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Biomedical subjects

K C Engel

Publications and source records attributed to K C Engel.

7 recordsLinked to original sources
PubMed2002

Oculocentric frames of reference for limb movement.

We have reviewed evidence that suggests that the target for limb motion is encoded in a retinocentric frame of reference. Errors in pointing that are elicited by an illusion that distorts the perceived motion of a target are strongly correlated with errors in gaze position. The modulations in the direction and speed of ocular smooth pursuit and of the hand show remarkable similarities, even though the inertia of the arm is much larger than that of the eye. We have suggested that ocular motion is constrained so that gaze provides an appropriate target signal for the hand. Finally, ocular and manual tracking deficits in patients with cerebellar ataxia are very similar. These deficits are also consistent with the idea that a gaze signal provides the target for hand motion; in some cases limb ataxia would be a consequence of optic ataxia rather than reflecting a deficit in the control of limb motion per se. These results, as well as neurophysiological data summarized here, have led us to revise a hypothesis we have previously put forth to account for the initial stages of sensorimotor transformations underlying targeted limb motions. In the original hypothesis, target location and initial arm posture were ultimately encoded in a common frame of reference tied to somatosensation, i.e. a body-centered frame of reference, and a desired change in posture was derived from the difference between the two. In our new scheme, a movement vector is derived from the difference between variables encoded in a retinocentric frame of reference. Accordingly, gaze, with its exquisite ability to stabilize a target image even under dynamic conditions, would be used as a reference signal. Consequently, this scheme would facilitate the processing of information under conditions in which the body and the target are moving relative to each other.

Animals↗
PubMed2001

The Duncker illusion and eye-hand coordination.

A moving background alters the perceived direction of target motion (the Duncker illusion). To test whether this illusion also affects pointing movements to remembered/extrapolated target locations, we constructed a display in which a target moved in a straight line and disappeared behind a band of moving random dots. Subjects were required to touch the spot where the target would emerge from the occlusion. The four directions of random-dot motion induced pointing errors that were predictable from the Duncker illusion. Because it has been previously established that saccadic direction is influenced by this illusion, gaze was subsequently recorded in a second series of experiments while subjects performed the pointing task and a similar task with eye-tracking only. In the pointing task, subjects typically saccaded to the lower border of the occlusion zone as soon as the target disappeared and then tried to maintain fixation at that spot. However, it was particularly obvious in the eye-tracking-only condition that horizontally moving random dots generally evoked an appreciable ocular following response, altering the gaze direction. Hand-pointing errors were related to the saccadic gaze error but were more highly correlated with final gaze errors (resulting from the initial saccade and the subsequent ocular following response). The results suggest a model of limb control in which gaze position can provide the target signal for limb movement.

Fixation, Ocular↗
PubMed2000

Manual tracking in two dimensions.

Manual tracking was studied by asking subjects to follow, with their finger, a target moving on a touch-sensitive video monitor. The target initially moved in a straight line at a constant speed and then, at a random point in time, made one abrupt change in direction. The results were approximated with a simple model according to which, after a reaction time, the hand moved in a straight line to intercept the target. Both the direction of hand motion and its peak speed could be predicted by assuming a constant time to intercept. This simple model was able to account for results obtained over a broad range of target speeds as well as the results of experiments in which both the speed and the direction of the target changed simultaneously. The results of an experiment in which the target acceleration was nonzero suggested that the error signals used during tracking are related to both speed and direction but poorly (if at all) to target acceleration. Finally, in an experiment in which target velocity remained constant along one axis but the perpendicular component underwent a step change, tracking along both axes was perturbed. This last finding demonstrates that tracking in two dimensions cannot be decomposed into its Cartesian components. However, an analytical model in a hand-centered frame of reference in which speed and direction are the controlled variables could account for much of the data.

Acceleration↗
PubMed2000

Similarity in the response of smooth pursuit and manual tracking to a change in the direction of target motion.

Subjects were asked to track, with their eyes or their hand, the movement of a target that maintained a constant speed and made a single, abrupt change in direction. The tracking speed and direction of motion after the step change in target direction were compared for the eyes and the hand. After removal of the saccades from the eye movement records, it was found that in both cases, there was a slow rotation from the initial direction to the new direction. For the eyes and the hand, it was found that this change in direction of movement occurred at a similar rate that was proportional to the magnitude of the abrupt change in target direction. This was further described by comparing the direction of pursuit tracking with the response of a second-order system to a step input. In addition, it was found that the speed of manual and pursuit tracking was modulated in a similar manner, with a reduction in tracking speed occurring before the change in tracking direction. This reduction in speed following the change in the direction of target motion was very similar for the hand and the eye, despite the large difference in the inertias of the two systems. Taken together, these data suggest that the neural mechanisms for smooth pursuit and manual tracking have common functional elements and that musculoskeletal dynamics do not appear to be a rate-limiting factor.

Analysis of Variance↗
PubMed1999

Oculomotor tracking in two dimensions.

Results from studies of oculomotor tracking in one dimension have indicated that saccades are driven primarily by errors in position, whereas smooth pursuit movements are driven primarily by errors in velocity. To test whether this result generalizes to two-dimensional tracking, we asked subjects to track a target that moved initially in a straight line then changed direction. We found that the general premise does indeed hold true; however, the study of oculomotor tracking in two dimensions provides additional insight. The first saccade was directed slightly in advance of target location at saccade onset. Thus its direction was related primarily to angular positional error. The direction of the smooth pursuit movement after the saccade was related linearly to the direction of target motion with an average slope of 0.8. Furthermore the magnitude and direction of smooth pursuit velocity did not change abruptly; consequently the direction of smooth pursuit appeared to rotate smoothly over time.

Humans↗
PubMed1997

Anticipatory and sequential motor control in piano playing.

Pianists were asked to play short excerpts from several pieces on an electronic keyboard. In each piece, there were two phrases whose first few notes were played identically with the right hand. Thereafter, the two phrases were played differently. The aim of the investigation was to ascertain whether or not hand and finger kinematics diverged prior to the depression of the last common note. Such a divergence would imply an anticipatory modification of sequential movements of the hand, akin to the phenomenon of coarticulation in speech. The lack of such a divergence would imply a strictly serial organization of movement sequences with one hand, as was found previously to be the case for typing. The time at which each key was depressed and released and the speed with which the key was depressed was recorded via a MIDI interface to a laboratory computer. The motion of the right wrist and of the fingers of the right hand was recorded optoelectronically. Piano playing can invoke anticipatory modifications of hand and finger kinematics. The time at which two patterns of movements diverged varied considerably from piece to piece. Playing an ascending scale with the requirement of a "thumb-under" maneuver could evoke an anticipatory modification as much as 500 ms in advance of the last common note. In another piece, keypresses appeared to be executed in a strict serial ordering and a third piece gave results intermediate between these two extremes. We interpret the results to suggest that a strict serial execution of a movement sequence is favored as long as this is compatible with the demands of the task.

Fingers↗