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

Spontaneous brain activity in patients with central retinal artery occlusion: a resting-state functional MRI study using machine learning

Central retinal artery occlusion (CRAO) is a serious eye condition that poses a risk to vision, resulting from the blockage of the central retinal artery. Because of the anatomical connection between the ocular artery, which derives from the internal carotid artery, and the anterior middle cerebral artery, hemodynamic alterations and sudden vision loss associated with CRAO may impact brain functionality. This study aimed to examine alterations in spontaneous neural activity among patients with CRAO by resting-state functional MRI. In addition, we selected the amplitude of low-frequency fluctuation (ALFF) and fractional amplitude of low-frequency fluctuation (fALFF) values as classification features for distinguishing CRAO from healthy controls (HCs) using a support vector machine classifier. A total of 18 patients diagnosed with CRAO and 18 HCs participated in the study. Resting-state brain function images and structural images were acquired from both groups. Aberrant changes in spontaneous brain functional activity among CRAO patients were investigated utilizing ALFF and fALFF analysis methods. Group differences in ALFF/fALFF values were assessed through a two-sample t -test. Subsequently, a machine learning classifier was developed to evaluate the clinical diagnostic potential of ALFF and fALFF values. In comparison to HCs, individuals with CRAO exhibited significantly higher ALFF values in the left cerebellum_6, vermis_7, left superior frontal gyrus, and left inferior frontal gyrus, triangular part. Conversely, the CRAO group displayed notably lower ALFF values in the left precuneus and left median cingulum gyri. Furthermore, higher fALFF values were observed in the left inferior frontal gyrus, triangular part, whereas lower fALFF values were noted in the right cerebellum_Crus2, left precuneus, right angular gyrus, left angular gyrus, right supramarginal gyrus, right superior parietal gyrus, and left precuneus. Utilizing the ALFF/fALFF values, the receiver operating characteristic curves (area under the curve) yielded 0.99 and 0.94 through machine learning analysis techniques. CRAO patients exhibit atypical neural activity in the brain, characterized by ALFF and fALFF values predominantly localized in the frontal, parietal, and cerebellar regions, which are closely linked to visual cognition and motor control impairments. Furthermore, ALFF and fALFF could serve as potential neuroimaging markers beyond the orbit among CRAO.

Oculomotor behavior can be adjusted on the basis of artificial feedback signals indicating externally caused errors

Whether a saccade is accurate and has reached the target cannot be evaluated during its execution, but relies on post-saccadic feedback. If the eye has missed the target object, a secondary corrective saccade has to be made to align the fovea with the target. If a systematic post-saccadic error occurs, adaptive changes to the oculomotor behavior are made, such as shortening or lengthening the saccade amplitude. Systematic post-saccadic errors are typically attributed internally to erroneous motor commands. The corresponding adaptive changes to the motor command reduce the error and the need for secondary corrective saccades, and, in doing so, restore accuracy and efficiency. However, adaptive changes to the oculomotor behavior also occur if a change in saccade amplitude is beneficial for task performance, or if it is rewarded. Oculomotor learning thus is more complex than reducing a post-saccadic position error. In the current study, we used a novel oculomotor learning paradigm and investigated whether human participants are able to adapt their oculomotor behavior to improve task performance even when they attribute the error externally. The task was to indicate the intended target object among several objects to a simulated human-machine interface by making eye movements. The participants were informed that the system itself could make errors. The decoding process depended on a distorted landing point of the saccade, resulting in decoding errors. Two different types of visual feedback were added to the post-saccadic scene and we compared how participants used the different feedback types to adjust their oculomotor behavior to avoid errors. We found that task performance improved over time, regardless of the type of feedback. Thus, error feedback from the simulated human-machine interface was used for post-saccadic error evaluation. This indicates that 1) artificial visual feedback signals and 2) externally caused errors might drive adaptive changes to oculomotor behavior.

Corticospinal Adaptation to Short-Term Horizontal Balance Perturbation Training

Sensorimotor training and strength training can improve balance control. Currently, little is known about how repeated balance perturbation training affects balance performance and its neural mechanisms. This study investigated corticospinal adaptation assessed by transcranial magnetic stimulation (TMS) and Hoffman-reflex (H-reflex) measurements during balance perturbation induced by perturbation training. Fourteen subjects completed three perturbation sessions (PS1, PS2, and PS3). The perturbation system operated at 0.25 m/s, accelerating at 2.5 m/s2 over a 0.3 m displacement in anterior and posterior directions. Subjects were trained by over 200 perturbations in PS2. In PS1 and PS3, TMS and electrical stimulation elicited motor evoked potentials (MEP) and H-reflexes in the right leg soleus muscle, at standing rest and two time points (40 ms and 140 ms) after perturbation. Body sway was assessed using the displacement and velocity of the center of pressure (COP), which showed a decrease in PS3. No significant changes were observed in MEP or H-reflex between sessions. Nevertheless, Δ MEP at 40 ms demonstrated a positive correlation with Δ COP, while Δ H-reflex at 40 ms demonstrated a negative correlation with Δ COP. Balance perturbation training led to less body sway and a potential increase in spinal-level involvement, indicating that movement automaticity may be suggested after perturbation training.

Multidimensional cerebellar computations for flexible kinematic control of movements

Both the environment and our body keep changing dynamically. Hence, ensuring movement precision requires adaptation to multiple demands occurring simultaneously. Here we show that the cerebellum performs the necessary multi-dimensional computations for the flexible control of different movement parameters depending on the prevailing context. This conclusion is based on the identification of a manifold-like activity in both mossy fibers (MFs, network input) and Purkinje cells (PCs, output), recorded from monkeys performing a saccade task. Unlike MFs, the PC manifolds developed selective representations of individual movement parameters. Error feedback-driven climbing fiber input modulated the PC manifolds to predict specific, error type-dependent changes in subsequent actions. Furthermore, a feed-forward network model that simulated MF-to-PC transformations revealed that amplification and restructuring of the lesser variability in the MF activity is a pivotal circuit mechanism. Therefore, the flexible control of movements by the cerebellum crucially depends on its capacity for multi-dimensional computations.

Mislocalization after inhibition of saccadic adaptation

Saccadic eye movements are often imprecise and result in an error between expected and actual retinal target location after the saccade. Repeated experience of this error produces changes in saccade amplitude to reduce the error and concomitant changes in apparent visual location. We investigated the relationship between these two plastic processes in a series of experiments. Following a recent paradigm of inhibition of saccadic adaptation, in which participants are instructed to look at the initial target position and to continue to look at that position even if the target were to move again, our participants nevertheless perceived a visual probe presented near the saccade target to be shifted in direction of the target error. The location percept of the target gradually shifted and diverged over time from the executed saccade. Our findings indicate that changes in perceived location can be the same even when changes in saccade amplitude differ according to instruction and can develop even when the amplitude of the saccades executed during the adaptation procedure does not change. There are two possible explanations for this divergence between the adaptation states of saccade amplitude and perceived location. Either the intrasaccadic target step might trigger updating of the association between pre- and post-saccadic target positions, causing the localization shift, or the saccade motor command adjusts together with the perceived location at a common adaptation site, downstream from which voluntary control is exerted upon the executed eye movement only.

Error inconsistency does not generally inhibit saccadic adaptation: Support for linear models of multi-gainfield adaptation

This study examined saccade adaptation induced by intrasaccadic target steps (ITS). By manipulating the ITS, we investigated potential effects of the consistency of the feedback error on saccade adaptation, which would provide evidence against the linearity of standard models of visuomotor adaptation. Previous studies addressing saccade adaptation arrived at different interpretations, but in these experiments only a single saccade amplitude was trained rather than a variety of saccade amplitudes in random order (mixed training). We extend previous studies by testing for effects of error consistency under additional control conditions described by the factors training protocol (single-amplitude/mixed), ITS direction (onward/backward), and adaptation phase (training/washout). Adaptation dynamics were assessed using a model of "multi-gainfield adaptation" developed by tailoring an existing linear model for visuomotor adaptation of movements with multiple target positions to gain adaptation of saccades with multiple amplitudes. The total adaptive change did not depend on the consistency of the ITS in either mixed or single-amplitude training. The initial adaptation speed was lower with inconsistent ITS. However, the effect on adaptation speed occurred only during amplitude reduction and not during enlargement or washout. These results corroborate the linearity of saccade adaptation in that the mean error is the main factor determining the total adaptive change, independent of error consistency. The multi-gainfield adaptation model was confirmed in that the retention rate and error sensitivity did not depend on the training protocol. The absence of effects of error consistency on saccade adaptation is relevant in the context of adaptive deficits in movement disorders.

Motor learning by selection in visual working memory

Motor adaptation maintains movement accuracy over the lifetime. Saccadic eye movements have been used successfully to study the mechanisms and neural basis of adaptation. Using behaviorally irrelevant targets, it has been shown that saccade adaptation is driven by errors only in a brief temporal interval after movement completion. However, under natural conditions, eye movements are used to extract information from behaviorally relevant objects and to guide actions manipulating these objects. In this case, the action outcome often becomes apparent only long after movement completion, outside the supposed temporal window of error evaluation. Here, we show that saccade adaptation can be driven by error signals long after the movement when using behaviorally relevant targets. Adaptation occurred when a task-relevant target appeared two seconds after the saccade, or when a retro-cue indicated which of two targets, stored in visual working memory, was task-relevant. Our results emphasize the important role of visual working memory for optimal movement control.

Investigating the effects of cerebellar transcranial direct current stimulation on saccadic adaptation and cortisol response

BACKGROUND

Transcranial Direct Current Stimulation (tDCS) over the prefrontal cortex has been shown to modulate subjective, neuronal and neuroendocrine responses, particularly in the context of stress processing. However, it is currently unknown whether tDCS stimulation over other brain regions, such as the cerebellum, can similarly affect the stress response. Despite increasing evidence linking the cerebellum to stress-related processing, no studies have investigated the hormonal and behavioural effects of cerebellar tDCS.

METHODS

This study tested the hypothesis of a cerebellar tDCS effect on mood, behaviour and cortisol. To do this we employed a single-blind, sham-controlled design to measure performance on a cerebellar-dependent saccadic adaptation task, together with changes in cortisol output and mood, during online anodal and cathodal stimulation. Forty-five participants were included in the analysis. Stimulation groups were matched on demographic variables, potential confounding factors known to affect cortisol levels, mood and a number of personality characteristics.

RESULTS

Results showed that tDCS polarity did not affect cortisol levels or subjective mood, but did affect behaviour. Participants receiving anodal stimulation showed an 8.4% increase in saccadic adaptation, which was significantly larger compared to the cathodal group (1.6%).

CONCLUSION

The stimulation effect on saccadic adaptation contributes to the current body of literature examining the mechanisms of cerebellar stimulation on associated function. We conclude that further studies are needed to understand whether and how cerebellar tDCS may module stress reactivity under challenge conditions.

Effects of lorazepam on prosaccades and saccadic adaptation

BACKGROUND

Benzodiazepines have reliable adverse effects on saccadic eye movements, but the impact of sex as a potential modulator of these effects is less clear. A recent study reported stronger adverse effects on the spatial consistency of saccades in females, which may reflect sex differences in cerebellar mechanisms.

AIMS

We aimed to further examine the role of sex as a potential modulator of benzodiazepine effects by employing the saccadic adaptation paradigm, which is known to be sensitive to cerebellar functioning.

METHODS

A total of n=50 healthy adults performed a horizontal step prosaccade task and a saccadic adaptation task under 0.5 mg lorazepam, 1 mg lorazepam and placebo in a double-blind, within-subjects design.

RESULTS

In the prosaccade task, lorazepam had adverse effects on measures of peak velocity, latency and spatial consistency. The administration of 0.5 mg lorazepam led to significant reductions in gain-decrease adaptation, while a dose of 1 mg did not impair adaptation learning. Gain-increase adaptation was generally less pronounced, and unaffected by the drug. There were no significant drug×sex interactions in either task.

CONCLUSIONS

We conclude that a low dose of lorazepam impairs gain-decrease adaptation independent of sex. At higher doses, however, increasing fatigue may facilitate adaptation and thus counteract the adverse effects observed at lower doses. With regards to prosaccades, our findings confirm peak velocity as well as latency and spatial measures as sensitive biomarkers of GABAergic effects.

Modulation of cerebellar brain inhibition during temporal adaptive learning in a coincident timing task

In the present study, we examined the role of the cerebellum in temporal adaptive learning during a coincident timing task, i.e., a baseball-like hitting task involving a moving ball presented on a computer monitor. The subjects were required to change the timing of their responses based on imposed temporal perturbations. Using paired-pulse transcranial magnetic stimulation, we measured cerebellar brain inhibition (CBI) before, during, and after the temporal adaptive learning. Reductions in CBI only occurred during and after the temporal adaptive learning, regardless of the direction of the temporal perturbations. In addition, the changes in CBI were correlated with the magnitude of the adaptation. Here, we showed that the cerebellum is essential for learning about and controlling the timing of movements during temporal adaptation. Furthermore, changes in cerebellar-primary motor cortex connectivity occurred during temporal adaptation, as has been previously reported for spatial adaptation.

Prism Adaptation Deficits in Schizophrenia

Recent clinical and neurobehavioral evidence suggests cerebellar dysfunction in schizophrenia (SZ). We used the prism adaptation motor task (PAT) to probe specific cerebellar circuits in the disorder. PAT requires cerebellum-dependent motor adaptation, perceptual remapping, and strategic control. A failure to engage in early corrective processes may indicate impairment within either the cerebellum or regions contributing to strategic components, such as the parietal lobe, while an inability to develop and retain a visuomotor shift with time strongly suggests cerebellar impairment. Thirty-one individuals with SZ and 31 individuals without a history of psychological disorders completed PAT. Subjects reached to a target before, during, and following prism exposure, while their movements were recorded using motion-sensing technology. The SZ group performed worse on conditions consisting of adaptation, post-adaptation, aftereffects, and reorientation, thereby demonstrating a failure to adapt to the same degree as healthy controls. SZ performance remained impaired even with visual feedback and did not differ from controls at baseline, suggesting the observed deficit is specific to adaptation. Results indicate that sensorimotor adaptation is impaired in SZ and implicate disturbances in cerebellar circuits.

Cognitive function and cerebellar morphometric changes relate to abnormal intra-cerebellar and cerebro-cerebellum functional connectivity in old adults

INTRODUCTION

Numerous structural studies have already reported volumetric reduction in cerebellum with aging. However, there are still limited studies particularly focusing on analysis of the cerebellar resting state FC in old adults. Even so, the least related studies were unable to include some important cerebellar lobules due to limited cerebellum segmentation methods.

OBJECTIVE

The purpose of this study is to explore cognitive function in relation to cerebellar lobular morphometry and cortico-cerebellar connectivity changes in old adults' lifespan by incorporating previously undetected cerebellar lobules.

METHODS

This study includes a sample of 264 old adults subdivided into five cognitively normal age groups (G1 through G5). Cerebellum Segmentation (CERES) software was used to obtain morphometric measures and brain masks of all the 24 cerebellar lobules. We then defined individual lobules as seed regions and mapped the whole-brain to get functional connectivity maps. To analyze age group differences in cortico-cerebellar connectivity and cerebellar lobular volume, we used one way ANOVA and post hoc analysis was performed for multiple comparisons using Bonferroni method.

RESULTS

Our results report cerebellar lobular volumetric reduction, disrupted intra-cerebellar connectivity and significant differences in cortico-cerebellar resting state FC across age groups. In addition, our results show that disrupted FC between left Crus-II and right ACC relates to well emotion regulation and cognitive decline and is associated with poor performance on TMT-B and logical memory tests in older adults.

CONCLUSION

Overall, our findings confirm that as humans get older and older, the cerebellar lobular volumes as well as the cortico-cerebellar functional connectivity are affected and hence reduces cognition.

Reactive saccade adaptation boosts orienting of visuospatial attention

Attention and saccadic eye movements are critical components of visual perception. Recent studies proposed the hypothesis of a tight coupling between saccadic adaptation (SA) and attention: SA increases the processing speed of unpredictable stimuli, while increased attentional load boosts SA. Moreover, their cortical substrates partially overlap. Here, we investigated for the first time whether this coupling in the reactive/exogenous modality is specific to the orienting system of attention. We studied the effect of adaptation of reactive saccades (RS), elicited by the double-step paradigm, on exogenous orienting, measured using a Posner-like detection paradigm. In 18 healthy subjects, the attentional benefit—the difference in reaction time to targets preceded by informative versus uninformative cues—in a control exposure condition was subtracted from that of each adaptation exposure condition (backward and forward); then, this cue benefit difference was compared between the pre- and post-exposure phases. We found that, the attentional benefit significantly increased for cued-targets presented in the left hemifield after backward adaptation and for cued-targets presented in the right hemifield after forward adaptation. These findings provide strong evidence in humans for a coupling between RS adaptation and attention, possibly through the activation of a common neuronal pool.

The effect of age and gender on anti-saccade performance: Results from a large cohort of healthy aging individuals

By 2050, the global population of people aged 65 years or older will triple. While this is accompanied with an increasing burden of age-associated diseases, it also emphasizes the need to understand the effects of healthy aging on cognitive processes. One such effect is a general slowing of processing speed, which is well documented in many domains. The execution of anti-saccades depends on a well-established brain-wide network ranging from various cortical areas and basal ganglia through the superior colliculus down to the brainstem saccade generators. To clarify the consequences of healthy aging as well as gender on the execution of reflexive and voluntary saccades, we measured a large sample of healthy, non-demented individuals (n = 731, aged 51-84 years) in the anti-saccade task. Age affected various aspects of saccade performance: The number of valid trials decreased with age. Error rate, saccadic reaction times (SRTs), and variability in saccade accuracy increased with age, whereas anti-saccade costs, accuracy, and peak velocity of anti-saccades and direction errors were not affected by age. Gender affected SRTs independent of age and saccade type with male participants having overall shorter SRTs. Our rigid and solid statistical testing using linear mixed-effect models provide evidence for a uniform slowing of processing speed independent of the actually performed eye movement. Our data do not support the assumption of a specific deterioration of frontal lobe functions with aging.

Saccadic Adaptation Boosts Ongoing Gamma Activity in a Subsequent Visuoattentional Task

Attention and saccadic adaptation (SA) are critical components of visual perception, the former enhancing sensory processing of selected objects, the latter maintaining the eye movements accuracy toward them. Recent studies propelled the hypothesis of a tight functional coupling between these mechanisms, possibly due to shared neural substrates. Here, we used magnetoencephalography to investigate for the first time the neurophysiological bases of this coupling and of SA per se. We compared visual discrimination performance of 12 healthy subjects before and after SA. Eye movements and magnetic signals were recorded continuously. Analyses focused on gamma band activity (GBA) during the pretarget period of the discrimination and the saccadic tasks. We found that GBA increases after SA. This increase was found in the right hemisphere for both postadaptation saccadic and discrimination tasks. For the latter, GBA also increased in the left hemisphere. We conclude that oculomotor plasticity involves GBA modulation within an extended neural network which persists after SA, suggesting a possible role of gamma oscillations in the coupling between SA and attention.

Role of the Vermal Cerebellum in Visually Guided Eye Movements and Visual Motion Perception

The cerebellar cortex is a crystal-like structure consisting of an almost endless repetition of a canonical microcircuit that applies the same computational principle to different inputs. The output of this transformation is broadcasted to extracerebellar structures by way of the deep cerebellar nuclei. Visually guided eye movements are accommodated by different parts of the cerebellum. This review primarily discusses the role of the oculomotor part of the vermal cerebellum [the oculomotor vermis (OMV)] in the control of visually guided saccades and smooth-pursuit eye movements. Both types of eye movements require the mapping of retinal information onto motor vectors, a transformation that is optimized by the OMV, considering information on past performance. Unlike the role of the OMV in the guidance of eye movements, the contribution of the adjoining vermal cortex to visual motion perception is nonmotor and involves a cerebellar influence on information processing in the cerebral cortex.

Functions of Memory Across Saccadic Eye Movements

Several times per second, humans make rapid eye movements called saccades which redirect their gaze to sample new regions of external space. Saccades present unique challenges to both perceptual and motor systems. During the movement, the visual input is smeared across the retina and severely degraded. Once completed, the projection of the world onto the retina has undergone a large-scale spatial transformation. The vector of this transformation, and the new orientation of the eye in the external world, is uncertain. Memory for the pre-saccadic visual input is thought to play a central role in compensating for the disruption caused by saccades. Here, we review evidence that memory contributes to (1) detecting and identifying changes in the world that occur during a saccade, (2) bridging the gap in input so that visual processing does not have to start anew, and (3) correcting saccade errors and recalibrating the oculomotor system to ensure accuracy of future saccades. We argue that visual working memory (VWM) is the most likely candidate system to underlie these behaviours and assess the consequences of VWM's strict resource limitations for transsaccadic processing. We conclude that a full understanding of these processes will require progress on broader unsolved problems in psychology and neuroscience, in particular how the brain solves the object correspondence problem, to what extent prior beliefs influence visual perception, and how disparate signals arriving with different delays are integrated.

Complex Spike Wars: a New Hope

The climbing fiber-Purkinje cell circuit is one of the most powerful and highly conserved in the central nervous system. Climbing fibers exert a powerful excitatory action that results in a complex spike in Purkinje cells and normal functioning of the cerebellum depends on the integrity of climbing fiber-Purkinje cell synapse. Over the last 50 years, multiple hypotheses have been put forward on the role of the climbing fibers and complex spikes in cerebellar information processing and motor control. Central to these theories is the nature of the interaction between the low-frequency complex spike discharge and the high-frequency simple spike firing of Purkinje cells. This review examines the major hypotheses surrounding the action of the climbing fiber-Purkinje cell projection, discussing both supporting and conflicting findings. The review describes newer findings establishing that climbing fibers and complex spikes provide predictive signals about movement parameters and that climbing fiber input controls the encoding of behavioral information in the simple spike firing of Purkinje cells. Finally, we propose the dynamic encoding hypothesis for complex spike function that strives to integrate established and newer findings.

Cortico-cerebellar network involved in saccade adaptation

Saccade adaptation is the learning process that ensures that vision and saccades remain calibrated. The central nervous system network involved in these adaptive processes remains unclear because of difficulties in isolating the learning process from the correlated visual and motor processes. Here we imaged the human brain during a novel saccade adaptation paradigm that allowed us to isolate neural signals involved in learning independent of the changes in the amplitude of corrective saccades usually correlated with adaptation. We show that the changes in activation in the ipsiversive cerebellar vermis that track adaptation are not driven by the changes in corrective saccades and thus provide critical supporting evidence for previous findings. Similarly, we find that activation in the dorsomedial wall of the contraversive precuneus mirrors the pattern found in the cerebellum. Finally, we identify dorsolateral and dorsomedial cortical areas in the frontal and parietal lobes that encode the retinal errors following inaccurate saccades used to drive recalibration. Together, these data identify a distributed network of cerebellar and cortical areas and their specific roles in oculomotor learning. NEW & NOTEWORTHY The central nervous system constantly learns from errors and adapts to keep visual targets and saccades in registration. We imaged the human brain while the gain of saccades adapted to a visual target that was displaced while the eye was in motion, inducing retinal error. Activity in the cerebellum and precuneus tracked learning, whereas parts of the dorsolateral and dorsomedial frontal and parietal cortex encoded the retinal error used to drive learning.

Learning from the past: A reverberation of past errors in the cerebellar climbing fiber signal

The cerebellum allows us to rapidly adjust motor behavior to the needs of the situation. It is commonly assumed that cerebellum-based motor learning is guided by the difference between the desired and the actual behavior, i.e., by error information. Not only immediate but also future behavior will benefit from an error because it induces lasting changes of parallel fiber synapses on Purkinje cells (PCs), whose output mediates the behavioral adjustments. Olivary climbing fibers, likewise connecting with PCs, are thought to transport information on instant errors needed for the synaptic modification yet not to contribute to error memory. Here, we report work on monkeys tested in a saccadic learning paradigm that challenges this concept. We demonstrate not only a clear complex spikes (CS) signature of the error at the time of its occurrence but also a reverberation of this signature much later, before a new manifestation of the behavior, suitable to improve it.

Purkinje Cell Representations of Behavior: Diary of a Busy Neuron

Fundamental for understanding cerebellar function is determining the representations in Purkinje cell activity, the sole output of the cerebellar cortex. Up to the present, the most accurate descriptions of the information encoded by Purkinje cells were obtained in the context of motor behavior and reveal a high degree of heterogeneity of kinematic and performance error signals encoded. The most productive framework for organizing Purkinje cell firing representations is provided by the forward internal model hypothesis. Direct tests of this hypothesis show that individual Purkinje cells encode two different forward models simultaneously, one for effector kinematics and one for task performance. Newer results demonstrate that the timing of simple spike encoding of motor parameters spans an extend interval of up to ±2 seconds. Furthermore, complex spike discharge is not limited to signaling errors, can be predictive, and dynamically controls the information in the simple spike firing to meet the demands of upcoming behavior. These rich, diverse, and changing representations highlight the integrative aspects of cerebellar function and offer the opportunity to generalize the cerebellar computational framework over both motor and non-motor domains.

Psychosocial stress affects the acquisition of cerebellar-dependent sensorimotor adaptation

Despite being overlooked in theoretical models of stress-related disorders, differences in cerebellar structure and function are consistently reported in studies of individuals exposed to current and early-life stressors. However, the mediating processes through which stress impacts upon cerebellar function are currently unknown. The aim of the current experiment was to test the effects of experimentally-induced acute stress on cerebellar functioning, using a classic, forward saccadic adaptation paradigm in healthy, young men and women. Stress induction was achieved by employing the Montreal Imaging Stress Task (MIST), a task employing mental arithmetic and negative social feedback to generate significant physiological and endocrine stress responses. Saccadic adaptation was elicited using the double-step target paradigm. In the experiment, 48 participants matched for gender and age were exposed to either a stress (n = 25) or a control (n = 23) condition. Saliva for cortisol analysis was collected before, immediately after, and 10, and 30 min after the MIST. Saccadic adaptation was assessed approximately 10 min after stress induction, when cortisol levels peaked. Participants in the stress group reported significantly more stress symptoms and exhibited greater total cortisol output compared to controls. The stress manipulation was associated with slower learning rates in the stress group, while control participants acquired adaptation faster. Learning rates were negatively associated with cortisol output and mood disturbance. Results suggest that experimentally-induced stress slowed acquisition of cerebellar-dependent saccadic adaptation, related to increases in cortisol output. These 'proof-of-principle' data demonstrate that stress modulates cerebellar-related functions.

Climbing fibers predict movement kinematics and performance errors

Requisite for understanding cerebellar function is a complete characterization of the signals provided by complex spike (CS) discharge of Purkinje cells, the output neurons of the cerebellar cortex. Numerous studies have provided insights into CS function, with the most predominant view being that they are evoked by error events. However, several reports suggest that CSs encode other aspects of movements and do not always respond to errors or unexpected perturbations. Here, we evaluated CS firing during a pseudo-random manual tracking task in the monkey (Macaca mulatta). This task provides extensive coverage of the work space and relative independence of movement parameters, delivering a robust data set to assess the signals that activate climbing fibers. Using reverse correlation, we determined feedforward and feedback CSs firing probability maps with position, velocity, and acceleration, as well as position error, a measure of tracking performance. The direction and magnitude of the CS modulation were quantified using linear regression analysis. The major findings are that CSs significantly encode all three kinematic parameters and position error, with acceleration modulation particularly common. The modulation is not related to "events," either for position error or kinematics. Instead, CSs are spatially tuned and provide a linear representation of each parameter evaluated. The CS modulation is largely predictive. Similar analyses show that the simple spike firing is modulated by the same parameters as the CSs. Therefore, CSs carry a broader array of signals than previously described and argue for climbing fiber input having a prominent role in online motor control.NEW & NOTEWORTHY This article demonstrates that complex spike (CS) discharge of cerebellar Purkinje cells encodes multiple parameters of movement, including motor errors and kinematics. The CS firing is not driven by error or kinematic events; instead it provides a linear representation of each parameter. In contrast with the view that CSs carry feedback signals, the CSs are predominantly predictive of upcoming position errors and kinematics. Therefore, climbing fibers carry multiple and predictive signals for online motor control.

A role for NMDAR-dependent cerebellar plasticity in adaptive control of saccades in humans

BACKGROUND

Saccade pulse amplitude adaptation is mediated by the dorsal cerebellar vermis and fastigial nucleus. Long-term depression at the parallel fibre-Purkinjie cell synapses has been suggested to provide a cellular mechanism for the corresponding learning process. The mechanisms and sites of this plasticity, however, are still debated.

OBJECTIVE

To test the role of cerebellar plasticity phenomena on adaptive saccade control.

METHODS

We evaluated the effect of continuous theta burst stimulation (cTBS) over the posterior vermis on saccade amplitude adaptation and spontaneous recovery of the initial response. To further identify the substrate of synaptic plasticity responsible for the observed adaptation impairment, subjects were pre-treated with memantine, an N-methyl-d-aspartate receptor (NMDAR) antagonist.

RESULTS

Amplitude adaptation was altered by cTBS, suggesting that cTBS interferes with cerebellar plasticity involved in saccade adaptation. Amplitude adaptation and spontaneous recovery were not affected by cTBS when recordings were preceded by memantine administration.

CONCLUSION

The effects of cTBS are NMDAR-dependent and are likely to involve long-term potentiation or long-term depression at specific synaptic connections of the granular and molecular layer, which could effectively take part in cerebellar motor learning.

The same oculomotor vermal Purkinje cells encode the different kinematics of saccades and of smooth pursuit eye movements

Saccades and smooth pursuit eye movements (SPEM) are two types of goal-directed eye movements whose kinematics differ profoundly, a fact that may have contributed to the notion that the underlying cerebellar substrates are separated. However, it is suggested that some Purkinje cells (PCs) in the oculomotor vermis (OMV) of monkey cerebellum may be involved in both saccades and SPEM, a puzzling finding in view of the different kinematic demands of the two types of eye movements. Such ‘dual’ OMV PCs might be oddities with little if any functional relevance. On the other hand, they might be representatives of a generic mechanism serving as common ground for saccades and SPEM. In our present study, we found that both saccade- and SPEM-related responses of individual PCs could be predicted well by linear combinations of eye acceleration, velocity and position. The relative weights of the contributions that these three kinematic parameters made depended on the type of eye movement. Whereas in the case of saccades eye position was the most important independent variable, it was velocity in the case of SPEM. This dissociation is in accordance with standard models of saccades and SPEM control which emphasize eye position and velocity respectively as the relevant controlled state variables.

The Errors of Our Ways: Understanding Error Representations in Cerebellar-Dependent Motor Learning

The cerebellum is essential for error-driven motor learning and is strongly implicated in detecting and correcting for motor errors. Therefore, elucidating how motor errors are represented in the cerebellum is essential in understanding cerebellar function, in general, and its role in motor learning, in particular. This review examines how motor errors are encoded in the cerebellar cortex in the context of a forward internal model that generates predictions about the upcoming movement and drives learning and adaptation. In this framework, sensory prediction errors, defined as the discrepancy between the predicted consequences of motor commands and the sensory feedback, are crucial for both on-line movement control and motor learning. While many studies support the dominant view that motor errors are encoded in the complex spike discharge of Purkinje cells, others have failed to relate complex spike activity with errors. Given these limitations, we review recent findings in the monkey showing that complex spike modulation is not necessarily required for motor learning or for simple spike adaptation. Also, new results demonstrate that the simple spike discharge provides continuous error signals that both lead and lag the actual movements in time, suggesting errors are encoded as both an internal prediction of motor commands and the actual sensory feedback. These dual error representations have opposing effects on simple spike discharge, consistent with the signals needed to generate sensory prediction errors used to update a forward internal model.

An fMRI study of training voluntary smooth circular eye movements

Despite a large number of recent studies, the promise of fMRI methods to produce valuable insights into motor skill learning has been restricted to sequence learning paradigms, or manual training paradigms where a relatively advanced capacity for sensory-motor integration and effector coordination already exists. We therefore obtained fMRIs from 16 healthy adults trained in a new paradigm that demanded voluntary smooth circular eye movements without a moving target. This aimed to monitor neural activation during two possible motor learning processes: (a) the smooth pursuit control system develops a new perceptual-motor relationship and successfully becomes involved in voluntary action in which it is not normally involved or (b) the saccadic system normally used for voluntary eye movement and which only exhibits linear action skill develops new dynamic coordinative control capable of smooth circular movement. Participants were able to improve within half an hour, typically demonstrating saccadic movement with progressively reduced amplitudes, which better approximated smooth circular movement. Activity in the inferior premotor cortex was significantly modulated and decreased during the progress of learning. In contrast, activations in dorsal premotor and parietal cortex along the intraparietal sulcus, the supplementary eye field and the anterior cerebellum did not change during training. Thus, the decrease of activity in inferior premotor cortex was critically related to the learning progress in visuospatial eye movement control.

Visuo-manual coordination in preterm infants without neurological impairments

The extent of and reasons for visuo-manual coordination deficits in moderate and late preterm born infants without neurological impairments are not well known. This paper presents a longitudinal study on the visuo-manual development of twelve preterm infants, born after 33-36 weeks of gestation without neurological complications, between the ages of 6 and 12 months. Visuo-manual integration and grasping were assessed using the Peabody Developmental Motor Scales, along with bimanual coordination and handedness tests. Visual function was examined once prior to the beginning of the study. Gross motor development was also evaluated every month. Preterm infants were compared to a control group of ten full-term infants according to corrected age. Compared to full-terms, the visual perception of preterm infants was close to normal, with only a measure of visual fixation lower than in full-terms. In contrast, preterm infants had delayed development of visuo-manual integration, grasping, bimanual coordination, and handedness even when compared using corrected age. Tonicity and gestational age at birth were the main variables associated to the delays. These results are discussed in terms of the possible factors underlying such delays. They need to be confirmed on a larger sample of preterm born children, and to be correlated with later development. This would allow developing markers of future neuropsychological impairments during childhood.

Oculomotor Adaptation Elicited By Intra-Saccadic Visual Stimulation: Time-Course of Efficient Visual Target Perturbation

Perception of our visual environment strongly depends on saccadic eye movements, which in turn are calibrated by saccadic adaptation mechanisms elicited by systematic movement errors. Current models of saccadic adaptation assume that visual error signals are acquired only after saccade completion, because the high speed of saccade execution disturbs visual processing (saccadic "suppression" and "mislocalization"). Complementing a previous study from our group, here we report that visual information presented during saccades can drive adaptation mechanisms and we further determine the critical time window of such error processing. In 15 healthy volunteers, shortening adaptation of reactive saccades toward a ±8° visual target was induced by flashing the target for 2 ms less eccentrically than its initial location either near saccade peak velocity ("PV" condition) or peak deceleration ("PD") or saccade termination ("END"). Results showed that, as compared to the "CONTROL" condition (target flashed at its initial location upon saccade termination), saccade amplitude decreased all throughout the "PD" and "END" conditions, reaching significant levels in the second adaptation and post-adaptation blocks. The results of nine other subjects tested in a saccade lengthening adaptation paradigm with the target flashing near peak deceleration ("PD" and "CONTROL" conditions) revealed no significant change of gain, confirming that saccade shortening adaptation is easier to elicit. Also, together with this last result, the stable gain observed in the "CONTROL" conditions of both experiments suggests that mislocalization of the target flash is not responsible for the saccade shortening adaptation demonstrated in the first group. Altogether, these findings reveal that the visual "suppression" and "mislocalization" phenomena related to saccade execution do not prevent brief visual information delivered "in-flight" from being processed to elicit oculomotor adaptation.

Deployment of spatial attention without moving the eyes is boosted by oculomotor adaptation

Vertebrates developed sophisticated solutions to select environmental visual information, being capable of moving attention without moving the eyes. A large body of behavioral and neuroimaging studies indicate a tight coupling between eye movements and spatial attention. The nature of this link, however, remains highly debated. Here, we demonstrate that deployment of human covert attention, measured in stationary eye conditions, can be boosted across space by changing the size of ocular saccades to a single position via a specific adaptation paradigm. These findings indicate that spatial attention is more widely affected by oculomotor plasticity than previously thought.

Context cue-dependent saccadic adaptation in rhesus macaques cannot be elicited using color

When the head does not move, rapid movements of the eyes called saccades are used to redirect the line of sight. Saccades are defined by a series of metrical and kinematic (evolution of a movement as a function of time) relationships. For example, the amplitude of a saccade made from one visual target to another is roughly 90% of the distance between the initial fixation point (T0) and the peripheral target (T1). However, this stereotypical relationship between saccade amplitude and initial retinal error (T1-T0) may be altered, either increased or decreased, by surreptitiously displacing a visual target during an ongoing saccade. This form of motor learning (called saccadic adaptation) has been described in both humans and monkeys. Recent experiments in humans and monkeys have suggested that internal (proprioceptive) and external (target shape, color, and/or motion) cues may be used to produce context-dependent adaptation. We tested the hypothesis that an external contextual cue (target color) could be used to evoke differential gain (actual saccade/initial retinal error) states in rhesus monkeys. We did not observe differential gain states correlated with target color regardless of whether targets were displaced along the same vector as the primary saccade or perpendicular to it. Furthermore, this observation held true regardless of whether adaptation trials using various colors and intrasaccade target displacements were randomly intermixed or presented in short or long blocks of trials. These results are consistent with hypotheses that state that color cannot be used as a contextual cue and are interpreted in light of previous studies of saccadic adaptation in both humans and monkeys.

Impairment of saccade adaptation in a patient with a focal thalamic lesion

The frequent jumps of the eyeballs-called saccades-imply the need for a constant correction of motor errors. If systematic errors are detected in saccade landing, the saccade amplitude adapts to compensate for the error. In the laboratory, saccade adaptation can be studied by displacing the saccade target. Functional selectivity of adaptation for different saccade types suggests that adaptation occurs at multiple sites in the oculomotor system. Saccade motor learning might be the result of a comparison between a prediction of the saccade landing position and its actual postsaccadic location. To investigate whether a thalamic feedback pathway might carry such a prediction signal, we studied a patient with a lesion in the posterior ventrolateral thalamic nucleus. Saccade adaptation was tested for reactive saccades, which are performed to suddenly appearing targets, and for scanning saccades, which are performed to stationary targets. For reactive saccades, we found a clear impairment in adaptation retention ipsilateral to the lesioned side and a larger-than-normal adaptation on the contralesional side. For scanning saccades, adaptation was intact on both sides and not different from the control group. Our results provide the first lesion evidence that adaptation of reactive and scanning saccades relies on distinct feedback pathways from cerebellum to cortex. They further demonstrate that saccade adaptation in humans is not restricted to the cerebellum but also involves cortical areas. The paradoxically strong adaptation for outward target steps can be explained by stronger reliance on visual targeting errors when prediction error signaling is impaired.

Changes in Purkinje cell simple spike encoding of reach kinematics during adaption to a mechanical perturbation

The cerebellum is essential in motor learning. At the cellular level, changes occur in both the simple spike and complex spike firing of Purkinje cells. Because simple spike discharge reflects the main output of the cerebellar cortex, changes in simple spike firing likely reflect the contribution of the cerebellum to the adapted behavior. Therefore, we investigated in Rhesus monkeys how the representation of arm kinematics in Purkinje cell simple spike discharge changed during adaptation to mechanical perturbations of reach movements. Monkeys rapidly adapted to a novel assistive or resistive perturbation along the direction of the reach. Adaptation consisted of matching the amplitude and timing of the perturbation to minimize its effect on the reach. In a majority of Purkinje cells, simple spike firing recorded before and during adaptation demonstrated significant changes in position, velocity, and acceleration sensitivity. The timing of the simple spike representations change within individual cells, including shifts in predictive versus feedback signals. At the population level, feedback-based encoding of position increases early in learning and velocity decreases. Both timing changes reverse later in learning. The complex spike discharge was only weakly modulated by the perturbations, demonstrating that the changes in simple spike firing can be independent of climbing fiber input. In summary, we observed extensive alterations in individual Purkinje cell encoding of reach kinematics, although the movements were nearly identical in the baseline and adapted states. Therefore, adaption to mechanical perturbation of a reaching movement is accompanied by widespread modifications in the simple spike encoding.

Educating the blind brain: a panorama of neural bases of vision and of training programs in organic neurovisual deficits

Vision is a complex function, which is achieved by movements of the eyes to properly foveate targets at any location in 3D space and to continuously refresh neural information in the different visual pathways. The visual system involves five main routes originating in the retinas but varying in their destination within the brain: the occipital cortex, but also the superior colliculus (SC), the pretectum, the supra-chiasmatic nucleus, the nucleus of the optic tract and terminal dorsal, medial and lateral nuclei. Visual pathway architecture obeys systematization in sagittal and transversal planes so that visual information from left/right and upper/lower hemi-retinas, corresponding respectively to right/left and lower/upper visual fields, is processed ipsilaterally and ipsialtitudinally to hemi-retinas in left/right hemispheres and upper/lower fibers. Organic neurovisual deficits may occur at any level of this circuitry from the optic nerve to subcortical and cortical destinations, resulting in low or high-level visual deficits. In this didactic review article, we provide a panorama of the neural bases of eye movements and visual systems, and of related neurovisual deficits. Additionally, we briefly review the different schools of rehabilitation of organic neurovisual deficits, and show that whatever the emphasis is put on action or perception, benefits may be observed at both motor and perceptual levels. Given the extent of its neural bases in the brain, vision in its motor and perceptual aspects is also a useful tool to assess and modulate central nervous system (CNS) in general.

Functional activity within the frontal eye fields, posterior parietal cortex, and cerebellar vermis significantly correlates to symmetrical vergence peak velocity: an ROI-based, fMRI study of vergence training

Convergence insufficiency (CI) is a prevalent binocular vision disorder with symptoms that include double/blurred vision, eyestrain, and headaches when engaged in reading or other near work. Randomized clinical trials support that Office-Based Vergence and Accommodative Therapy with home reinforcement leads to a sustained reduction in patient symptoms. However, the underlying neurophysiological basis for treatment is unknown. Functional activity and vergence eye movements were quantified from seven binocularly normal controls (BNC) and four CI patients before and after 18 h of vergence training. An fMRI conventional block design of sustained fixation vs. vergence eye movements stimulated activity in the frontal eye fields (FEF), the posterior parietal cortex (PPC), and the cerebellar vermis (CV). Comparing the CI patients' baseline measurements to the post-vergence training data sets with a paired t-test revealed the following: (1) the percent change in the BOLD signal in the FEF, PPC, and CV significantly increased (p < 0.02), (2) the peak velocity from 4° symmetrical convergence step responses increased (p < 0.01) and (3) patient symptoms assessed using the CI Symptom Survey (CISS) improved (p < 0.05). CI patient measurements after vergence training were more similar to levels observed within BNC. A regression analysis revealed the peak velocity from BNC and CI subjects before and after vergence training was significantly correlated to the percent BOLD signal change within the FEF, PPC, and CV (r = 0.6; p < 0.05). Results have clinical implications for understanding the behavioral and neurophysiological changes after vergence training in patients with CI, which may lead to the sustained reduction in visual symptoms.

The cerebellum for jocks and nerds alike

Historically the cerebellum has been implicated in the control of movement. However, the cerebellum's role in non-motor functions, including cognitive and emotional processes, has also received increasing attention. Starting from the premise that the uniform architecture of the cerebellum underlies a common mode of information processing, this review examines recent electrophysiological findings on the motor signals encoded in the cerebellar cortex and then relates these signals to observations in the non-motor domain. Simple spike firing of individual Purkinje cells encodes performance errors, both predicting upcoming errors as well as providing feedback about those errors. Further, this dual temporal encoding of prediction and feedback involves a change in the sign of the simple spike modulation. Therefore, Purkinje cell simple spike firing both predicts and responds to feedback about a specific parameter, consistent with computing sensory prediction errors in which the predictions about the consequences of a motor command are compared with the feedback resulting from the motor command execution. These new findings are in contrast with the historical view that complex spikes encode errors. Evaluation of the kinematic coding in the simple spike discharge shows the same dual temporal encoding, suggesting this is a common mode of signal processing in the cerebellar cortex. Decoding analyses show the considerable accuracy of the predictions provided by Purkinje cells across a range of times. Further, individual Purkinje cells encode linearly and independently a multitude of signals, both kinematic and performance errors. Therefore, the cerebellar cortex's capacity to make associations across different sensory, motor and non-motor signals is large. The results from studying how Purkinje cells encode movement signals suggest that the cerebellar cortex circuitry can support associative learning, sequencing, working memory, and forward internal models in non-motor domains.

Adaptation of scanning saccades co-occurs in different coordinate systems

Plastic changes of saccades (i.e., following saccadic adaptation) do not transfer between oppositely directed saccades, except when multiple directions are trained simultaneously, suggesting a saccadic planning in retinotopic coordinates. Interestingly, a recent study in human healthy subjects revealed that after an adaptive increase of rightward-scanning saccades, both leftward and rightward double-step, memory-guided saccades, triggered toward the adapted endpoint, were modified, revealing that target location was coded in spatial coordinates (Zimmermann et al. 2011). However, as the computer screen provided a visual frame, one alternative hypothesis could be a coding in allocentric coordinates. Here, we questioned whether adaptive modifications of saccadic planning occur in multiple coordinate systems. We reproduced the paradigm of Zimmermann et al. (2011) using target light-emitting diodes in the dark, with and without a visual frame, and tested different saccades before and after adaptation. With double-step, memory-guided saccades, we reproduced the transfer of adaptation to leftward saccades with the visual frame but not without, suggesting that the coordinate system used for saccade planning, when the frame is visible, is allocentric rather than spatiotopic. With single-step, memory-guided saccades, adaptation transferred to leftward saccades, both with and without the visual frame, revealing a target localization in a coordinate system that is neither retinotopic nor allocentric. Finally, with single-step, visually guided saccades, the classical, unidirectional pattern of amplitude change was reproduced, revealing retinotopic coordinate coding. These experiments indicate that the same procedure of adaptation modifies saccadic planning in multiple coordinate systems in parallel-each of them revealed by the use of different saccade tasks in postadaptation.

The cerebellum and visual perceptual learning: evidence from a motion extrapolation task

Visual perceptual learning is widely assumed to reflect plastic changes occurring along the cerebro-cortical visual pathways, including at the earliest stages of processing, though increasing evidence indicates that higher-level brain areas are also involved. Here we addressed the possibility that the cerebellum plays an important role in visual perceptual learning. Within the realm of motor control, the cerebellum supports learning of new skills and recalibration of motor commands when movement execution is consistently perturbed (adaptation). Growing evidence indicates that the cerebellum is also involved in cognition and mediates forms of cognitive learning. Therefore, the obvious question arises whether the cerebellum might play a similar role in learning and adaptation within the perceptual domain. We explored a possible deficit in visual perceptual learning (and adaptation) in patients with cerebellar damage using variants of a novel motion extrapolation, psychophysical paradigm. Compared to their age- and gender-matched controls, patients with focal damage to the posterior (but not the anterior) cerebellum showed strongly diminished learning, in terms of both rate and amount of improvement over time. Consistent with a double-dissociation pattern, patients with focal damage to the anterior cerebellum instead showed more severe clinical motor deficits, indicative of a distinct role of the anterior cerebellum in the motor domain. The collected evidence demonstrates that a pure form of slow-incremental visual perceptual learning is crucially dependent on the intact cerebellum, bearing the notion that the human cerebellum acts as a learning device for motor, cognitive and perceptual functions. We interpret the deficit in terms of an inability to fine-tune predictive models of the incoming flow of visual perceptual input over time. Moreover, our results suggest a strong dissociation between the role of different portions of the cerebellum in motor versus non-motor functions, with only the posterior lobe being responsible for learning in the perceptual domain.

Adaptation of naturally paced saccades

In the natural environment, humans make saccades almost continuously. In many eye movement experiments, however, observers are required to fixate for unnaturally long periods of time. The resulting long and monotonous experimental sessions can become especially problematic when collecting data in a clinical setting, where time can be scarce and subjects easily fatigued. With this in mind, we tested whether the well-studied motor learning process of saccade adaptation could be induced with a dramatically shortened intertrial interval. Observers made saccades to targets that stepped left or right either ∼250 ms or ∼1,600 ms after the saccade landed. In experiment I, we tested baseline saccade parameters to four different target amplitudes (5°, 10°, 15°, and 20°) in the two timing settings. In experiments II and III, we adapted 10° saccades via 2° intrasaccadic steps either backwards or forwards, respectively. Seven subjects performed eight separate adaptation sessions (2 intertrial timings × 2 adaptation direction × 2 session trial lengths). Adaptation proceeded remarkably similarly in both timing conditions across the multiple sessions. In the faster-paced sessions, robust adaptation was achieved in under 2 min, demonstrating the efficacy of our approach to streamlining saccade adaptation experiments. Although saccade amplitudes were similar between conditions, the faster-paced condition unexpectedly resulted in significantly higher peak velocities in all subjects. This surprising finding demonstrates that the stereotyped "main sequence" relationship between saccade amplitude and peak velocity is not as fixed as originally thought.

Similarities in error processing establish a link between saccade prediction at baseline and adaptation performance

Adaptive processes are crucial in maintaining the accuracy of body movements and rely on error storage and processing mechanisms. Although classically studied with adaptation paradigms, evidence of these ongoing error-correction mechanisms should also be detectable in other movements. Despite this connection, current adaptation models are challenged when forecasting adaptation ability with measures of baseline behavior. On the other hand, we have previously identified an error-correction process present in a particular form of baseline behavior, the generation of predictive saccades. This process exhibits long-term intertrial correlations that decay gradually (as a power law) and are best characterized with the tools of fractal time series analysis. Since this baseline task and adaptation both involve error storage and processing, we sought to find a link between the intertrial correlations of the error-correction process in predictive saccades and the ability of subjects to alter their saccade amplitudes during an adaptation task. Here we find just such a relationship: the stronger the intertrial correlations during prediction, the more rapid the acquisition of adaptation. This reinforces the links found previously between prediction and adaptation in motor control and suggests that current adaptation models are inadequate to capture the complete dynamics of these error-correction processes. A better understanding of the similarities in error processing between prediction and adaptation might provide the means to forecast adaptation ability with a baseline task. This would have many potential uses in physical therapy and the general design of paradigms of motor adaptation.

Engineering animal models of dystonia

Dystonia is a neurological disorder characterized by abnormal involuntary movements that are prolonged and often cause twisting and turning. Several genetically modified worms, fruit flies, and rodents have been generated as models of genetic dystonias, in particular DYT1, DYT11, and DYT12 dystonias. Although these models do not show overt dystonic symptoms, the rodent models exhibit motor deficits in specialized behavioral tasks, such as the rotarod and beam-walking tests. For example, in a rodent model of DYT12 dystonia, which is generally stress triggered, motor deficits are observed only after the animal is stressed. Moreover, in a rodent model of DYT1 dystonia, the motor and electrophysiological deficits can be rescued by trihexyphenidyl, a common anticholinergic medication used to treat dystonic symptoms in human patients. Biochemically, the DYT1 and DYT11 animal models also share some similarities to patients, such as a reduction in striatal D2 dopamine receptor and binding activities. In addition, conditional knockout mouse models for DYT1 and DYT11 dystonia demonstrate that loss of the causal dystonia-related proteins in the striatum leads to motor deficits. Interestingly, loss of the DYT1 dystonia causal protein in Purkinje cells shows an improvement in motor performance, suggesting that gene therapy targeting of the cerebellum or intervention in its downstream pathways may be useful. Finally, recent studies using DYT1 dystonia worm and mouse models led to a potential novel therapeutic agent, which is currently undergoing clinical trials. These results indicate that genetic animal models are powerful tools to elucidate the pathophysiology and to further develop new therapeutics for dystonia.

Effects of structural and functional cerebellar lesions on sensorimotor adaptation of saccades

The cerebellum is critically involved in the adaptation mechanisms that maintain the accuracy of goal-directed acts such as saccadic eye movements. Two categories of saccades, each relying on different adaptation mechanisms, are defined: reactive (externally triggered) saccades and voluntary (internally triggered) saccades. The contribution of the medio-posterior part of the cerebellum to reactive saccades adaptation has been clearly demonstrated, but the evidence that other parts of the cerebellum are also involved is limited. Moreover, the cerebellar substrates of voluntary saccades adaptation have only been marginally investigated. Here, we addressed these two questions by investigating the adaptive capabilities of patients with cerebellar or pre-cerebellar stroke. We recruited three groups of patients presenting focal lesions located, respectively, in the supero-anterior cerebellum, the infero-posterior cerebellum and the lateral medulla (leading to a Wallenberg syndrome including motor dysfunctions similar to those resulting from lesion of the medio-posterior cerebellum). Adaptations of reactive saccades and of voluntary saccades were tested during separate sessions in all patients and in a group of healthy participants. The functional lesion of the medio-posterior cerebellum in Wallenberg syndrome strongly impaired the adaptation of both reactive and voluntary saccades. In contrast, patients with lesion in the supero-anterior part of the cerebellum presented a specific adaptation deficit of voluntary saccades. Finally, patients with an infero-posterior cerebellar lesion showed mild adaptation deficits. We conclude that the medio-posterior cerebellum is critical for the adaptation of both saccade categories, whereas the supero-anterior cerebellum is specifically involved in the adaptation of voluntary saccades.

The reference frames in saccade adaptation

Saccade adaptation is a mechanism that adjusts saccade landing positions if they systematically fail to reach their intended target. In the laboratory, saccades can be shortened or lengthened if the saccade target is displaced during execution of the saccade. In this study, saccades were performed from different positions to an adapted saccade target to dissociate adaptation to a spatiotopic position in external space from a combined retinotopic and spatiotopic coding. The presentation duration of the saccade target before saccade execution was systematically varied, during adaptation and during test trials, with a delayed saccade paradigm. Spatiotopic shifts in landing positions depended on a certain preview duration of the target before saccade execution. When saccades were performed immediately to a suddenly appearing target, no spatiotopic adaptation was observed. These results suggest that a spatiotopic representation of the visual target signal builds up as a function of the duration the saccade target is visible before saccade execution. Different coordinate frames might also explain the separate adaptability of reactive and voluntary saccades. Spatiotopic effects were found only in outward adaptation but not in inward adaptation, which is consistent with the idea that outward adaptation takes place at the level of the visual target representation, whereas inward adaptation is achieved at a purely motor level.

A vermal Purkinje cell simple spike population response encodes the changes in eye movement kinematics due to smooth pursuit adaptation

Smooth pursuit adaptation (SPA) is an example of cerebellum-dependent motor learning that depends on the integrity of the oculomotor vermis (OMV). In an attempt to unveil the neuronal basis of the role of the OMV in SPA, we recorded Purkinje cell simple spikes (PC SS) of trained monkeys. Individual PC SS exhibited specific changes of their discharge patterns during the course of SPA. However, these individual changes did not provide a reliable explanation of the behavioral changes. On the other hand, the population response of PC SS perfectly reflected the changes resulting from adaptation. Population vector was calculated using all cells recorded independent of their location. A population code conveying the behavioral changes is in full accordance with the anatomical convergence of PC axons on target neurons in the cerebellar nuclei. Its computational advantage is the ease with which it can be adjusted to the needs of the behavior by changing the contribution of individual PC SS based on error feedback.

Brain processing of visual information during fast eye movements maintains motor performance

Movement accuracy depends crucially on the ability to detect errors while actions are being performed. When inaccuracies occur repeatedly, both an immediate motor correction and a progressive adaptation of the motor command can unfold. Of all the movements in the motor repertoire of humans, saccadic eye movements are the fastest. Due to the high speed of saccades, and to the impairment of visual perception during saccades, a phenomenon called “saccadic suppression”, it is widely believed that the adaptive mechanisms maintaining saccadic performance depend critically on visual error signals acquired after saccade completion. Here, we demonstrate that, contrary to this widespread view, saccadic adaptation can be based entirely on visual information presented during saccades. Our results show that visual error signals introduced during saccade execution–by shifting a visual target at saccade onset and blanking it at saccade offset–induce the same level of adaptation as error signals, presented for the same duration, but after saccade completion. In addition, they reveal that this processing of intra-saccadic visual information for adaptation depends critically on visual information presented during the deceleration phase, but not the acceleration phase, of the saccade. These findings demonstrate that the human central nervous system can use short intra-saccadic glimpses of visual information for motor adaptation, and they call for a reappraisal of current models of saccadic adaptation.

Long term retention of saccadic adaptation is induced by a dark environmental context

Under many circumstances, motor memory needs to be retained for a long period of time to enable accurate behavior. Since the first introduction of the saccadic adaptation paradigm in 1960s, saccadic adaptation protocols have been widely used to study the mechanisms of motor learning and motor memory. However, previous studies reported that the effect of saccadic adaptation on the oculomotor system was rather short (minutes to hours) in human and non-human primates. Here we ask whether the fast decay of the effects of saccadic adaptation is due to the influence of environmental context. To test this hypothesis, we asked human subjects to perform a saccadic adaptation task in a very dark environment. Our data showed that saccade gain remained at the post-adaptation level 24-72h after exposure to the saccadic adaptation task without significant recovery, and that the effect of saccadic adaptation on saccade gain could still be found 2 months later, much longer than previously reported. Our results indicate a vital role for environmental context in the retention of saccadic adaptation.

A role for the parietal cortex in sensorimotor adaptation of saccades

Sensorimotor adaptation ensures movement accuracy despite continuously changing environment and body. Adaptation of saccadic eye movements is a classical model of sensorimotor adaptation. Beside the well-established role of the brainstem-cerebellum in the adaptation of reactive saccades (RSs), the cerebral cortex has been suggested to be involved in the adaptation of voluntary saccades (VSs). Here, we provide direct evidence for a causal involvement of the parietal cortex in saccadic adaptation. First, the posterior intraparietal sulcus (pIPS) was identified in each subject using functional magnetic resonance imaging (fMRI). Then, a saccadic adaptation paradigm was used to progressively reduce the amplitude of RSs and VSs, while single-pulse transcranial magnetic stimulation (spTMS) was applied over the right pIPS. The perturbations of pIPS resulted in impairment for the adaptation of VSs, selectively when spTMS was applied 60 ms after saccade onset. In contrast, the adaptation of RSs was facilitated by spTMS applied 90 ms after saccade initiation. The differential effect of spTMS relative to saccade types suggests a direct interference with pIPS activity for the VS adaptation and a remote interference with brainstem-cerebellum activity for the RS adaptation. These results support the hypothesis that the adaptation of VSs and RSs involves different neuronal substrates.

Fall prevention modulates decisional saccadic behavior in aging

As society ages and frequency of falls increases in older adults, counteracting motor decline is a challenging issue for developed countries. Physical activity based on aerobic and strength training as well as motor activity based on skill learning both help benefit balance and reduce the risk of falls, as assessed by clinical or laboratory measures. However, how such programs influence motor control is a neglected issue. This study examined the effects of fall prevention (FP) training on saccadic control in older adults. Saccades were recorded in 12 participants aged 64–91 years before and after 2.5 months training in FP. Traditional analysis of saccade timing and dynamics was performed together with a quantitative analysis using the LATER model, enabling us to examine the underlying motor control processes. Results indicated that FP reduced the rate of anticipatory and express saccades in inappropriate directions and enhanced that of express saccades in the appropriate direction, resulting in decreased latency and higher left-right symmetry of motor responses. FP reduced within-participant variability of saccade duration, amplitude, and peak velocity. LATER analysis suggested that FP modulates decisional thresholds, extending our knowledge of motor training influence on central motor control. We introduce the Threshold Interval Modulation with Early Release-Rate of rIse Deviation with Early Release (TIMER-RIDER) model to account for the results.