Damage to cerebellocortical pathways after closed head injury: a behavioural and magnetic resonance imaging study

Real and imagined sensory feedback have comparable effects on action anticipation

The forward model monitors the success of sensory feedback to an action and links it to an efference copy originating in the motor system. The Readiness Potential (RP) of the electroencephalogram has been denoted as a neural signature of the efference copy. An open question is whether imagined sensory feedback works similarly to real sensory feedback. We investigated the RP to audible and imagined sounds in a button-press paradigm and assessed the role of sound complexity (vocal vs. non-vocal sound). Sensory feedback (both audible and imagined) in response to a voluntary action modulated the RP amplitude time-locked to the button press. The RP amplitude increase was larger for actions with expected sensory feedback (audible and imagined) than those without sensory feedback, and associated with N1 suppression for audible sounds. Further, the early RP phase was increased when actions elicited an imagined vocal (self-voice) compared to non-vocal sound. Our results support the notion that sensory feedback is anticipated before voluntary actions. This is the case for both audible and imagined sensory feedback and confirms a role of overt and covert feedback in the forward model.

Changes in motor preparation affect the sensory consequences of voice production in voice hearers

BACKGROUND

Auditory verbal hallucinations (AVH) are a cardinal symptom of psychosis but are also present in 6-13% of the general population. Alterations in sensory feedback processing are a likely cause of AVH, indicative of changes in the forward model. However, it is unknown whether such alterations are related to anomalies in forming an efference copy during action preparation, selective for voices, and similar along the psychosis continuum. By directly comparing psychotic and nonclinical voice hearers (NCVH), the current study specifies whether and how AVH proneness modulates both the efference copy (Readiness Potential) and sensory feedback processing for voices and tones (N1, P2) with event-related brain potentials (ERPs).

METHODS

Controls with low AVH proneness (n = 15), NCVH (n = 16) and first-episode psychotic patients with AVH (n = 16) engaged in a button-press task with two types of stimuli: self-initiated and externally generated self-voices or tones during EEG recordings.

RESULTS

Groups differed in sensory feedback processing of expected and actual feedback: NCVH displayed an atypically enhanced N1 to self-initiated voices, while N1 suppression was reduced in psychotic patients. P2 suppression for voices and tones was strongest in NCVH, but absent for voices in patients. Motor activity preceding the button press was reduced in NCVH and patients, specifically for sensory feedback to self-voice in NCVH.

CONCLUSIONS

These findings suggest that selective changes in sensory feedback to voice are core to AVH. These changes already show in preparatory motor activity, potentially reflecting changes in forming an efference copy. The results provide partial support for continuum models of psychosis.

Task-Dependent Recruitment of Modality-Specific and Multimodal Regions during Conceptual Processing

Conceptual knowledge is central to cognitive abilities such as word comprehension. Previous neuroimaging evidence indicates that concepts are at least partly composed of perceptual and motor features that are represented in the same modality-specific brain regions involved in actual perception and action. However, it is unclear to what extent the retrieval of perceptual-motor features and the resulting engagement of modality-specific regions depend on the concurrent task. To address this issue, we measured brain activity in 40 young and healthy participants using functional magnetic resonance imaging, while they performed three different tasks-lexical decision, sound judgment, and action judgment-on words that independently varied in their association with sounds and actions. We found neural activation for sound and action features of concepts selectively when they were task-relevant in brain regions also activated during auditory and motor tasks, respectively, as well as in higher-level, multimodal regions which were recruited during both sound and action feature retrieval. For the first time, we show that not only modality-specific perceptual-motor areas but also multimodal regions are engaged in conceptual processing in a flexible, task-dependent fashion, responding selectively to task-relevant conceptual features.

Approach to acute ataxia in childhood: diagnosis and evaluation

Ataxia refers to motor incoordination that is usually most prominent during movement or when a child is attempting to maintain a sitting posture. The first part of the review focuses on the anatomic localization of ataxia--both within the nervous system and without--using a combination of historical features and physical findings. The remainder of the review discusses etiological considerations that vary depending on the age group under consideration. In infancy, certain specific diseases, such as opsoclonus myoclonus ataxia syndrome, must receive special mention because the underlying disease process may be amenable to surgical intervention. In the toddler- and school-age groups, certain conditions (such as stroke and acute cerebellitis) require immediate recognition and imaging, whereas others (such as post-infectious ataxia and concussion) require close follow-up. Finally, mention must be made of diseases outside of the central nervous system that can present with ataxia, such as Guillain-Barré syndrome.

Motor abilities in autism: a review using a computational context

Altered motor behaviour is commonly reported in Autism Spectrum Disorder, but the aetiology remains unclear. Here, we have taken a computational approach in order to break down motor control into different components and review the functioning of each process. Our findings suggest abnormalities in two areas--poor integration of information for efficient motor planning, and increased variability in basic sensory inputs and motor outputs. In contrast, motor learning processes are relatively intact and there is inconsistent evidence for deficits in predictive control. We suggest future work on motor abilities in autism should focus on sensorimotor noise and on higher level motor planning, as these seem to have a significant role in causing motor difficulties for autistic individuals.

Self-reported gait unsteadiness in mildly impaired neurological patients: an objective assessment through statistical gait analysis

Background

Self-reported gait unsteadiness is often a problem in neurological patients without any clinical evidence of ataxia, because it leads to reduced activity and limitations in function. However, in the literature there are only a few papers that address this disorder. The aim of this study is to identify objectively subclinical abnormal gait strategies in these patients.

Methods

Eleven patients affected by self-reported unsteadiness during gait (4 TBI and 7 MS) and ten healthy subjects underwent gait analysis while walking back and forth on a 15-m long corridor. Time-distance parameters, ankle sagittal motion, and muscular activity during gait were acquired by a wearable gait analysis system (Step32, DemItalia, Italy) on a high number of successive strides in the same walk and statistically processed. Both self-selected gait speed and high speed were tested under relatively unconstrained conditions. Non-parametric statistical analysis (Mann–Whitney, Wilcoxon tests) was carried out on the means of the data of the two examined groups.

Results

The main findings, with data adjusted for velocity of progression, show that increased double support and reduced velocity of progression are the main parameters to discriminate patients with self-reported unsteadiness from healthy controls. Muscular intervals of activation showed a significant increase in the activity duration of the Rectus Femoris and Tibialis Anterior in patients with respect to the control group at high speed.

Conclusions

Patients with a subjective sensation of instability, not clinically documented, walk with altered strategies, especially at high gait speed. This is thought to depend on the mechanisms of postural control and coordination. The gait anomalies detected might explain the symptoms reported by the patients and allow for a more focused treatment design. The wearable gait analysis system used for long distance statistical walking assessment was able to detect subtle differences in functional performance monitoring, otherwise not detectable by common clinical examinations.

Is a cerebellar deficit the underlying cause of reading disabilities?

This study investigated whether children with dyslexia differed in their performance on reading, phonological, rapid naming, motor, and cerebellar-related tasks and automaticity measures compared to reading age (RA)-matched and chronological age (CA)-matched control groups. Participants were 51 children attending mainstream English elementary schools in Quebec. All participants completed measures of IQ, word and nonword reading fluency, elision, nonword decoding, rapid naming, bead threading, peg moving, toe tapping, postural stability, and muscle tone. Results from both group contrasts and analyses at the individual case level did not provide support for claims of motor-cerebellar involvement in either typical or atypical reading acquisition. Results were more consistent with a phonological core process account of both typical reading and reading difficulty. Phonological deficits for children with dyslexia compared to RA-matched controls were, however, only evident in group contrasts. Findings thus also have important implications for identifying at-risk readers among their same-aged peers.

Cerebellar ataxia: pathophysiology and rehabilitation

This series of articles for rehabilitation in practice aims to cover a knowledge element of the rehabilitation medicine curriculum. Nevertheless they are intended to be of interest to a multidisciplinary audience. The competency addressed in this article is 'The trainee consistently demonstrates a knowledge of management approaches for specific impairments including spasticity, ataxia.'

Visuokinesthetic perception of hand movement is mediated by cerebro-cerebellar interaction between the left cerebellum and right parietal cortex

Combination of visual and kinesthetic information is essential to perceive bodily movements. We conducted behavioral and functional magnetic resonance imaging experiments to investigate the neuronal correlates of visuokinesthetic combination in perception of hand movement. Participants experienced illusory flexion movement of their hand elicited by tendon vibration while they viewed video-recorded flexion (congruent: CONG) or extension (incongruent: INCONG) motions of their hand. The amount of illusory experience was graded by the visual velocities only when visual information regarding hand motion was concordant with kinesthetic information (CONG). The left posterolateral cerebellum was specifically recruited under the CONG, and this left cerebellar activation was consistent for both left and right hands. The left cerebellar activity reflected the participants' intensity of illusory hand movement under the CONG, and we further showed that coupling of activity between the left cerebellum and the "right" parietal cortex emerges during this visuokinesthetic combination/perception. The "left" cerebellum, working with the anatomically connected high-order bodily region of the "right" parietal cortex, participates in online combination of exteroceptive (vision) and interoceptive (kinesthesia) information to perceive hand movement. The cerebro-cerebellar interaction may underlie updating of one's "body image," when perceiving bodily movement from visual and kinesthetic information.

Changes in intracortical circuits of the human motor cortex following theta burst stimulation of the lateral cerebellum

OBJECTIVE

The cerebellum takes part in several motor functions through its influence on the motor cortex (M1). Here, we applied the theta burst stimulation (TBS) protocol, a novel form of repetitive Transcranial Magnetic Stimulation (rTMS) over the lateral cerebellum. The aim of this study was to test whether TBS of the lateral cerebellum could be able to modulate the excitability of the contralateral M1 in healthy subjects.

METHODS

Motor-evoked potentials (MEPs) amplitude, short intracortical inhibition (SICI), long intracortical inhibition (LICI) and short intracortical facilitation (SICF) were tested in the M1 before and after cerebellar continuous TBS (cTBS) or intermittent TBS (iTBS).

RESULTS

We found that cTBS induced a reduction of SICI and an increase of LICI. On the other hand, cerebellar iTBS reduced LICI. MEPs amplitude also differently vary following cerebellar stimulation with cTBS or iTBS, resulting in a decrease by the former and an increase by the latter.

CONCLUSIONS

Although the interpretation of these data remains highly speculative, these findings reveal that the cerebellar cortex undergoes bidirectional plastic changes that modulate different intracortical circuits within the contralateral primary motor cortex.

SIGNIFICANCE

Long-lasting modifications of these pathways could be useful to treat various pathological conditions characterized by an altered cortical excitability.

Fear of falling after brain injury

Objectives: To investigate the prevalence and nature of fear of falling in a sample of people with severe acquired brain injury.

Design: A descriptive study.

Setting: A regional inpatient neurological rehabilitation unit.

Participants: One hundred and five adults with acquired brain injury of mixed aetiology.

Procedure: All 105 participants were rated by observers who were asked to judge the degree to which fear behaviour interfered with rehabilitation therapy (activity limitation). Eighty-two participants also rated themselves. They were asked to report the degree of distress caused by fear. Both participants and observers were asked to describe the focus of any reported fear. Two stepwise logistic regression analyses were carried out to identify variables that predicted fear giving rise to significant activity limitation and fear giving rise to significant subjective distress.

Main measures: Self and observer rating scales designed and constructed specifically for the study.

Results: Raters reported significant fear-related activity limitation in 12—15% of participants. Significant fear-related subjective distress was reported by 40% of participants. Fear of falling, fear of physical harm and fear of not making sufficient rehabilitation progress dominated the reports of both observers and participants. The variables predicting significant activity limitation were premorbid alcohol misuse, low functional ability and the occurrence of a fall since onset. The variables predicting significant subjective distress were poor motor coordination and organization, and good verbal comprehension.

Conclusion: Fear of falling is a clinically significant phenomenon in younger adults recovering from severe acquired brain injury. Fear sufficient to cause high degrees of subjective distress was often not evident to observers. Proactive questioning about fear of falling is therefore advisable when working clinically with this group.

Motor deficits and recovery during the first year following mild closed head injury

OBJECTIVE

This study examined motor impairments over 1 year following mild closed head injury (CHI). It is the first study to serially assess long-term oculomotor and upper-limb visuomotor function following mild head trauma.

METHODS

Thirty-seven patients with mild CHI and 37 matched controls were compared at 1 week, 3 months and 6 months and 31 available pairs at 12 months post-injury on measures of saccades, oculomotor smooth pursuit, upper-limb visuomotor function and neuropsychological performance. Symptomatic recovery was sampled using the Rivermead Postconcussion Symptoms Questionnaire.

RESULTS

At 1 week, the group with CHI reported high levels of post-concussional symptoms and exhibited prolonged saccade latencies, increased directional errors, decreased saccade accuracy and impaired fast sinusoidal smooth pursuit concomitant with increased arm movement reaction time, decreased arm movement speed and decreased motor accuracy on upper-limb visuomotor tracking tasks. Neuropsychological testing identified deficits only in verbal learning and speed of processing while attention, short-term/working memory and general cognitive performance were preserved. At 3 and 6 months, the group with CHI continued to show deficits on several oculomotor and upper-limb visuomotor measures in combination with some deficits on verbal learning and improved, yet abnormal, levels of post-concussional symptoms. At 12 months, the group with CHI had no cognitive impairment but residual deficits in eye and arm motor function and continued to show elevated levels of post-concussional symptoms.

CONCLUSIONS

The findings indicate that multiple motor systems are measurably impaired up to 12 months following mild CHI and that instrumented motor assessment may provide sensitive and objective markers of cerebral dysfunction during recovery from mild head trauma independent of neuropsychological assessment and patient self-report.

Depth perception in cerebellar and basal ganglia disease

There is increasing evidence that the cerebellum and the basal ganglia serve not only a role in motor control but also in visual perception. Patients with Parkinson's disease (PD) as well as patients with cerebellar lesions exhibit impairments of vision that are not fully explained by ocular motor deficits. It is less clear to which extent these visual deficits contribute to an impaired control of visually guided movements. This study examined whether a dysfunction of the cerebellum or the basal ganglia induces impairments in depth perception, which affect action. We employed an illusionary display, the Ames trapezoidal window, to determine the ability of PD patients (n=10) and patients with spinocerebellar ataxia (SCA) (n=6) to process depth cues when estimating object slant. Participants either pointed to the edges of the window (motor judgement) or verbally indicated the perceived orientation of the display (verbal judgement). To control for ocular and limb motor deficits, participants judged the slant of a non-illusionary display in a second task. Slant estimation of the non-illusionary window was not impaired in either patient group when compared to control subjects (all P>0.2). In contrast, SCA as well as PD patients exhibited significantly greater slant estimation errors than controls when pointing to the illusionary window (P=0.005). In addition, both patient groups made larger errors than controls in their verbal judgements during binocular viewing of the illusion (P=0.005), but not during monocular viewing (P>0.2). In sum, the present findings point towards a role for both the basal ganglia and cerebellum for the processing of visual information about depth. Since the deficits were seen both in the context of action and perception and were only partially reconciled by the availability of binocular depth cues, we conclude that basal ganglia as well as cerebellar disease may affect the visual perception of depth.

Acute transient cerebellar dysfunction and stuttering following mild closed head injury

INTRODUCTION

A wide range of pathologies can cause cerebellar dysfunction but there have been few reports of transient dysfunction after mild head injury. There are none describing stuttering as an acute symptom after such injuries.

CASE REPORT

We report a 12-year-old boy who presented with headache, stuttering speech and truncal and bilateral peripheral cerebellar dysfunction after falling prone from his skateboard. Computed tomographic brain imaging showed normal results. All clinical symptoms and signs had resolved within 24 h obviating further investigation. A follow up clinical interview and examination 4 weeks later revealed no recurrence of cerebellar or speech dysfunction.

CONCLUSION

This is the first reported case of acute transient cerebellar dysfunction and stuttering after mild closed head injury. Neurogenic causes for stuttering are increasingly being recognised.

The effect of rTMS over the cerebellum in normal human volunteers on peg-board movement performance

Low frequency rTMS over the paramedian part of the right cerebellum was used to test the effects of TMS-induced disruption of the cerebellum on performance of the 10-hole pegboard task. A test group (n = 14) showed significantly increased movement times lasting about 3 min after the 5-min 1 Hz rTMS train, compared to a control group who received no rTMS (n = 14), tested in a parallel group design. The increase was greatest for the hand ipsilateral to the stimulation, but the difference between the two hands was not statistically significant. These results suggest that the rTMS affects cerebellar excitability and cause a short-lasting bilateral change in sensory-motor performance.

Recovery of ambulation after traumatic brain injury

OBJECTIVES

To identify variables that are predictive of independent ambulation after traumatic brain injury (TBI) and to define the time course of recovery.

DESIGN

Retrospective review of consecutive admissions of patients with severe TBI over a 32-month period.

SETTING

Brain injury unit in an acute, inpatient rehabilitation hospital.

PARTICIPANTS

Of 264 patients screened, 116 met criteria that included the ability to participate in motor and functional evaluation on admission to acute rehabilitation, and the absence of other neurologic disorders or fractures that affect one's ability to ambulate.

INTERVENTION

Inpatient rehabilitation on a specialized TBI unit by an interdisciplinary team.Main outcome measures Recovery of independent ambulation and time to recover independent ambulation.

RESULTS

Of eligible patients, 73.3% achieved independent ambulation by latest follow-up (up to 5.1 mo). Patients who achieved independent ambulation were significantly younger (P<.05), had better gait scores on admission (P<.05), and tended to be less severely injured-based on duration of posttraumatic amnesia (PTA; P=.058)-than those who did not ambulate independently. There were no differences in recovery based on neuropathologic profile. Mean time to independent ambulation +/- standard deviation was 5.7+/-4.3 weeks; of those achieving independent ambulation, 82.4% did so by 2 months and 94.1% by 3 months. If not independent by 3 months postinjury, patients had a 13.9% chance of recovery. Multivariate regression analysis generated prediction models for time to independent ambulation, using admission FIM instrument scores and age (38% of variance); initial gait score, loss of consciousness, and age (40% of variance); or initial gait score and PTA (58% of variance), when restricted to just those patients with diffuse axonal injury.

CONCLUSIONS

Most patients with severe TBI achieved independent ambulation; the vast majority did so within 3 months postinjury. Functional measures, injury severity measures, and age can help guide prognosis and expectations for time to recover.

Supplementary motor area provides an efferent signal for sensory suppression

Voluntary actions produce suppression of neural activity in sensory areas, and reduced levels of conscious sensation. Recent computational models of motor control have linked sensory suppression to motor prediction: an efferent signal from motor areas may cancel the sensory reafferences predicted as a consequence of movement. Direct evidence for the efferent mechanism in sensory suppression has been lacking. We investigated the perceived size of finger-muscle twitches (MEPs) evoked by TMS in eight normal subjects. Subjects freely chose on each trial whether to make or withhold a voluntary flexion of the right index finger, in synchrony with an instructional stimulus. A test MEP occurred at the instructed time of action. The subject then relaxed and a second reference MEP occurred a few seconds later. Subjects judged which of the two MEPs was larger. Subjects perceived the first test MEP to be smaller in trials where they made voluntary actions than on trials where they did not, demonstrating sensory suppression. On randomly selected trials, a conditioning prepulse was delivered over the supplementary motor area (SMA) 10 ms before the pulse producing the test MEP. The SMA prepulse reduced and almost abolished the sensory suppression effect in voluntary action trials. We suggest the SMA may provide an efferent signal which is used by other brain areas to modulate somatosensory activity during self-generated movement.

Neuronal activity related to the visual representation of arm movements in the lateral cerebellar cortex

Testing the hypothesis that the lateral cerebellum forms a sensory representation of arm movements, we investigated cortical neuronal activity in two monkeys performing visually guided step-tracking movements with a manipulandum. A virtual target and cursor image were viewed co-planar with the manipulandum. In the normal task, manipulandum and cursor moved in the same direction; in the mirror task, the cursor was left-right reversed. In one monkey, 70- and 200-ms time delays were introduced on cursor movement. Significant task-related activity was recorded in 31 cells in one animal and 142 cells in the second: 10.2% increased activity before arm movements onset, 77.1% during arm movement, and 12.7% after the new position was reached. To test for neural representation of the visual outcome of movement, firing rate modulation was compared in normal and mirror step-tracking. Most task-related neurons (68%) showed no significant directional modulation. Of 70 directionally sensitive cells, almost one-half (n = 34, 48%) modulated firing with a consistent cursor movement direction, many fewer responding to the manipulandum direction (n = 9, 13%). For those "cursor-related" cells tested with delayed cursor movement, increased activity onset was time-locked to arm movement and not cursor movement, but activation duration was extended by an amount similar to the applied delay. Hence, activity returned to baseline about when the delayed cursor reached the target. We conclude that many cells in the lateral cerebellar cortex signaled the direction of cursor movement during active step-tracking. Such a predictive representation of the arm movement could be used in the guidance of visuo-motor actions.

The cerebellum and the timing of coordinated eye and hand tracking

The cerebellum is known to have important functions in motor control, coordination, motor learning, and timing. It may have other "higher" functions as well, up to and including cognitive processing independent of motor behavior. In this article, we will review some of the evidence from functional imaging, lesion studies, electrophysiological recordings, and anatomy which support the theory that the cerebellum provides a "forward model" of the motor system. This forward model would be used for control of movement; it could also underlie a cerebellar role in coordination. In this role, the forward model would generate time-specific signals predicting the motion of each motor effector, essential for predictive control of, for example, eye and hand movements. Data are presented from human eye and hand tracking that support this. Tracking performance is better if eye and hand follow the same spatial trajectory, but better still if the eye leads the hand by about 75 to 100 ms. This suggests that information from the ocular control system feeds into the manual control system to assist its tracking.

Changes in the human brain during rhythm learning

Subjects were scanned with PET while they learned a complex arbitrary rhythm, paced by visual cues. In the comparison condition, the intervals were varied randomly. The behavioral results showed that the subjects decreased their response time with training, thus becoming more accurate in responding to the pacing cues at the appropriate time. There were learning-related increases in the posterior lateral cerebellum (lobule HVIIa), intraparietal and medial parietal cortex, presupplementary motor area (pre-SMA), and lateral premotor cortex. Learning-related decreases were found in the prestriate and inferior temporal cortex, suggesting that with practice the subjects increasingly came to depend on internal rather than external cues to time their responses. There were no learning-related increases in the basal ganglia. It is suggested that it is the neocortical-cerebellar loop that is involved in the timing and coordination of responses.

Functional anatomy of nonvisual feedback loops during reaching: a positron emission tomography study

Reaching movements performed without vision of the moving limb are continuously monitored, during their execution, by feedback loops (designated nonvisual). In this study, we investigated the functional anatomy of these nonvisual loops using positron emission tomography (PET). Seven subjects had to "look at" (eye) or "look and point to" (eye-arm) visual targets whose location either remained stationary or changed undetectably during the ocular saccade (when vision is suppressed). Slightly changing the target location during gaze shift causes an increase in the amount of correction to be generated. Functional anatomy of nonvisual feedback loops was identified by comparing the reaching condition involving large corrections (jump) with the reaching condition involving small corrections (stationary), after subtracting the activations associated with saccadic movements and hand movement planning [(eye-arm-jumping minus eye-jumping) minus (eye-arm-stationary minus eye-stationary)]. Behavioral data confirmed that the subjects were both accurate at reaching to the stationary targets and able to update their movement smoothly and early in response to the target jump. PET difference images showed that these corrections were mediated by a restricted network involving the left posterior parietal cortex, the right anterior intermediate cerebellum, and the left primary motor cortex. These results are consistent with our knowledge of the functional properties of these areas and more generally with models emphasizing parietal-cerebellar circuits for processing a dynamic motor error signal.

Increased response to visual feedback of drug-induced dyskinetic movements in advanced Parkinson's disease

To investigate the response to visual feedback of involuntary movements which have a frequency composition similar to cerebellar tremor but are not caused by cerebellar damage, we have tested six advanced Parkinson's disease (PD) patients with drug-induced dyskinetic movements using visually guided wrist tracking tasks. Tracking performance was assessed under three visual conditions: (1) both guiding target and movement cursor were displayed continuously; (2) the target display was turned off for the second half of each trial; or (3) the cursor display, but not the target, was turned off for the second half of each trial. The response to visual feedback of drug-induced dyskinetic movements at 1-5 Hz in these advanced PD patients were significantly increased than in normal controls. This suggests that increased response to visual feedback might be a common feature of low frequency involuntary movements and not directly caused by cerebellar damages.

Functional anatomy of nonvisual feedback loops during reaching: a positron emission tomography study

Reaching movements performed without vision of the moving limb are continuously monitored, during their execution, by feedback loops (designated nonvisual). In this study, we investigated the functional anatomy of these nonvisual loops using positron emission tomography (PET). Seven subjects had to "look at" (eye) or "look and point to" (eye-arm) visual targets whose location either remained stationary or changed undetectably during the ocular saccade (when vision is suppressed). Slightly changing the target location during gaze shift causes an increase in the amount of correction to be generated. Functional anatomy of nonvisual feedback loops was identified by comparing the reaching condition involving large corrections (jump) with the reaching condition involving small corrections (stationary), after subtracting the activations associated with saccadic movements and hand movement planning [(eye-arm-jumping minus eye-jumping) minus (eye-arm-stationary minus eye-stationary)]. Behavioral data confirmed that the subjects were both accurate at reaching to the stationary targets and able to update their movement smoothly and early in response to the target jump. PET difference images showed that these corrections were mediated by a restricted network involving the left posterior parietal cortex, the right anterior intermediate cerebellum, and the left primary motor cortex. These results are consistent with our knowledge of the functional properties of these areas and more generally with models emphasizing parietal-cerebellar circuits for processing a dynamic motor error signal.

Dissociation of 'on-line' and 'off-line' visuomotor control of the arm by focal lesions in the cerebellum and brainstem

Visuomotor control of the arm was assessed in a single case study of a subject with focal lesions in the cerebellum and brainstem. A dissociation between 'on-line' and 'off-line' visuomotor control was revealed: impairments in 'on-line' visuomotor control included inaccuracy of tracking velocity, increase in spatial pointing variability and a delay in simple reaction time; whereas the patient was able to adapt to a gain change in 'off-line' visual feedback during a pointing task, and his adaptation was less affected than that of control subjects by trial-to-trial random fluctuations in 'off-line' visual feedback. We conclude that focal damage in the cerebellar peduncles may be principally responsible for this dissociation.

Coordinating actions

The motor system composes complex actions by combining simpler submovements. This presumably involves sharing information about the progress of one submovement with the centres controlling another submovement, to ensure that the second happens in an appropriate relation to the first. This process is called coordination. In this paper I discuss evidence that coordinating actions indeed involves an active process of sharing information about the current state of movements. Coordination appears to be qualitatively different from the process of reacting to external stimuli. This may reflect the importance of predictive representations in coordination. Finally, the processes underlying coordination appear to be organized in a response-specific fashion, as a number of relatively independent circuits. The development and tuning of these circuits may, in part, be what makes an action "skilled".

Analysis of action tremor and impaired control of movement velocity in multiple sclerosis during visually guided wrist-tracking tasks

We investigated the relationship between action tremor (AT) and impaired control of movement velocity (MV) in visually guided tracking tasks, in normal subjects and in patients with multiple sclerosis (MS) with or without motor deficits. The effects of withdrawing visual feedback of either the target or the cursor were then investigated. Visually cued simple reaction times (SRTs) were also measured. The effects of thalamotomy on motor performance in these tasks were evaluated in seven patients. In the MS patients with tremor, there was no correlation between AT and impairment in control of MV, but the latter was highly correlated with an increased delay in SRT. Withdrawal of visually guiding cues increased the error significantly in MV, but reduced AT by approximately 30% in magnitude. Frequency analysis indicated that the AT had two components: (a) non-visual-dependent, oscillatory movements, mainly at 4 Hz; and (2) visual-dependent, repetitive movements, with significant power at 1-2 Hz. Thalamotomy significantly reduced AT but hardly improved accuracy in MV. These results suggest that visual feedback of a spatial mismatch signal may provoke a visually dependent repetitive movement contributing to AT. Conduction delays along either the cortico-cerebello-cortical or the proprioceptive pathways and impaired working memory caused by MS may be responsible for the movement disorders in these patients.

Forward Models for Physiological Motor Control

Based on theoretical and computational studies it has been suggested that the central nervous system (CNS) internally simulates the behaviour of the motor system in planning, control and learning. Such an internal "forward" model is a representation of the motor system that uses the current state of the motor system and motor command to predict the next state. We will outline the uses of such internal models for solving several fundamental computational problems in motor control and then review the evidence for their existence and use by the CNS. Finally we speculate how the location of an internal model within the CNS may be identified. Copyright 1996 Elsevier Science Ltd.