Biomedical subjects

P van Donkelaar

Publications and source records attributed to P van Donkelaar.

At least 19 recordsLinked to original sources

PubMed2005

Attentional deficits in concussion.

PRIMARY OBJECTIVE: The purpose of the present study was to examine deficits in the alerting, orienting and executive components of attention in individuals who have recently suffered a concussion. RESEARCH DESIGN: A group design was used in which the performance by individuals with concussion was compared to control subjects matched for age, height, weight and activity level. METHODS AND PROCEDURES: Participants completed the Attentional Network Test (ANT) that breaks down attention into alerting, orienting and executive components. Reaction time and response accuracy were the dependent variables. MAIN OUTCOMES AND RESULTS: It was found that only the orienting and executive components of attention were affected by concussion, whereas the alerting component was normal. Furthermore, participants with concussion required a significantly longer time than controls to initiate correct responses. CONCLUSIONS: These results suggest that the orienting and executive components of attention are most susceptible to the effects of concussion.

Adolescent↗

PubMed2000

Eye-hand coordination to visual versus remembered targets.

It has been suggested that the basal ganglia preferentially contribute to movements made to remembered targets, whereas the cerebellum preferentially contributes to movements based on visual cues. Thus, it is possible that eye-hand coordination may differ in these two types of movement. To examine this issue we compared the response characteristics of combined eye and hand movements made towards visual versus remembered targets. In addition, the influence of the eye movement on the hand movement was investigated by comparing the effects of visual fixation in each task. Our results demonstrated that hand movement amplitude was greater when the hand movements were produced in isolation versus in combination with an eye movement. This was true regardless of whether the movement was made to a visual or a remembered target. This suggests that the integration of eye position information into the manual motor response occurs at a common neural site for both tasks. By contrast, the timing between saccade and hand onsets and offsets differed in the two conditions. This is consistent with the idea that the timing inherent in eye-hand coordination is the result of separate processing within either the basal ganglia or cerebellar systems. Taken together, the results from this study demonstrate that certain processes underlying eye-hand coordination during movements to visual versus remembered targets share a common neural substrate whereas others function independently.

Adult↗

PubMed2000

The role of proprioception and attention in a visuomotor adaptation task.

The role of proprioception in the control and adaptation of visuomotor relationships is still unclear. We have studied a deafferented subject, IW, and control subjects in a task in which they used single joint elbow extension to move to a visual target, with visual feedback of the terminal position provided by a cursor displayed in the plane of their movements. We report the differences in movement accuracy between the deafferented subject and controls in the normal task and when challenged with a cognitive load, counting backwards. All subjects were less accurate when counting; this was a small effect for the controls (<10% change) but much greater for the deafferented subject (>60% change). We also examined changes in movement kinematics when the instructed amplitude was altered via a changed gain between final arm position and presentation of the feedback cursor. The deafferented subject maintained temporal movement parameters stable and altered amplitude by scaling force (i.e. changed peak velocity), whereas the controls scaled both movement velocity and duration. Finally, we compared the subjects' adaptation of movement amplitude after a period of exposure to the changed visuomotor gain. The deafferented subject was able to adapt, but his adaptation was severely impaired by the counting task. These results suggest that proprioception is not an absolute requirement for adaptation to occur. Instead, proprioception has a more subtle role to play in the adjustment to visuomotor perturbations. It has an important role in the control of reaching movements, while in the absence of proprioception, attention appears necessary to monitor movements.

Adaptation, Physiological↗

PubMed2000

Temporary inactivation in the primate motor thalamus during visually triggered and internally generated limb movements.

To better understand the contribution of cerebellar- and basal ganglia-receiving areas of the thalamus [ventral posterolateral nucleus, pars oralis (VPLo), area X, ventral lateral nucleus, pars oralis (VLo), or ventral anterior nucleus, pars parvicellularis (VApc)] to movements based on external versus internal cues, we temporarily inactivated these individual nuclei in two monkeys trained to make visually triggered (VT) and internally generated (IG) limb movements. Infusions of lignocaine centered within VPLo caused hemiplegia during which movements of the contralateral arm rarely were performed in either task for a short period of time ( approximately 5-30 min). When VT responses were produced, they had prolonged reaction times and movement times and a higher incidence of trajectory abnormalities compared with responses produced during the preinfusion baseline period. In contrast, those IG responses that were produced remained relatively normal. Infusions centered within area X never caused hemiplegia. The only deficits observed were an increase in reaction time and movement amplitude variability and a higher incidence of trajectory abnormalities during VT trials. Every other aspect of both the VT and IG movements remained unchanged. Infusions centered within VLo reduced the number of movements attempted during each block of trials. This did not appear to be due to hemiplegia, however, as voluntary movements easily could be elicited outside of the trained tasks. The other main deficit resulting from inactivation of VLo was an increased reaction time in the VT task. Finally, infusions centered within VApc caused IG movements to become slower and smaller in amplitude, whereas VT movements remained unchanged. Control infusions with saline did not cause any consistent deficits. This pattern of results implies that VPLo and VLo play a role in the production of movements in general regardless of the context under which they are performed. They also suggest that VPLo contributes more specifically to the execution of movements that are visually triggered and guided, whereas area X contributes specifically to the initiation of such movements. In contrast, VApc appears to play a role in the execution of movements based on internal cues. These results are consistent with the hypothesis that specific subcircuits within the cerebello- and basal ganglio-thalamo-cortical systems preferentially contribute to movements based on external versus internal cues.

Analysis of Variance↗

PubMed2000

Changes in motion perception following oculomotor smooth pursuit adaptation.

The hypothesis that oculomotor smooth pursuit (SP) adaptation is accompanied by alterations in velocity perception was tested by assessing coherence thresholds, using random-dot kinematograms before and after the adaptation paradigm. The results showed that the sensitivity to coherent motion at 10 deg/sec (the initial target velocity during adaptation) was reduced after the SP adaptation, ending up at a level that was between those normally observed for velocities of 10 and 20 deg/sec. This is consistent with an overestimation of the velocity of the coherent motion and suggests that SP adaptation alters not only the oculomotor output, but also the perception of target velocity.

Acceleration↗

PubMed1999

Spatiotemporal modulation of attention during smooth pursuit eye movements.

The present investigation examined how attention is distributed across both space and time during smooth pursuit eye movements. This was accomplished by measuring manual button pressing latencies to the sudden appearance of a peripheral target during the onset, maintenance, or offset of the pursuit response to a step-ramp target motion. The results showed that manual response latencies were shorter for stimuli flashed ahead of the pursuit target than for those presented in its wake. In addition, the latencies to the peripheral target appearance were shorter overall during pursuit maintenance compared to pursuit onset or offset. Taken together, these results indicate that attention is significantly modulated both in space and time during smooth pursuit eye movements.

Adult↗

PubMed1999

Pointing movements are affected by size-contrast illusions.

The influence that the perceived size of visual targets has on the characteristics of pointing movements was investigated in the present study. A size-contrast illusion, known as the Ebbinghaus or Tichener circles, was employed. In this illusion, a target circle surrounded by several smaller circles is perceived to be larger than a target circle of the same physical size surrounded by several larger circles. Movement times of open-loop pointing responses directed to the perceptually smaller target circle were significantly longer than the movement times of pointing responses directed to the perceptually larger target circle. The extent of this difference was similar to that observed when pointing responses were directed at physically different-sized target circles that were not surrounded by other circles. In addition, when the perceptually smaller circle was enlarged so that it appeared to be the same size as the perceptually larger circle, the movement times became equivalent. This evidence supports the contention that the relative rather than the absolute size of the target has a major impact on the control and execution of pointing movements. Such a conclusion contradicts those made previously concerning grasping movements made under similar conditions and implies that pointing responses are more directly influenced by visual perceptual processing than grasping responses.

Adult↗

PubMed1999

Neuronal activity in the primate motor thalamus during visually triggered and internally generated limb movements.

Single-unit recordings were made from the basal-ganglia- and cerebellar-receiving areas of the thalamus in two monkeys trained to make arm movements that were either visually triggered (VT) or internally generated (IG). A total of 203 neurons displaying movement-related changes in activity were examined in detail. Most of these cells (69%) showed an increase in firing rate in relation to the onset of movement and could be categorized according to whether they fired in the VT task exclusively, in the IG task exclusively, or in both tasks. The proportion of cells in each category was found to vary between each of the cerebellar-receiving [oral portion of the ventral posterolateral nucleus (VPLo) and area X] and basal-ganglia-receiving [oral portion of the ventral lateral nucleus (VLo) and parvocellular portion of the ventral anterior nucleus (VApc)] nuclei that were examined. In particular, in area X the largest group of cells (52%) showed an increase in activity during the VT task only, whereas in VApc the largest group of cells (53%) fired in the IG task only. In contrast to this, relatively high degree of task specificity, in both VPLo and VLo the largest group of cells ( approximately 55%) burst in relation to both tasks. Of the cells that were active in both tasks, a higher proportion were preferentially active in the VT task in VPLo and area X, and the IG task in VLo and VApc. In addition, cells in all four nuclei became active earlier relative to movement onset in the IG task compared with the VT task. These results demonstrate that functional distinctions do exist in the cerebellar- and basal-ganglia-receiving portions of the primate motor thalamus in relation to the types of cues used to initiate and control movement. These distinctions are most clear in area X and VApc, and are much less apparent in VPLo and VLo.

Animals↗

PubMed1998

Saccade amplitude influences pointing movement kinematics.

The amplitude of open-loop pointing movements to step displacements in target position is influenced by the amplitude of simultaneously produced saccadic eye movements. The time course over which this occurs was addressed in the present study. Analysis of the pointing kinematics showed that saccade amplitude had its effect only during the initial acceleration of the hand. Moreover, the magnitude of the initial acceleration was correlated with the difference in the onset times of the eye and hand movements: the closer in time the saccadic and pointing responses were initiated the larger the initial hand acceleration. Taken together, these results demonstrate that saccades influence the kinematics of simultaneously produced limb movements but only over a limited time frame.

Acceleration↗

PubMed1997

Eye-hand interactions during goal-directed pointing movements.

Saccadic eye and hand movements made to step displacements in target position were measured under conditions designed to dissociate the output of the ocular and manual motor systems. This was accomplished by having subjects look and point, either with or without vision of the hand (closed or open loop, respectively) at peripheral targets starting from independent initial positions. The results showed that the amplitude of open loop pointing responses increased in size when accompanied by saccades that were larger than the required hand movement. Providing the subject with visual feedback of the hand during the response or asking them to visually fixate caused this effect to disappear. Taken together, this pattern of results suggests that when vision of the hand is unavailable the programming of saccade metrics influences the control of simultaneously produced pointing movements in an on-line manner.

Adult↗

PubMed1997

The role of ocular muscle proprioception during modifications in smooth pursuit output.

The output of the smooth pursuit (SP) system can be increased by adding a portion of the recorded eye motion onto target motion, producing a situation analogous to that occurring with weakened ocular muscles. This change is most likely the result of alterations in the signals that code eye and target motion. We have assessed the contribution of one such signal, that arising from ocular proprioception, to the modification process during monocular SP by preventing the motion of the non-viewing eye with a suction scleral lens. The large increases normally observed for SP velocity following the modification period were substantially reduced under these conditions. Similar alterations were also observed in a manual tracking task. These results demonstrate that ocular proprioceptive signals serve to stabilize the output of the SP system following perturbations, via the recoding of eye and target motion.

Humans↗

PubMed1995

Encoding the position of a flashed visual target after passive body rotations.

The capacity of the central nervous system (CNS) for processing vestibular signals during passive whole-body rotations to update the internal representation of a visual target position in relation to the body was assessed. Results showed that subjects mislocalized previously presented visual targets after body rotations in complete darkness. Detailed analysis of the results suggested that the large target mislocalization stemmed not only from a systematic underestimation of rotation magnitude but also from the incapacity of the CNS to use the vestibular signals to accurately update the internal representation of the target position in relation to the body after passive rotations.

Adult↗

PubMed1995

Mechanisms underlying functional recovery following stroke.

This article reviews recent evidence from animal experiments indicating that there is considerable potential for reorganization of representations and functions in in sensory and motor cortex following localized lesions or various manipulations of peripheral target structures. Three major mechanisms for this plastic reorganization are considered: unmasking of existing but functionally inactive pathways, sprouting of fibers from surviving neurons and formation of new synapses, and redundancy of CNS circuitry allowing alternative pathways to take over functions. Studies using positron emission tomography or transcranial magnetic stimulation suggest that similar forms of neuroplasticity may occur in the human brain and could contribute to functional recovery following stroke. The potential therapeutic implications are discussed.

Animals↗

PubMed1994

Adaptive modification of oculomotor pursuit influences manual tracking responses.

We have addressed the question of whether adaptively modifying the oculomotor response to a visual pursuit stimulus has an influence on a related manual tracking response. Subjects used their unseen right hand to track targets moving at constant velocities while visually fixating a stationary LED. Manual tracking performance was compared before and after a 20 min period during which smooth pursuit eye movements alone were adaptively enhanced by adding 50% of the instantaneous eye position signal to target position. Compared with the preadaptation trials, hand gain was markedly increased during the postadaptation period. These results imply that the adaptation occurred at a level common to both motor systems, probably in CNS structures concerned with visual motion processing.

Adaptation, Physiological↗

PubMed1994

The contribution of retinal and extraretinal signals to manual tracking movements.

We have assessed the contribution made by retinal and extraretinal signals when subjects used their hand to track targets moving at constant velocities. Comparisons were made between responses produced under the following conditions: (1) with full vision of the hand and unrestricted movement of the eyes, (2) without vision of the hand or (3) while visually fixating a stationary LED. Target velocity was varied in a pseudo-random order across trials. In each condition response latency decreased as target velocity was increased. There was a approximately 24 ms increase in latency when vision of the hand was removed or eye movements were restricted. Under normal conditions, subjects were able to accurately catch up to and match target velocity with their hand. When vision of the hand was removed, subjects lagged behind the target but were able to match target velocity. This deficit was eliminated when vision of the hand was made available for the beginning of the response. When subjects were required to visually fixate they could catch up to the target with their hand, but subsequently produced a steady state hand velocity that was greater than target velocity. When the LED was positioned such that the target started in the peripheral visual field, the overestimation of target velocity was evident from the beginning of the response: subjects produced initial accelerations with their hand that were significantly greater than in normal conditions. Finally, normal responses were produced when subjects were required to visually pursue a second target that moved at the same speed and in the same direction as the main target.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1994

Interactions between the eye and hand motor systems: disruptions due to cerebellar dysfunction.

1. We tested the hypothesis that interactions occur between eye and hand movements produced in conjunction. This was accomplished by having human subjects with cerebellar dysfunction and age-matched controls perform two tasks: 1) tracking a moving target with the hand and 2) performing a pointing movement to intercept the target. Our prediction was that the inaccuracies that are characteristic of eye and hand movements generated in isolation by cerebellar subjects would be accentuated in each system during combined eye-hand tasks. 2. The cerebellar subjects took longer to respond to the onset of target motion in both tasks. This was true for both the eyes and hand, regardless of whether the eye and hand movements were generated in isolation or in conjunction with each other. 3. The cerebellar subjects also displayed a larger degree of error and/or variability in their hand movements than the control subjects. A significant amount of this increased variability was due to systematic changes in the trajectory of the hand during the critical periods leading up to and after each ocular saccade. These systematic changes were consistent with an overestimation of target velocity in the perifoveal visual field. 4. The increased variability of the cerebellar subjects' hand movements was markedly reduced by restricting eye movements. A similar reduction in variability occurred when vision of the hand was restricted in the tracking task. This effect was accompanied by improved eye movements. 5. For both sets of subjects the eye movements were affected by the hand movements produced in the tracking task. In particular, eye movement accuracy was improved in the controls and degraded in the cerebellar subjects when compared with the eye movements generated in isolation. In contrast, no changes were observed in the interception task. 6. Taken together, these results imply that a reciprocal interaction occurs between the eye and hand motor systems and/or that common "upstream" sites influence each of these systems in a similar manner. The functional anatomy and neurophysiological characteristics of several sites where such interactions may take place are discussed.

Adult↗

PubMed1992

Control strategies in directing the hand to moving targets.

We have evaluated the use of visual information about the movement of a target in two tasks--tracking and interceptions--involving multi-joint reaching movements with the arm. Target velocity was either varied in a pseudorandom order (random condition) or was kept constant (predictable condition) across trials. Response latency decreased as target velocity increased in each condition. A simple model that assumes that latency is the sum of two components--the time taken for target motion to be detected, and a fixed processing time--provides a good fit to the data. Results from a step-ramp experiment, in which the target stepped a small distance immediately preceding the onset of the ramp motion, were consistent with this model. The characteristics of the first 100 ms of the response depended on the amount of information about target motion available to the subject. In the tracking task with randomly varied target velocities, the initial changes in hand velocity were largely independent of target velocity. In contrast, when the velocity was predictable the initial hand velocity depended on target velocity. Analogously, the initial changes in the direction of hand motion in the interception task were independent of target velocity in the random condition, but depended on target velocity in the predictable condition. The time course for development of response dependence was estimated by controlling the amount of visual information about target velocity available to the subject before the onset of limb movement. The results suggest that when target velocity was random, hand movement started before visual motion processing was complete. The response was subsequently adjusted after target velocity was computed. Subjects displayed idiosyncratic strategies during the catch-up phase in the tracking task. The peak hand velocity depended on target velocity and was similar for all subjects. The time at which the peak occurred, in contrast, varied substantially among subjects. In the interception task the hand paths were straighter in the predictable than in the random condition. This appeared to be the result of making adjustments in movement direction in the former condition to correct for initially inappropriate responses.

Acoustic Stimulation↗

PubMed1991

The effects of changing movement velocity and complexity on response preparation: evidence from latency, kinematic, and EMG measures.

If movement control is afforded through the advance planning, or preprogramming, of upcoming actions, then one of the behavioral outcomes should be an increase in reaction time (RT) as the movement becomes more complex. In some situations, however, RT does not increase across levels of complexity, rather it remains invariant. In these cases, on-line preparation is typically inferred. That is, the sequence is said to be prepared in parts throughout the movement, as opposed to entirely beforehand. Given that there is some planning occurring during the sequence, then evidence of this process should be apparent within the movement itself. Three such dependent variables appear to provide such evidence. Specifically, the number of times the underlying accelerations cross the zero line within the movement, the number of 'significant deviations' within the acceleration trace, and the length of time for which the muscles are active (as measured by EMG) in relation to the duration of the movement. In the present experiment, then, these variables were measured in addition to the time required to prepare and initiate a movement performed under conditions conducive to either preprogramming or online preparation. Specifically, the movements were either completed as fast as possible, or at a considerably slower, more controlled speed. Each of the dependent variables displayed evidence of preprogramming in the movements completed at the fast velocity, and on-line preparation in the slower paced movements. Thus, in the slow condition, subjects appeared to rely more heavily on on-line prepared adjustments to produce an accurate outcome. The convergence attained between the various dependent measures lends power to the conclusions regarding hypothesized modes of control within the different speeds of movement.

Arm↗