Adaptation of eye and hand movements to target displacements of different size

Previous work has documented that the direction of eye and hand movements can be adaptively modified using the double-step paradigm. Here we report that both motor systems adapt not only to small direction steps (5° gaze angle) but also to large ones (28° gaze angle). However, the magnitude of adaptation did not increase with step size, and the relative magnitude of adaptation therefore decreased from 67% with small steps to 15% with large steps. This decreasing efficiency of adaptation may reflect the participation of directionally selective neural circuits in double-step adaptation

Sensory Processing of Motor Inaccuracy Depends on Previously Performed Movement and on Subsequent Motor Corrections: A Study of the Saccadic System

When goal-directed movements are inaccurate, two responses are generated by the brain: a fast motor correction toward the target and an adaptive motor recalibration developing progressively across subsequent trials. For the saccadic system, there is a clear dissociation between the fast motor correction (corrective saccade production) and the adaptive motor recalibration (primary saccade modification). Error signals used to trigger corrective saccades and to induce adaptation are based on post-saccadic visual feedback. The goal of this study was to determine if similar or different error signals are involved in saccadic adaptation and in corrective saccade generation. Saccadic accuracy was experimentally altered by systematically displacing the visual target during motor execution. Post-saccadic error signals were studied by manipulating visual information in two ways. First, the duration of the displaced target after primary saccade termination was set at 15, 50, 100 or 800 ms in different adaptation sessions. Second, in some sessions, the displaced target was followed by a visual mask that interfered with visual processing. Because they rely on different mechanisms, the adaptation of reactive saccades and the adaptation of voluntary saccades were both evaluated. We found that saccadic adaptation and corrective saccade production were both affected by the manipulations of post-saccadic visual information, but in different ways. This first finding suggests that different types of error signal processing are involved in the induction of these two motor corrections. Interestingly, voluntary saccades required a longer duration of post-saccadic target presentation to reach the same amount of adaptation as reactive saccades. Finally, the visual mask interfered with the production of corrective saccades only during the voluntary saccades adaptation task. These last observations suggest that post-saccadic perception depends on the previously performed action and that the differences between saccade categories of motor correction and adaptation occur at an early level of visual processing

Cortical and cerebellar activation induced by reflexive and voluntary saccades

Reflexive saccades are driven by visual stimulation whereas voluntary saccades require volitional control. Behavioral and lesional studies suggest that there are two separate mechanisms involved in the generation of these two types of saccades. This study investigated differences in cerebral and cerebellar activation between reflexive and self-paced voluntary saccadic eye movements using functional magnetic resonance imaging. In two experiments (whole brain and cerebellum) using the same paradigm, differences in brain activations induced by reflexive and self-paced voluntary saccades were assessed. Direct comparison of the activation patterns showed that the frontal eye fields, parietal eye field, the motion-sensitive area (MT/V5), the precuneus (V6), and the angular and the cingulate gyri were more activated in reflexive saccades than in voluntary saccades. No significant difference in activation was found in the cerebellum. Our results suggest that the alleged separate mechanisms for saccadic control of reflexive and self-paced voluntary are mainly observed in cerebral rather than cerebellar areas

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

Longitudinal Visuomotor Development in a Malaria Endemic Area: Cerebral Malaria and Beyond

Paediatric cerebral malaria is the most serious complication of Plasmodium falciparum infection. While the majority recover, long-term cognitive impairment has been highlighted as a significant and neglected problem. Persistent or serious deficits in processes such as attention or behavioural inhibition should be manifest in changes to performance on oculomotor tasks. Therefore we investigated the impact of cerebral malaria on the development of reflexive pro-saccades and antisaccades. In a longitudinal study, 47 children previously admitted with retinopathy-confirmed cerebral malaria (mean age at admission 54 months), were compared with 37 local healthy controls (mean ages at first study visit 117 and 110 months respectively). In each of three or four test sessions, over a period of up to 32 months, participants completed 100 prosaccade tasks and 100 antisaccade tasks. Eye movements were recorded using infrared reflectance oculography; prosaccade, correct antisaccade and error prosaccade latency, and antisaccade directional error rate were calculated. Hierarchical linear modelling was used to investigate the effect of age and the influence of cerebral malaria on these parameters. Data were also collected from an independent, older group (mean age 183 months) of 37 local healthy participants in a separate cross-sectional study. Longitudinal data exhibited the expected decrease in latency with age for all saccade types, and a decrease in the antisaccade directional error rate. Hierarchical linear modelling confirmed that age had a statistically significant effect on all parameters (p< = 0.001). However, there were no statistically significant differences between the cerebral malaria and control groups. Combining groups, comparison with the literature demonstrated that antisaccade directional error rate for the Malawi sample was significantly higher than expected, while latencies for all saccade types were indistinguishable from published. The high directional error rate was also confirmed in the older, healthy Malawian participants from the cross sectional study. Our observation of similar oculomotor performance in cerebral malaria and control groups at long follow-up periods suggests that cerebral malaria survivors are not at a generally increased risk of persistent cognitive deficits. Our data raise questions about the prevailing hypothesis that cerebral malaria has gross impacts on the development of processes such as attention and behavioural inhibition. More importantly, our novel finding of a clear difference in antisaccade performance between all of the Malawi participants and published data suggests that the Malawian paediatric population as a whole faces serious challenges to cognitive development beyond cerebral malaria

Separate Neural Substrates in the Human Cerebellum for Sensory-motor Adaptation of Reactive and of Scanning Voluntary Saccades

International audienceSensory-motor adaptation processes are critically involved in maintaining accurate motor behavior throughout life. Yet their underlying neural substrates and task-dependency bases are still poorly understood. We address these issues here by studying adaptation of saccadic eye movements, a well-established model of sensory-motor plasticity. The cerebellum plays a major role in saccadic adaptation but it has not yet been investigated whether this role can account for the known specificity of adaptation to the saccade type (e.g., reactive versus voluntary). Two patients with focal lesions in different parts of the cerebellum were tested using the double-step target paradigm. Each patient was submitted to two separate sessions: one for reactive saccades (RS) triggered by the sudden appearance of a visual target and the second for scanning voluntary saccades (SVS) performed when exploring a more complex scene. We found that a medial cerebellar lesion impaired adaptation of reactive-but not of voluntary-saccades, Cerebellu

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

International audienceThe cerebellum is critically involved in the adaptation mechanisms that maintain the accuracy of goaldirected 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 ofvoluntary 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 ofthe 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, patientswith 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

Right-hemispheric dominance for visual remapping in humans

We review evidence showing a right-hemispheric dominance for visuo-spatial processing and representation in humans. Accordingly, visual disorganization symptoms (intuitively related to remapping impairments) are observed in both neglect and constructional apraxia. More specifically, we review findings from the intervening saccade paradigm in humans—and present additional original data—which suggest a specific role of the asymmetrical network at the temporo-parietal junction (TPJ) in the right hemisphere in visual remapping: following damage to the right dorsal posterior parietal cortex (PPC) as well as part of the corpus callosum connecting the PPC to the frontal lobes, patient OK in a double-step saccadic task exhibited an impairment when the second saccade had to be directed rightward. This singular and lateralized deficit cannot result solely from the patient's cortical lesion and, therefore, we propose that it is due to his callosal lesion that may specifically interrupt the interhemispheric transfer of information necessary to execute accurate rightward saccades towards a remapped target location. This suggests a specialized right-hemispheric network for visuo-spatial remapping that subsequently transfers target location information to downstream planning regions, which are symmetrically organized

Long-term sensorimotor and therapeutical effects of a mild regime of prism adaptation in spatial neglect. A double-blind RCT essay

Spatial neglect (SN) is commonly associated with poor functional outcome. Adaptation to a rightward optical deviation of vision has been shown to benefit to SN rehabilitation. The neurophysiological foundations and the optimal modalities of prism adaptation (PA) therapy however remain to be validated. This study is aimed at exploring the long-term sensory-motor, cognitive and functional effects produced by weekly PA sessions over a period of four weeks. A double-blind, monocentric randomized and controlled trial (RCT) was carried out. Twenty patients with left SN secondary to stroke were included, 10 in the prism group and 10 in the control group. The sensory-motor effects of PA were evaluated by measurement of manual and visual straight-ahead, and also by precision of pointing without visual feedback before and after each PA session. The functional independence measure (FIM) was evaluated before and at 1, 3 and 6 months after PA, while SN severity was assessed using the Behavioural Inattention Test (BIT) before and 6 months after PA. Before the intervention, only manual straight-ahead pointing constituted a reproducible sensory-motor measurement. During prism exposure, a questionnaire showed that not a single patient were aware of the direct effects of optical deviation on pointing movement performance. The sensory-motor after-effects produced by the PA produced a more rapid reduction of the rightward manual straight-ahead, which was secondarily followed by visual straight-ahead. These sensory-motor effects helped to clarify the action mechanisms of PA on SN. At the conclusion of the 6-month follow-up, the two groups showed similar improvement, indicating that a weekly PA session over 4 weeks was not sufficient to produce long-term functional benefit. This improvement was correlated with the evolution of visual straight-ahead, which can be proposed as a marker for patients outcome