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Biomedical subjects

O Bock

Publications and source records attributed to O Bock.

At least 55 records · Page 3Linked to original sources
PubMed1997

Effects of a tilted visual background on human sensory-motor coordination.

The present study investigated the effects of a tilted visual background on perceived hand orientation, and on the execution of aimed arm movements. Subjects were seated in a room tilted about their mid-sagittal axis to the left or right. They were asked to indicate the gravitational vertical or the body midline by rotating their supported or free, unseen hand about the longitudinal forearm axis. They were further asked to draw vertical lines with their unseen arm, and to point with the hand at visual targets. Our results indicate that if the hand is stationary, tilted environments induce an illusory hand and body tilt in the opposite direction; the effects on the hand is substantially smaller than that on the body. We found no evidence for illusory hand tilt with line drawing, and pointing movements were not noticeably modified by background tilt. We concluded that the latter two tasks provide dynamic cues about hand orientation, which remain veridical in tilted environments, and can be utilized for fast corrections of motor commands.

Adult↗
PubMed1997

Visuo-motor adaptation: evidence for a distributed amplitude control system.

We investigated the constraints for visuo-motor adaptation in human pointing movements. Subjects pointed at sequentially presented visual targets while visual feedback about their finger position was either absent (pre- and post-period), or was manipulated such as to require a gradual reduction of response amplitude (per-period). We found that response amplitudes were smaller during the post- than during the pre-period, which documents the existence of adaptation to distorted visual feedback. We further found that adaptation can transfer fully to untrained amplitudes (Exp. 1), although the amount of transfer may be reduced if trained and untrained amplitudes are substantially different (Exp. 2). However, selective adaptation of one amplitude but not another can also be yielded if the paradigm explicitly asks for it (Exp. 3), and if the two amplitudes differ by more than about 10 cm (Exp. 4). We conclude from these findings that the adapted mechanism consists of amplitude-specific elements, tuned to amplitude spans of some 10 cm.

Adaptation, Physiological↗
PubMed1996

Performance of a simple aiming task in hypergravity: I. overall accuracy.

BACKGROUND: Visuo-motor performance is known to be affected by exposure to hyper-gravity (hyper-G), but the underlying mechanisms remain to be determined; the present study investigated the role of target mislocalization. METHOD: Subjects pointed before, during and after exposure to hyper-G at targets without seeing their hand. Target positions were displayed: a) throughout each pointing response; b) before response onset; or c) in normal gravity prior to a set of movements. RESULTS AND CONCLUSIONS: For all display conditions, subjects pointed higher in hyper-G than in normal gravity from the first movement on. We attribute the discrepancy between this finding and previous results (8, 12) to different movement strategies. The effects of hyper-G on pointing performance were small, but sustained when targets were displayed before or throughout each movement, but they were large and transient when targets were memorized in normal-G. We conclude that too-high pointing in hyper-G cannot be simply explained by the "elevator illusion," and propose a tentative interpretation based on known perceptual deficits.

Adult↗
PubMed1996

Performance of a simple aiming task in hypergravity: II. detailed response characteristics.

BACKGROUND: Literature proposes three hypotheses for impaired movement execution in hyper-G. The present study attempted to discriminate between these hypotheses by comparing kinematic characteristics and final accuracy of pointing movements in different gravity levels. METHOD: Subjects pointed without seeing their hand at targets presented before, during and after exposure to hyper-G. RESULTS: After factoring out movement amplitude, peak vertical velocity and the skewness of velocity profiles tended to increase, while movement duration tended to decrease with increasing G-level. Further, final response position was slightly less modulated by target position in hyper-G than in normal-G. CONCLUSION: Although not all findings reached statistical significance, the observed pattern of results corroborates the hypothesis (2) that the motor system re-interprets hyper-G as increased arm weight.

Adult↗
PubMed1996

Grasping of virtual objects in changed gravity.

BACKGROUND: Little is known about the effects of changed gravity on the execution of grasping movements, even though such movements play an important role in normal motor behavior of humans. HYPOTHESIS: The formation of an adequate grip aperture is impaired in changed gravity. METHOD: During parabolic flight, five subjects grasped mirror-viewed virtual targets with their thumb and index finger. From their video-taped responses, we determined grip aperture as the distance between the two fingertips. RESULTS: In changed gravity, the final grip aperture was about 15% smaller than in normal gravity, and the peak grip aperture was about 30% less modulated by target size. Both findings were similar in hyper-G and in micro-G. CONCLUSIONS: We conclude that (virtual) grasping in changed gravity is affected by a deterioration of visual and/or proprioceptive signals, or by the increased computational burden of controlling movements in unusual force environments.

Adult↗
PubMed1995

On the role of extraretinal signals for saccade generation.

We investigated the accuracy of sequential saccadic eye movements, executed without visual feed-back. We found evidence that the final error of one saccade is corrected during the next, which supports the existence of extraretinal inputs to the saccadic generator. The corrections, however, were incomplete, which suggests that extraretinal signals are only partially effective.

Eye Movements↗
PubMed1994

Scaling of joint torque during planar arm movements.

The present study scrutinized the "Motor Program" concept for aimed arm movements. Human subjects pointed at visual targets in a horizontal plane, with movements of varying starting positions, amplitudes and directions. We recorded movement kinematics and subsequently calculated the shoulder and elbow joint torque profiles. Our results indicate that the shape of torque profiles is rather uniform across movements and joints. We defined the size of those profiles by six "landmark variables", which could be subsequently reduced to three factors using factor analysis: one factor represented torque magnitude and two represented different aspects of torque timing. Additional analyses indicated that total torque duration is an important controlled signal. Our findings conform with the view that movements are executed by playing back scaled versions of prototypical joint torque profiles.

Adult↗
PubMed1994

Joint position sense in simulated changed-gravity environments.

The proprioceptive position sense was investigated in an elbow-angle matching task in which the right forearm was moved passively by an experimenter and the left forearm actively by the subject. The right forearm could be immersed in water or loaded with a weight, to simulate micro- and hypergravity. We found that in simulated microgravity, matching performance was more variable than in normal gravity, and the right forearm deviated systematically upwards. The latter finding was limited to near-horizontal forearm positions (i.e., where the anticipated effects of gravity are strongest), and could be "reset" by intermittent visual feedback. The observed impairments of the proprioceptive position sense may adversely affect the manual performance of astronauts. In simulated hypergravity, we found no changes of variability and no systematic deviations. This outcome confirms our previous results that weight compensation is efficient even when only static (i.e., gravitational) cues are available.

Adult↗
PubMed1993

Localization of objects in the peripheral visual field.

We investigated visual localization by asking humans to point at visual objects without vision of their hand. The objects were luminous discs, presented stereoscopically at different distances, eccentricities and meridians with respect to the subjects' straight-ahead. Final pointing position was recorded by an electromagnetic search-coil technique. We found that the eccentricity of pointing responses towards peripheral targets was larger when subjects fixated straight-ahead rather than looked at the targets. This outcome confirmed our previous finding that target eccentricity in the peripheral visual field is overestimated. We further found that overestimation increased less than proportionally with target eccentricity, which suggests that the local magnification factor gradually declines in the visual periphery. A quantitative analysis indicated that the magnification factor is about 1.5 at the fovea, and approaches 1.0 at 10 degrees visual angle. Thus, our data support the hypothesis of a peri-foveal magnification effect which gradually subsides with increasing eccentricity. The observed magnification was similar for the horizontal and the vertical meridian. We found that the egocentric distance of pointing responses depends not only on the distance of the object pointed at, but also on the distance of a second object in the visual field. This outcome was in quantitative agreement with the predictions of Foley's model of interactive distance evaluation. Response depth, i.e. the difference in the response distances towards the two objects, was larger if both objects appeared near the center of the visual field rather than if one object appeared in the visual periphery.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

Early stages of load compensation in human aimed arm movements.

We investigated the kinematics of pointing movements in human subjects while varying the weight load carried by their hand. In accordance with our previous study, we found that movement velocity decreased and duration increased with load size, while skewness of velocity profiles and final pointing position were load-independent. When the load was changed without advance information about the new load size, kinematics of the first movement deviated initially (within the first 135 ms) from those of the preceding and following movements. Two types of deviation were found. Firstly, the initial rise of movement velocity was slower, independent of the new load size; we attributed this finding to a purposeful motor strategy to "probe" the new relationship between force and acceleration, and thus to determine the new load size. Secondly, the initial portions of movement trajectories deviated downwards after a load increase and upwards after a load decrease, depending in a graded way on the change of load size; this finding probably reflects incomplete load compensation. Deviating movement kinematics were found only for the first movement after a load change. This suggests that information required for appropriate load compensation is determined during the first movement, is stored in memory, and is available for the execution of subsequent movements. No deviating kinematics were found if prior to the first response after a load change, subjects moved the hand orthogonally to the direction of pointing or suspended the load against gravity. This suggests that the stored information is not specific for movements of similar spatial characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Acceleration↗
PubMed1993

Error accumulation and error correction in sequential pointing movements.

Human subjects pointed, without seeing their arm, at visual targets presented in repeated sequences in a frontal plane. Required movement direction could change within the sequence by 0, 45, 90, 135 or 180 degrees. Hand position was recorded contact-free in three dimensions (3D). From the recordings, the pointing errors towards each target were transformed into a Cartesian coordinate system with the x-axis representing the mean direction of all movements towards that target. We then investigated the relationship between successive errors by applying linear regression analysis separately to the three Cartesian error components. For the x-component, we found that successive errors were positively correlated throughout the experiment, which confirms our previous finding that errors in sequential pointing tend to accumulate (Bock and Eckmiller 1986; Bock et al. 1990). Correlation dropped by nearly 50% following a direction change of 90 degrees or more, suggesting that accumulation is reduced but not abolished by large changes in movement direction. The slope of the regression line averaged 0.6, which indicates the existence of a complementary trend towards error correction, contributing about 40% towards motor performance. Changes of movement direction affected slope and correlation in a closely similar way, suggesting that reduced accumulation is paralleled by increased correction. For the y- and z-components, we found that successive errors were positively correlated as well, but were not reduced following even large direction changes. This apparent discrepancy can be resolved by assuming separate neural mechanisms for amplitude and for direction control, differing in their sensitivity to direction changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗
PubMed1992

Adaptation of aimed arm movements to sensorimotor discordance: evidence for direction-independent gain control.

Human subjects pointed, without sight of their arm, at visual targets presented on a mirror-viewed monitor screen. During the adaptation period of each experiment, the position of the pointing fingertip was continuously recorded and displayed on the screen along with the targets. This visual feedback was not always veridical; rather, it was manipulated to require a gradual modification of the pointing response gain throughout the adaptation period. No visual feedback at all was available during the pre- and postadaptation periods of each experiment. The adaptive effect was determined as difference between pre- and postadaptation gains. In Expt. A, visual feedback during the adaptation period prescribed a gradual reduction of the horizontal response gain without specifying the gain for other directions; the adaptive effect was found to generalize uniformly to all movement directions. Expt. B1 prescribed a reduction of the horizontal, and an unchanged vertical gain component: in spite of this differential requirement, the adaptive effect was again uniform for all directions. Expt. B2 prescribed a reduction of the horizontal, and an increase of the vertical gain component: we found a reduced gain for all directions, with a mild direction-dependence in the magnitude of the adaptive effect. In a modified version of Expt. B2, no intermanual transfer of the adaptive effect was found. Expt. C1-3 prescribed gain reduction for target directions within 15, 30, or 45 degrees around the horizontal, and gain increase for all other directions: we found little or no adaptive effects under such conditions. From the above findings, we concluded that the adapted system controls movement gain largely independent of movement direction. This mechanism responds readily to requirements for gain reduction, but not gain increase. No evidence for an organization of the arm motor system in direction-selective channels was found, in contrast to findings on the saccadic control system in a paradigm similar to our Expt. A8. This discrepancy supports the view that arm and eye movements are controlled by distinct mechanisms.

Adaptation, Psychological↗
PubMed1992

Motor control prior to movement onset: preparatory mechanisms for pointing at visual targets.

The present study investigated the mechanisms involved in the preparation of pointing movements in humans. We provided visual precues on the location of the upcoming target, and registered the effect of these precues on the reaction time (RT = interval between target appearance and movement onset). Generally, precues were found to reduce RT, suggesting that some aspects of the preparatory process have been advanced in time. In Exp. 1, precues fully specified the direction required for the upcoming movement while indicating only a range of movement amplitudes; in Exp. 2, precues fully specified the amplitude and indicated a range of directions. In both experiments, RT was shorter than in control trials without precues, and gradually increased with the size of the precued amplitude or direction range. This result suggests that the preparation of either parameter is possible without knowing the precise value of the other, i.e. amplitude and direction are not prepared in a fixed order. Furthermore, our results are consistent with the view that movement preparation includes a progressive contraction of the precued range towards the final value. The speed of this process can be estimated as 0.31 cm/ms for amplitude, and 1.7 deg/ms for direction ranges. In Exp. 3 and 4, precues indicated both amplitude and direction as ranges only. The size of the amplitude range was held constant while the size of the direction range was varied (Exp. 3), or vice versa (Exp. 4). Under these conditions, RT increased with the size of the varied range.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

The characteristics of arm movements executed in unusual force environments.

Human subjects pointed at stationary visual targets without sight of their arm while the force environment was varied by applying weight or spring loads to the hand. The path travelled by the finger, pointing accuracy, and the shape of the finger velocity profile remained invariant across all force environments after a single practice trial. However, the magnitude and duration of the velocity profile depended consistently on the presence and size of a weight load. In contrast, velocity was not affected by spring loads. An analysis of movement dynamics in our study indicated that inertial and gravitational load components were compensated by separate mechanisms, the former employing time- and the latter magnitude scaling of muscle force profiles. The presence of such separate mechanisms led us to predict little problems for movement dynamics in weightlessness, which was indeed confirmed in a study on pointing movements aboard the KC-135 aircraft.

Arm↗
PubMed1992

Accuracy of aimed arm movements in changed gravity.

We studied the accuracy of aimed arm movements in normal gravity, and during the hypergravity (hyper-G) and microgravity (micro-G) episodes of KC-135 parabolic flights. Subjects pointed at mirror-viewed targets without sight of their arm, and final pointing position was measured by a digitizing pad. Compared with the normal gravity (normal-G) baseline, subjects pointed consistently higher in hyper-G, and still higher in micro-G. Results were not different if subjects viewed targets only during normal-G and pointed at their memorized position under changed gravity (changed-G); this suggests that the "elevator illusion" played a minor role in our study. The observed impairments were attributed to degraded proprioceptive feedback and/or inappropriate motor programs in changed-G. Pointing accuracy improved movement-to-movement but not parabola-to-parabola, indicating that prolonged exposure is needed for sustained adaptation.

Aerospace Medicine↗
PubMed1990

Load compensation in human goal-directed arm movements.

We analysed the execution of multijoint pointing movements in humans while weight or spring loads were applied to the pointing hand. Visual feedback on arm and hand position was excluded. Movement paths, final positions, and normalized velocity profiles were found to be load-independent, except for the very first movement after a load change. With increasing size of a weight load movement velocity decreased, and movement duration increased by the same factor, i.e. the velocity profiles were rescaled in magnitude and time. In contrast, under a spring load movement velocity and duration were not different from no-load controls. These findings led us to propose a new hypothesis on load compensation by the motor system. We suggest that an important controlled variable is a fictional force acting externally on the hand, and that the inertia- and gravity-related components of this force are controlled separately; then, loads are compensated by time scaling of the inertia-related, and magnitude scaling of the gravity-related component. The predictions of this hypothesis regarding movement paths and velocities under weight and spring loads are in good quantitative agreement with our experimental data. When specifically asked to do so, our subjects were able to generate velocity profiles under a weight load that were not different from those under no-load conditions, which suggests that alternative control strategies are available when needed.

Biomechanical Phenomena↗
PubMed1990

Control of arm movements in a 2-dimensional pointing task.

The present study analyses in humans the control principles of sequential, unpracticed pointing movements in a 2-dimensional space. Our data reveal that variable pointing errors add up within such sequences. This finding supports the hypothesis that movement amplitude rather than position is the controlled variable of the investigated movements.

Attention↗