and caudal contralateral SC, and disynaptic excitation from the rostral ipsilateral SC. These reticular neurons with caudal fastigial monosynaptic excitation were not activated antidromically from the contralateral abducens nucleus, but most of them were reticulospinal neurons (RSNs) that were activated antidromically from the cervical cord. RSNs in the caudal NRPc received very weak monosynaptic excitation from only the contralateral caudal FN, and received either monosynaptic excitation only from the contralateral caudal SC, or monosynaptic and disynaptic excitation from the contralateral caudal and ipsilateral rostral SC, respectively. These results suggest that the caudal FN helps to control also head movements via RSNs targeted by the SC, and these RSNs with SC topographic input play different functional roles in head movements.

Trigeminal high-frequency stimulation produces short- and long-term modification of reflex blink gain

Reflex blinks provide a model system for investigating motor learning in normal and pathological states. We investigated whether high-frequency stimulation (HFS) of the supraorbital branch of the trigeminal nerve before the R2 blink component (HFS-B) decreases reflex blink gain in alert rats. As with humans (Mao JB, Evinger C. J Neurosci 21: RC151, 2001), HFS-B significantly reduced blink size in the first hour after treatment for rats. Repeated days of HFS-B treatment produced long-term depression of blink circuits. Blink gain decreased exponentially across days, indicating a long-term depression of blink circuits. Additionally, the HFS-B protocol became more effective at depressing blink amplitude across days of treatment. This depression was not habituation, because neither long- nor short-term blink changes occurred when HFS was presented after the R2. To investigate whether gain modifications produced by HFS-B involved cerebellar networks, we trained rats in a delay eyelid conditioning paradigm using HFS-B as the unconditioned stimulus and a tone as the conditioned stimulus. As HFS-B depresses blink circuits and delay conditioning enhances blink circuit activity, occlusion should occur if they share neural networks. Rats acquiring robust eyelid conditioning did not exhibit decreases in blink gain, whereas rats developing low levels of eyelid conditioning exhibited weak, short-term reductions in blink gain. These results suggested that delay eyelid conditioning and long-term HFS-B utilize some of the same cerebellar circuits. The ability of repeated HFS-B treatment to depress trigeminal blink circuit activity long term implied that it may be a useful protocol to reduce hyperexcitable blink circuits that underlie diseases like benign essential blepharospasm.

Updating of visual space across horizontal saccades in cerebellar and thalamic lesion patients

Efference copies of motor commands are used to update visual space across saccades, ultimately ensuring transsaccadic constancy of space. Thalamic lesions have been shown to impair efference copy-based saccadic updating in an oculomotor context, i.e., when two successive saccades are required. Moreover, the cerebellum has also been discussed as one possible source of saccade-related efference copy signals. The present study aimed to investigate the effects of thalamic and cerebellar lesions on saccadic updating in a perceptual context. To this end, seven patients with focal cerebellar lesions, seven patients with focal thalamic lesions and 11 healthy controls completed a perceptual localisation task in which the position of a target had to be updated across a single horizontal saccade, while saccade-related event-related potentials (ERPs) were recorded. Contrary to the expectations, localisation precision in both patient groups did not differ from the respective controls. A positive ERP component with centroparietal distribution occurring from about 300 to 500 ms after saccade onset in the updating condition was observed equally pronounced in controls and thalamic lesion patients. In cerebellar lesion patients, there was evidence of a reduction of this relative positivity in the updating condition, particularly for leftward saccades. This finding suggests that cerebellar damage altered the neural processes underlying saccadic updating in a perceptual context without causing overt behavioural deficits.

Saccade adaptation abnormalities implicate dysfunction of cerebellar-dependent learning mechanisms in Autism Spectrum Disorders (ASD)

The cerebellar vermis (lobules VI-VII) has been implicated in both postmortem and neuroimaging studies of autism spectrum disorders (ASD). This region maintains the consistent accuracy of saccadic eye movements and plays an especially important role in correcting systematic errors in saccade amplitudes such as those induced by adaptation paradigms. Saccade adaptation paradigms have not yet been used to study ASD. Fifty-six individuals with ASD and 53 age-matched healthy controls performed an intrasaccadic target displacement task known to elicit saccadic adaptation reflected in an amplitude reduction. The rate of amplitude reduction and the variability of saccade amplitude across 180 adaptation trials were examined. Individuals with ASD adapted slower than healthy controls, and demonstrated more variability of their saccade amplitudes across trials prior to, during and after adaptation. Thirty percent of individuals with ASD did not significantly adapt, whereas only 6% of healthy controls failed to adapt. Adaptation rate and amplitude variability impairments were related to performance on a traditional neuropsychological test of manual motor control. The profile of impaired adaptation and reduced consistency of saccade accuracy indicates reduced neural plasticity within learning circuits of the oculomotor vermis that impedes the fine-tuning of motor behavior in ASD. These data provide functional evidence of abnormality in the cerebellar vermis that converges with previous reports of cellular and gross anatomic dysmorphology of this brain region in ASD.

New supervised learning theory applied to cerebellar modeling for suppression of variability of saccade end points

A new supervised learning theory is proposed for a hierarchical neural network with a single hidden layer of threshold units, which can approximate any continuous transformation, and applied to a cerebellar function to suppress the end-point variability of saccades. In motor systems, feedback control can reduce noise effects if the noise is added in a pathway from a motor center to a peripheral effector; however, it cannot reduce noise effects if the noise is generated in the motor center itself: a new control scheme is necessary for such noise. The cerebellar cortex is well known as a supervised learning system, and a novel theory of cerebellar cortical function developed in this study can explain the capability of the cerebellum to feedforwardly reduce noise effects, such as end-point variability of saccades. This theory assumes that a Golgi-granule cell system can encode the strength of a mossy fiber input as the state of neuronal activity of parallel fibers. By combining these parallel fiber signals with appropriate connection weights to produce a Purkinje cell output, an arbitrary continuous input-output relationship can be obtained. By incorporating such flexible computation and learning ability in a process of saccadic gain adaptation, a new control scheme in which the cerebellar cortex feedforwardly suppresses the end-point variability when it detects a variation in saccadic commands can be devised. Computer simulation confirmed the efficiency of such learning and showed a reduction in the variability of saccadic end points, similar to results obtained from experimental data.

Short-term saccadic adaptation in the macaque monkey: a binocular mechanism

Saccadic eye movements are rapid transfers of gaze between objects of interest. Their duration is too short for the visual system to be able to follow their progress in time. Adaptive mechanisms constantly recalibrate the saccadic responses by detecting how close the landings are to the selected targets. The double-step saccadic paradigm is a common method to simulate alterations in saccadic gain. While the subject is responding to a first target shift, a second shift is introduced in the middle of this movement, which masks it from visual detection. The error in landing introduced by the second shift is interpreted by the brain as an error in the programming of the initial response, with gradual gain changes aimed at compensating the apparent sensorimotor mismatch. A second shift applied dichoptically to only one eye introduces disconjugate landing errors between the two eyes. A monocular adaptive system would independently modify only the gain of the eye exposed to the second shift in order to reestablish binocular alignment. Our results support a binocular mechanism. A version-based saccadic adaptive process detects postsaccadic version errors and generates compensatory conjugate gain alterations. A vergence-based saccadic adaptive process detects postsaccadic disparity errors and generates corrective nonvisual disparity signals that are sent to the vergence system to regain binocularity. This results in striking dynamical similarities between visually driven combined saccade-vergence gaze transfers, where the disparity is given by the visual targets, and the double-step adaptive disconjugate responses, where an adaptive disparity signal is generated internally by the saccadic system.

Visuomotor cerebellum in human and nonhuman primates

In this paper, we will review the anatomical components of the visuomotor cerebellum in human and, where possible, in non-human primates and discuss their function in relation to those of extracerebellar visuomotor regions with which they are connected. The floccular lobe, the dorsal paraflocculus, the oculomotor vermis, the uvula-nodulus, and the ansiform lobule are more or less independent components of the visuomotor cerebellum that are involved in different corticocerebellar and/or brain stem olivocerebellar loops. The floccular lobe and the oculomotor vermis share different mossy fiber inputs from the brain stem; the dorsal paraflocculus and the ansiform lobule receive corticopontine mossy fibers from postrolandic visual areas and the frontal eye fields, respectively. Of the visuomotor functions of the cerebellum, the vestibulo-ocular reflex is controlled by the floccular lobe; saccadic eye movements are controlled by the oculomotor vermis and ansiform lobule, while control of smooth pursuit involves all these cerebellar visuomotor regions. Functional imaging studies in humans further emphasize cerebellar involvement in visual reflexive eye movements and are discussed.

Cerebellar lesions alter performance monitoring on the antisaccade task--an event-related potentials study

Error processing is associated with distinct event-related potential components (ERPs), i.e. the error-related negativity (ERN) which occurs within approximately 150 ms and is typically more pronounced than the correct-response negativity (CRN), and the error positivity (Pe) emerging from about 200 to 400 ms after an erroneous response. The short latency of the ERN suggests that the internal error monitoring system acts on rapidly available central information such as an efference copy signal rather than slower peripheral feedback. The cerebellum has been linked to an internal forward-model which enables online performance monitoring by predicting the sensory consequences of actions, most probably by making use of efference copies. In the present study it was hypothesized that the cerebellum is involved in the fast evaluation of saccadic response accuracy as reflected by the ERN. Error processing on an antisaccade task was investigated in eight patients with focal vascular lesions to the cerebellum and 22 control subjects using ERPs. While error rates were comparable between groups, saccadic reaction times (SRTs) were enhanced in the patients, and the error-correct difference waveforms showed reduced amplitudes for patients relative to controls in the ERN time window. Notably, this effect was mainly driven by an increased CRN in the patients. In the later Pe time window, the difference signal yielded higher amplitudes in patients compared to controls mainly because of smaller Pe amplitudes on correct trials in patients. The altered ERN/CRN pattern suggests that the cerebellum is critically involved in fast classification of saccadic accuracy. Largely intact performance accuracy together with increased SRTs and the altered Pe pattern may indicate a compensatory mechanism presumably related to slower, more conscious aspects of error processing in the patients.

Acquisition of neural learning in cerebellum and cerebral cortex for smooth pursuit eye movements

We evaluated the emergence of neural learning in the frontal eye fields (FEF(SEM)) and the floccular complex of the cerebellum while monkeys learned a precisely timed change in the direction of pursuit eye movement. For each neuron, we measured the time course of changes in neural response across a learning session that comprised at least 100 repetitions of an instructive change in target direction. In both areas, the average population learning curves tracked the behavioral changes with high fidelity, consistent with possible roles in driving learning. However, the learning curves of individual neurons sometimes bore little relation to the smooth, monotonic progression of behavioral learning. In the FEF(SEM), neural learning was episodic. For individual neurons, learning appeared at different times during the learning session and sometimes disappeared by the end of the session. Different FEF(SEM) neurons expressed maximal learning at different times relative to the acquisition of behavioral learning. In the floccular complex, many Purkinje cells acquired learned simple-spike responses according to the same time course as behavioral learning and retained their learned responses throughout the learning session. A minority of Purkinje cells acquired learned responses late in the learning session, after behavioral learning had reached an asymptote. We conclude that learning in single neurons can follow a very different time course from behavioral learning. Both the FEF(SEM) and the floccular complex contain representations of multiple temporal components of learning, with different neurons contributing to learning at different times during the acquisition of a learned movement.

The role of the cerebellum in saccadic adaptation as a window into neural mechanisms of motor learning

How does the nervous system guide the muscular periphery during the acquisition of a new motor skill? This is a fundamental question for researchers trying to understand the neural basis of motor learning. Recent advances in studying a valuable example of short-term motor learning, namely the adaptation of saccadic eye movements, have revealed neuronal processes in the cerebellum that underlie the unfolding of the learned behavior. In this review, we describe the latest findings from electrophysiology studies of saccadic adaptation and how they can generalize to more elaborate examples of cerebellum-dependent adaptation of movements. We focus our discussion on the plastic changes that are observed in the firing properties of Purkinje cells during the acquisition of the wanted motor response and describe how the altered activity of these neurons modifies the dynamics of the cerebellar microcircuitry. We finally demonstrate how such task-related modifications in the cerebellum are appropriate to fine-tune extracerebellar pre-motor structures and induce the learned behavior.

Differences in saccade dynamics between spinocerebellar ataxia 2 and late-onset cerebellar ataxias

The cerebellum is implicated in maintaining the saccadic subsystem efficient for vision by minimizing movement inaccuracy and by learning from endpoint errors. This ability is often disrupted in degenerative cerebellar diseases, as demonstrated by saccade kinetic abnormalities. The study of saccades in these patients may therefore provide insights into the neural substrate underlying saccadic motor control. We investigated the different extent of saccade dynamic abnormalities in spinocerebellar ataxia type 2 and late-onset cerebellar ataxias, genetically undefined and with prevalent cerebellar atrophy. Reflexive and voluntary saccades of different amplitude (10°-18°) were studied in seven patients with spinocerebellar ataxia 2, eight patients with late-onset cerebellar ataxia and 25 healthy controls. Quantitative analysis of saccade parameters and measures of saccade accuracy were performed. Detailed neurological, neurophysiological and magnetic resonance imaging assessment was obtained for each patient. Genetic and laboratory screening for spinocerebellar ataxias and other forms of late-onset cerebellar ataxias were also performed. A lower peak saccade velocity and longer duration was observed in patients with spinocerebellar ataxia 2 with respect to those with late-onset cerebellar ataxia and controls. Unlike subjects with spinocerebellar ataxia 2, patients with late-onset cerebellar ataxia showed main sequence relationships to similar saccades made by normal subjects. Saccades were significantly more inaccurate, namely hypometric, in late-onset cerebellar ataxia than in spinocerebellar ataxia 2 and inaccuracy increased with saccade amplitude. The percentage of hypometric primary saccades and of larger secondary corrective saccades were consistently higher in late-onset cerebellar ataxia than in spinocerebellar ataxia 2 and controls. No other significant differences were found between groups. Two different mechanisms were adopted to redirect the fovea as fast and/or accurately as possible to peripheral targets by the two groups of cerebellar patients. Patients with spinocerebellar ataxia 2 maintained accuracy using slow saccades with longer duration. This reflects prevalent degenerative processes affecting the pontine burst generator and leading to saccade velocity failure. On the other hand, patients with late-onset cerebellar ataxia reached the target with a number of fast inaccurate, mostly hypometric saccades. Different degrees of cerebellar oculomotor vermis involvement may account for differences in optimizing the trade-off between velocity and accuracy in the two groups. In addition, as suggested by spinocerebellar patients having slow saccades that are no longer ballistic, visual feedback might be continuously available during the movement execution to guide the eye to its target.

The absence of eye muscle fatigue indicates that the nervous system compensates for non-motor disturbances of oculomotor function

The physical properties of our bodies are subject to change (due to fatigue, heavy equipment, injury or aging) as we move around in the surrounding environment. The traditional definition of motor adaptation dictates that a mechanism in our brain needs to compensate for such alterations by appropriately modifying neural motor commands, if the vitally important accuracy of executed movements is to be preserved. In this article we describe how a repetitive eye movement task brings about changes in eye saccade kinematics that compromise accurate motor performance in the absence of a proper compensatory response. Surgical lesions in animals and human patient studies have previously demonstrated that an intact cerebellum is necessary for the compensation to arise and prevent the occurrence of hypometric movements. Here we identified the dynamic properties of the eye plant by recording from abducens motoneurons responsible for the required movement and measured the muscle response to microstimulation of the abducens nucleus in rhesus monkeys. The ensuing results demonstrate that the muscular periphery remains intact during the fatiguing eye movement task, while internal sources of noise (drowsiness, attentional modulation, neuronal fatigue etc.) must be responsible for a diminished oculomotor performance. This finding leads to the important realization that while supervising the accuracy of our movements, the nervous system takes additionally into account and adapts to any disruptive processes within the brain itself, clearly unrelated to the dynamical behavior of muscles or the environment. The existence of this supplementary mechanism forces a reassessment of traditional views of cerebellum-dependent motor adaptation.

The locus of motor activity in the superior colliculus of the rhesus monkey is unaltered during saccadic adaptation

The location of motor-related activity in the deeper layers of the superior colliculus (SC) is thought to generate a desired displacement command specifying the amplitude and direction of saccadic eye movements. However, the amplitude of saccadic eye movements made to visual targets can be systematically altered by surreptitiously moving the target location after the saccade has been initiated. Depending on whether the target is moved closer to or further from the fixation location, adaptation of saccade amplitude results in movements that are either smaller or larger than control movements. It remains an open question whether the SC specifies the desired movement to the original target location or whether SC activity specifies the vector of the amplitude-altered movement that is observed as adaptation progresses. We investigated this question by recording the activity of saccade-related burst neurons in the SC of head-restrained rhesus monkeys during both backward and forward saccadic adaptation. During adaptation in each direction, we find no evidence that is consistent with a change in the locus of SC activity despite changes in saccade amplitude; the location of SC motor-related activity does not appear to be remapped during either forward or backward saccadic adaptation. These data are inconsistent with hypotheses that propose a key role for the SC in mediating the changes in saccade amplitude observed during adaptation.

Saccade adaptation as a model of learning in voluntary movements

Motor learning ensures the accuracy of our daily movements. However, we know relatively little about its mechanisms, particularly for voluntary movements. Saccadic eye movements serve to bring the image of a visual target precisely onto the fovea. Their accuracy is maintained not by on-line sensory feedback but by a learning mechanism, called saccade adaptation. Recent studies on saccade adaptation have provided valuable additions to our knowledge of motor learning. This review summarizes what we know about the characteristics and neural mechanisms of saccade adaptation, emphasizing recent findings and new ideas. Long-term adaptation, distinct from its short-term counterpart, seems to be present in the saccadic system. Accumulating evidence indicates the involvement of the oculomotor cerebellar vermis as a learning site. The superior colliculus is now suggested not only to generate saccade commands but also to issue driving signals for motor learning. These and other significant contributions have advanced our understanding of saccade adaptation and motor learning in general.

Saccade and vestibular ocular motor adaptation

PURPOSE

This paper focuses on motor learning within the saccadic and vestibulo-ocular reflex (VOR) oculomotor systems, vital for our understanding how the brain keeps these subsystems calibrated in the presence of disease, trauma, and the changes that invariably accompany normal development and aging. We will concentrate on new information related to multiple time scales of saccade motor learning, adaptation of the VOR during high-velocity impulses, and the role of saccades in VOR adaptation. The role of the cerebellum in both systems is considered.

METHODS

Review of data involving saccade and VOR motor learning.

RESULTS

Data supports learning within the saccadic and VOR oculomotor systems is influenced by 1). Multiple time scales, with different rates of both learning and forgetting (seconds, minutes, hours, days, and months). In the case of forgetting, relearning on a similar task may be faster. 2). Pattern of training, learning and forgetting are not similarly achieved. Different contexts require different motor behaviors and rest periods between training sessions can be important for memory consolidation.

CONCLUSIONS

The central nervous system has the difficult task of determining where blame resides when motor performance is impaired (the credit assignment problem). Saccade and VOR motor learning takes place at multiple levels within the nervous system, from alterations in ion channel and membrane properties on single neurons, to more complex changes in neural circuit behavior and higher-level cognitive processes including prediction.

Head-free gaze shifts provide further insights into the role of the medial cerebellum in the control of primate saccadic eye movements

This study examines how signals generated in the oculomotor cerebellum could be involved in the control of gaze shifts, which rapidly redirect the eyes from one object to another. Neurons in the caudal fastigial nucleus (cFN), the output of the oculomotor cerebellum, discharged when monkeys made horizontal head-unrestrained gaze shifts, composed of an eye saccade and a head movement. Eighty-seven percent of our neurons discharged a burst of spikes for both ipsiversive and contraversive gaze shifts. In both directions, burst end was much better timed with gaze end than was burst start with gaze start, was well correlated with eye end, and was poorly correlated with head end or the time of peak head velocity. Moreover, bursts accompanied all head-unrestrained gaze shifts whether the head moved or not. Therefore we conclude that the cFN is not part of the pathway that controls head movement. For contraversive gaze shifts, the early part of the burst was correlated with gaze acceleration. Thereafter, the burst of the neuronal population continued throughout the prolonged deceleration of large gaze shifts. For a majority of neurons, gaze duration was correlated with burst duration; for some, gaze amplitude was less well correlated with the number of spikes. Therefore we suggest that the population burst provides an acceleration boost for high acceleration (smaller) contraversive gaze shifts and helps maintain the drive required to extend the deceleration of large contraversive gaze shifts. In contrast, the ipsiversive population burst, which is less well correlated with gaze metrics but whose peak rate occurs before gaze end, seems responsible primarily for terminating the gaze shift.

Changes in simple spike activity of some Purkinje cells in the oculomotor vermis during saccade adaptation are appropriate to participate in motor learning

Adaptation of saccadic eye movements provides an excellent motor learning model to study theories of neuronal plasticity. When primates make saccades to a jumping target, a backward step of the target during the saccade can make it appear to overshoot. If this deception continues for many trials, saccades gradually decrease in amplitude to go directly to the back-stepped target location. We used this adaptation paradigm to evaluate the Marr-Albus hypothesis that such motor learning occurs at the Purkinje (P)-cell of the cerebellum. We recorded the activity of identified P-cells in the oculomotor vermis, lobules VIc and VII. After documenting the on and off error directions of the complex spike activity of a P-cell, we determined whether its saccade-related simple spike (SS) activity changed during saccade adaptation in those two directions. Before adaptation, 57 of 61 P-cells exhibited a clear burst, pause, or a combination of both for saccades in one or both directions. Sixty-two percent of all cells, including two of the four initially unresponsive ones, behaved differently for saccades whose size changed because of adaptation than for saccades of similar sizes gathered before adaptation. In at least 42% of these, the changes were appropriate to decrease saccade amplitude based on our current knowledge of cerebellum and brainstem saccade circuitry. Changes in activity during adaptation were not compensating for the potential fatigue associated with performing many saccades. Therefore, many P-cells in the oculomotor vermis exhibit changes in SS activity specific to adapted saccades and therefore appropriate to induce adaptation.

Effect of reading on surface electromyogram recordings in patients with blepharospasm

PURPOSE

To compare the surface electromyogram recordings between patients with benign essential blepharospasm and controls while maintaining primary gaze and reading in downgaze.

METHODS

A university-based, prospective case series of 7 benign essential blepharospasm patients and 7 normal patients. Bilateral lower eyelid preseptal orbicularis oculi muscle potentials were recorded via surface electromyogram and video while performing a standardized 60-second task that was divided in 3 equal 20-second subtasks. Specifically, patients were asked to maintain primary gaze for the first 20-second interval, to read quietly for the second interval, and again maintain primary gaze for the third 20-second interval. Fourteen trials capturing bilateral recordings were performed in each group; qualitative and quantitative analysis of the surface electromyogram recordings was performed and compared both between patient groups and between subtasks. Blepharospasm patients were also asked to describe the effect of reading on their spasms. Specifically, they were asked before the study if reading lessened or worsened their spasms and then were asked poststudy if their symptoms lessened or worsened during the reading task.

RESULTS

The mean surface electromyogram potential of the orbicularis muscle was significantly less in normal eyes when compared with blepharospasm eyes for each task. Furthermore, the mean surface electromyogram potential during the reading task was significantly reduced when compared with the primary gaze task in all eyes. Only blepharospasm patients noticed this reduction subjectively.

CONCLUSIONS

Although conventional wisdom regarding blepharospasm suggests that reading is an exacerbating factor, short durations of reading seem to relieve spasms. This may be due to the effect of downgaze on the blink reflex and may offer possible areas of investigation for novel blepharospasm treatments.

Electrical microstimulation of the fastigial oculomotor region in the head-unrestrained monkey

It has been shown that inactivation of the caudal fastigial nucleus (cFN) by local injection of muscimol leads to inaccurate gaze shifts in the head-unrestrained monkey and that the gaze dysmetria is primarily due to changes in the horizontal amplitude of eye saccades in the orbit. Moreover, changes in the relationship between amplitude and duration are observed for only the eye saccades and not for the head movements. These results suggest that the cFN output primarily influences a neural network involved in moving the eyes in the orbit. The present study further tested this hypothesis by examining whether head movements could be evoked by electrical microstimulation of the saccade-related region in the cFN. Long stimulation trains (200-300 ms) evoked staircase gaze shifts that were ipsi- or contralateral, depending on the stimulated site. These gaze shifts were small in amplitude and were essentially accomplished by saccadic movements of the eyes. Head movements were observed in some sites but their amplitudes were very small (mean=2.4 degrees). The occurrence of head movements and their amplitude were not enhanced by increasing stimulation frequency or intensity. In several cases, electrically evoked gaze shifts exhibited an eye-head coupling that was different from that observed in visually triggered gaze shifts. This study provides additional observations suggesting that the saccade-related region in the cFN modulates the generation of eye movements and that the deep cerebellar output region involved in influencing head movements is located elsewhere.

Changes in control of saccades during gain adaptation

In a typical short-term saccadic adaptation protocol, the target moves intrasaccadically either toward (gain-down) or away (gain-up) from initial fixation, causing the saccade to complete with an endpoint error. A central question is how the motor system adapts in response to this error: are the motor commands changed to bring the eyes to a different goal, akin to a remapping of the target, or is adaptation focused on the processes that monitor the ongoing motor commands and correct them midflight, akin to changes that act via internal feedback? Here, we found that, in the gain-down paradigm, the brain learned to produce a smaller amplitude saccade by altering the trajectory of the saccade. The adapted saccades had reduced peak velocities, reduced accelerations, shallower decelerations, and increased durations compared with a control saccade of equal amplitude. These changes were consistent with a change in an internal feedback that acted as a forward model. However, in the gain-up paradigm, the brain learned to produce a larger amplitude saccade with trajectories that were identical with those of control saccades of equal amplitude. Therefore, whereas the gain-down paradigm appeared to induce adaptation via an internal feedback that controlled saccades midflight, the gain-up paradigm induced adaptation primarily via target remapping. Our simulations explained that, for each condition, the specific adaptation produced a saccade that brought the eyes to the target with the smallest motor costs.

Head-unrestrained gaze adaptation in the rhesus macaque

The ability to adjust the amplitude of gaze shifts in response to persistent visual errors ("gaze adaptation") has been investigated primarily by introducing visual errors at the end of saccades produced by head-restrained primates. Very little is known about the behavior and neural mechanisms underlying gaze adaptation when the head is free to move. We tested alternative hypotheses about the signals that are altered during gaze adaptation by increasing (25 degrees --> 50 degrees ; "forward adaptation") or decreasing (50 degrees --> 25 degrees ; "backward adaptation") the size of large, head-unrestrained gaze shifts. In our three rhesus monkey subjects, changes to primary gaze shift amplitude occurred regardless of the particular combinations of eye and head movements that made up the amplitude-altered gaze shifts. The relative changes to eye and head movements that occurred during adaptation could be predicted based on the magnitude of gaze adaptation and the positions of the eyes in the orbits at gaze onset. These results are consistent with the hypothesis that gaze adaptation occurs at the level of a gaze shift command and inconsistent with hypotheses based on the assumption that gaze adaptation results from alterations to eye- and/or head-specific signals.

Cerebellar-dependent motor learning is based on pruning a Purkinje cell population response

The improvement of motor behavior, based on experience, is a form of learning that is critically dependent on the cerebellum. A well studied example of cerebellar motor learning is short-term saccadic adaptation (STSA). In STSA, information on saccadic errors is used to improve future saccades. The information optimizing saccade metrics is conveyed by Purkinje cells simple spikes (PC-SS) because they are the critical input to the premotor circuits for saccades. We recorded PC-SS of monkeys undergoing STSA to reveal the code used for improving behavior. We found that the discharge of individual PC-SS was unable to account for the behavioral changes. The PC-SS population burst (PB), however, exhibited changes that closely paralleled the qualitatively different changes of saccade kinematics associated with gain-increase and gain-decrease STSA, respectively. Gain-increase STSA, characterized by an increase in saccade duration, replicates the relationship between saccade duration and the end of the PB valid for unadapted saccades. In contrast, gain-decrease STSA, which sports normal saccade duration but reduced saccadic velocity, is characterized by a PB that ends well before the adapted saccade. This suggests that the duration of normal as well as gain-increased saccades is determined by appropriately setting the end of PB end. However, the duration of gain-decreased saccades is apparently not modified by the cerebellum because the PB signals ends too early to determine saccade end. In summary, STSA, and most probably cerebellar-dependent learning in general, is based on optimizing the shape of a PC-SS population response.

Saccadic adaptation in lateral medullary and cerebellar infarction

To determine the adaptive capability of saccadic eye movements, and its association with enduring saccadic dysmetria in cerebellar and lateral medullary infarction (LMI), we investigated saccadic accuracy and adaptation in 15 patients with cerebellar or lateral medullary infarction, compared with those of 7 patients with diffuse cerebellar atrophy and 11 normal subjects. Saccade adaptation was elicited by a 37.5% backward target step during the primary saccade in both horizontal directions. Horizontal preadaptive saccadic gains were decreased in patients with cerebellar infarction, and contralesionally in patients with LMI. In contrast, adaptive saccadic gain change was reduced in patients with diffuse cerebellar atrophy and cerebellar infarction. Saccadic hypometria and reduced saccadic adaptability were dissociated in the majority of the patients with cerebellar infarctions; seven of the eight patients with cerebellar infarction showed saccadic hypometria and only three of them showed reduced saccadic adaptation, uni- or bilaterally in two with bilateral infarctions and ipsilesionally in one with unilateral infarction. The most commonly affected structure on MRI was the cerebellar hemisphere in the patients either with saccadic hypometria or with reduced saccadic adaptation. All patients with unilateral LMI exhibited normal saccadic gain adaptation in both directions, including those patients with enduring saccadic ipsipulsion. Our results suggest that the cerebellar hemispheres as well as the dorsal vermis and fastigial nucleus may be involved in the control of saccadic accuracy and adaptation. Reduced saccadic adaptation and persisting dysmetria are not tightly linked to each other in the cerebellar or lateral medullary lesions.

Adaptive control of saccades via internal feedback

Ballistic movements like saccades require the brain to generate motor commands without the benefit of sensory feedback. Despite this, saccades are remarkably accurate. Theory suggests that this accuracy arises because the brain relies on an internal forward model that monitors the motor commands, predicts their sensory consequences, and corrects eye trajectory midflight. If control of saccades relies on a forward model, then the forward model should adapt whenever its predictions fail to match sensory feedback at the end of the movement. Using optimal feedback control theory, we predicted how this adaptation should alter saccade trajectories. We trained subjects on a paradigm in which the horizontal target jumped vertically during the saccade. With training, the final position of the saccade moved toward the second target. However, saccades became increasingly curved, i.e., suboptimal, as oculomotor commands were corrected on-line to steer the eye toward the second target. The adaptive response had two components: (1) the motor commands that initiated the saccades changed slowly, aiming the saccade closer to the jumped target. The adaptation of these earliest motor commands displayed little forgetting during the rest periods. (2) Late in saccade trajectory, another adaptive response steered it still closer to the jumped target, producing curvature. Adaptation of these late motor commands showed near-complete forgetting during the rest periods. The two components adapted at different timescales, with the late-acting component displaying much faster rates. It appears that in controlling saccades, the brain relies on an internal feedback that has the characteristics of a fast-adapting forward model.

Visual versus motor vector inversions in the antisaccade task: a behavioral investigation with saccadic adaptation

In the antisaccade task, subjects must execute an eye movement away from a visual target. Correctly executing an antisaccade requires inhibiting a prosaccade toward the visual target and programming a movement to the opposite side. This movement could be based on the inversion of the visual vector, corresponding to the distance between the fixation point and the visual target, or the motor vector of the unwanted prosaccade. We dissociated the two vectors by means of saccadic adaptation. Adaptation can be observed when systematic targeting errors are caused by the displacement of the visual target during the saccade. Adaptation progressively modifies saccade amplitude (defined by the motor vector) such that it becomes appropriate to the postsaccadic stimulus position and thus different from the visual vector of the target. If antisaccade preparation depended on visual vector inversion, rightward prosaccade adaptation should not transfer to leftward antisaccades (which are based on the same visual vector) but should transfer to rightward antisaccades (which are based on a visual vector inside the adaptation field). If antisaccade preparation depended on motor vector inversion, rightward prosaccade adaptation should transfer to leftward antisaccades (which are based on the same, adapted motor vector) but should not transfer to rightward antisaccades (which are based on a nonadapted motor vector). The results are in line with the first hypothesis, showing that vector inversion precedes saccadic adaptation and suggesting that antisaccade preparation depends on the inversion of the visual target vector.

Complex spike activity signals the direction and size of dysmetric saccade errors

The cerebellar oculomotor vermis (OMV) receives inputs from both the superior colliculus (SC) via the nucleus reticularis tegmenti pontis as mossy fibres and the inferior olive as climbing fibres. Lesion studies show that the OMV is necessary for the saccade amplitude adaptation that corrects persistent motor errors. In this study, we examined whether the complex spike (CS) activity due to climbing fibre inputs could serve as an error signal to drive saccade adaptation. When there was an error during behaviourally induced saccade dysmetrias, the probability of CS occurrence depended on the direction and size of the error. If this CS activity actually drives saccade adaptation, we speculate that adaptation should be equally efficient in all directions and that the course of adaptation could have two operating modes.

Premotor inhibitory neurons carry signals related to saccade adaptation in the monkey

Cerebellar output changes during motor learning. How these changes cause alterations of motoneuron activity and movement remains an unresolved question for voluntary movements. To answer this question, we examined premotor neurons for saccadic eye movement. Previous studies indicate that cells in the fastigial oculomotor region (FOR) within the cerebellar nuclei on one side exhibit a gradual increase in their saccade-related discharge as the amplitude of ipsiversive saccades adaptively decreases. This change in FOR activity could cause the adaptive change in saccade amplitude because neurons in the FOR project directly to the brain stem region containing premotor burst neurons (BNs). To test this possibility, we recorded the activity of saccade-related burst neurons in the area that houses premotor inhibitory burst neurons (IBNs) and examined their discharge during amplitude-reducing adaptation elicited by intrasaccadic target steps. We specifically analyzed their activity for off-direction (contraversive) saccades, in which the IBN activity would increase to reduce saccade size. Before adaptation, 29 of 42 BNs examined discharged, at least occasionally, for contraversive saccades. As the amplitude of contraversive saccades decreased adaptively, half of BNs with off-direction spike activity showed an increase in the number of spikes (14/29) or an earlier occurrence of spikes (7/14). BNs that were silent during off-direction saccades before adaptation remained silent after adaptation. These results indicate that the changes in the off-direction activity of BNs are closely related to adaptive changes in saccade size and are appropriate to cause these changes.

Adaptation of Voluntary Saccades, But Not of Reactive Saccades, Transfers to Hand Pointing Movements

Studying the transfer of visuomotor adaptation from a given effector (e.g., the eye) to another (e.g., the hand) allows us to question whether sensorimotor processes influenced by adaptation are common to both effector control systems and thus to address the level where adaptation takes place. Previous studies have shown only very weak transfer of the amplitude adaptation of reactive saccades—i.e., produced automatically in response to the sudden appearance of visual targets—to hand pointing movements. Here we compared the amplitude of hand pointing movements recorded before and after adaptation of either reactive or voluntary saccades, produced either in a saccade sequence task or in a single saccade task. No transfer to hand pointing movements was found after adaptation of reactive saccades. In contrast, a substantial transfer to the hand was obtained following adaptation of voluntary saccades produced in sequence. Large amounts of transfer between the two saccade types were also found. These results demonstrate that the visuomotor processes influenced by saccadic adaptation depend on the type of saccades and that, in the case of voluntary saccades, they are shared by hand pointing movements. Implications for the neurophysiological substrates of the adaptation of reactive and voluntary saccades are discussed.

Activity changes in monkey superior colliculus during saccade adaptation

Saccades are eye movements that are used to foveate targets rapidly and accurately. Their amplitude must be adjusted continually, throughout life, to compensate for movement inaccuracies due to maturation, pathology, or aging. One possible locus for such saccade adaptation is the superior colliculus (SC), the relay for cortical commands to the premotor brain stem generator for saccades. However, previous stimulation and recording studies have disagreed as to whether saccade adaptation occurs up- or downstream of the SC. Therefore we have reexamined the behavior of SC burst neurons during saccade adaptation under conditions that were optimized to produce the biggest possible change in neuronal activity. We show that behavioral adaptation of saccade amplitude was associated with significant increases or decreases, in the number of spikes in the burst and/or changes in the shape of the movement field in 35 of 43 SC neurons tested. Of the 35, 29 had closed movement fields and 14 were classified indeterminate because the movement field could not be definitively diagnosed. Changes in the number of spikes occurred gradually during adaptation and resulted from correlated changes in burst lead and duration without consistent changes in peak burst rate. These data indicate that the great majority of SC neurons show a change in discharge in association with saccade amplitude adaptation. Based on these and previous results, we speculate that the site for saccade adaptation resides in the SC or that the SC is the final common pathway for adaptive changes that occur elsewhere in the saccade system.

Saccadic adaptation in Chiari type II malformation

BACKGROUND

Saccadic adaptation corrects errors in saccadic amplitude. Experimentally-induced saccadic adaptation provides a method for studying motor learning. The cerebellum is a major participant in saccadic adaptation. Chiari type II malformation (CII) is a developmental deformity of the cerebellum and brainstem that is associated with spina bifida. We investigated the effects of CII on saccadic adaptation.

METHOD

We measured eye movements using an infrared eye tracker in 21 subjects with CII (CII group) and 39 typically developing children (control group), aged 8-19 years. Saccadic adaptation was induced experimentally using targets that stepped horizontally 120 to the right and then stepped backward 3 degrees during saccades.

RESULTS

Saccadic adaptation was achieved at the end of the adaptation phase in participants in each group. Saccadic amplitude gain decreased by 6.9% in the CII group and 9.3% in the control group. The groups did not differ significantly (p = 0.27). Amplitude gain reduction was significantly less in the CII participants who had multiple shunt revisions. Regression analyses revealed no effects of spinal lesion level, presence of nystagmus, or cerebellar vermis dysmorphology on saccadic adaptation.

CONCLUSION

The neural circuits involved in saccadic adaptation appear to be functionally intact in CII.