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Biomedical subjects

L S Jakobson

Publications and source records attributed to L S Jakobson.

9 recordsLinked to original sources
PubMed1992

The role of binocular vision in prehension: a kinematic analysis.

This study examined the contribution of binocular vision to the control of human prehension. Subjects reached out and grasped oblong blocks under conditions of either monocular or binocular vision. Kinematic analyses revealed that prehensile movements made under monocular viewing differed substantially from those performed under binocular conditions. In particular, grasping movements made under monocular viewing conditions showed longer movement times, lower peak velocities, proportionately longer deceleration phases, and smaller grip apertures than movements made under binocular viewing. In short, subjects appeared to be underestimating the distance of objects (and as a consequence, their size) under monocular viewing. It is argued that the differences in performance between the two viewing conditions were largely a reflection of differences in estimates of the target's size and distance obtained prior to movement onset. This study provides the first clear kinematic evidence that binocular vision (stereopsis and possibly vergence) makes a significant contribution to the accurate programming of prehensile movements in humans.

Adult↗
PubMed1991

A neurological dissociation between perceiving objects and grasping them.

Studies of the visual capacity of neurological patients have provided evidence for a dissociation between the perceptual report of a visual stimulus and the ability to direct spatially accurate movements toward that stimulus. Some patients with damage to the parietal lobe, for example, are unable to reach accurately towards visual targets that they unequivocally report seeing. Conversely, some patients with extensive damage to primary visual cortex can make accurate pointing movements or saccades toward a stimulus presented in their 'blind' scotoma. But in investigations of visuomotor control in patients with visual disorders, little consideration has been given to complex acts such as manual prehension. Grasping a three-dimensional object requires knowledge not only of the object's spatial location, but also of its form, orientation and size. We have examined a patient with a profound disorder in the perception of such object qualities. Our quantitative analyses demonstrate strikingly accurate guidance of hand and finger movements directed at the very objects whose qualities she fails to perceive. These data suggest that the neural substrates for the visual perception of object qualities such as shape, orientation and size are distinct from those underlying the use of those qualities in the control of manual skills.

Adult↗
PubMed1991

Factors affecting higher-order movement planning: a kinematic analysis of human prehension.

Past studies of the kinematics of human prehension have shown that varying object size affects the maximum opening of the hand, while varying object distance affects the kinematic profile of the reaching limb. These data contributed to the formulation of a theory that the reaching and grasping components of human prehension reflect the output of two independent, though temporally coupled, motor programs (Jeannerod 1984). In the first experiment of the present study, subjects were required to reach out and grasp objects, with or without on-line, visual feedback. Object size and distance were covaried in a within-subjects design, and it was found that both grip formation and reach kinematics were affected by the manipulation of either variable. These data suggest that the control mechanisms underlying transport of the limb and grip formation are affected by similar task constraints. It was also observed that when visual feedback was unavailable after movement onset subjects showed an exaggerated opening of their hands, although grip size continued to be scaled for object size. The question remained as to whether the larger opening of the hand during no-feedback trials reflected the lack of opportunity to fine-tune the opening of the hand on-line, or the adoption of a strategy designed to increase tolerance for initial programming errors. To address this question, a second experiment was carried out in which we manipulated the predictability of visual feedback by presenting feedback and no-feedback trials in a random order. In contrast to the situation in which feedback and no-feedback trials were presented in separate blocks of trials (Exp. 1), in the randomly-ordered series of trials presented in Exp. 2, subjects always behaved as if they were reaching without vision, even on trials where visual feedback was continuously available. These findings suggest that subjects adopt different strategies on the basis of the predictability of visual feedback, although there is nothing to suggest that this takes place at a conscious, or voluntary, level. The results of both experiments are consistent with the notion of a hierarchically-organized motor control center, responsible for optimizing performance under a variety of conditions through the coordination of different effector systems and the anticipation of operating constraints.

Adult↗
PubMed1991

A kinematic analysis of reaching and grasping movements in a patient recovering from optic ataxia.

A detailed, kinematic analysis revealed subtle deficits in midline pointing and prehension in a patient showing good clinical signs of recovery from optic ataxia associated with bilateral parietooccipital damage. Relative to control subjects, the patient tended to misreach to the left with her right hand, and to the right with her left hand on a pointing task. While reach kinematics were otherwise normal in the pointing task, they were markedly disturbed in a prehension task, in which reaching and grasping movements must be integrated. In addition, difficulties in making fine postural adjustments to the hands were still evident 17 months post-injury. These findings suggest an important role for the posterior parietal lobes in programming goal-directed manual movements, and have implications for current theories of motor control and visual perception.

Aged↗
PubMed1990

Kinematic analysis of limb movements in neuropsychological research: subtle deficits and recovery of function.

In this brief review, we argue that a detailed analysis of the spatial and temporal characteristics of visually guided limb movements can provide important insights into the nature of neuropsychological deficits and the functional organization of the brain. In particular, we will show how kinematic analysis of a simple aiming movement can reveal differences that might not be apparent clinically in the behaviour of patients with unilateral brain lesions. We will argue that the subtle but distinctive deficits that follow damage to the left or right hemisphere can be related to the clinical syndromes of manual apraxia and hemispatial neglect, respectively. The fact that these deficits are apparent only with kinematic analysis demonstrates the potential of these techniques for evaluating recovery of function.

Attention↗
PubMed1989

Trajectories of reaches to prismatically-displaced targets: evidence for "automatic" visuomotor recalibration.

The present study examined the kinematics of unrestricted reaches to prismatically-displaced targets. The kinematic analysis allowed us (1) to document how and where in the reach adjustments were made to compensate for the prismatic displacement. (2) to detail the changes that occur in the characteristics of reaches during the course of adaptation to the prisms, and (3) to look at the effects of providing information (or not) to the subject about the presence and nature of the prismatic distortion. The experiment differed from classic studies of prism adaptation in that subjects were permitted full visual feedback of their moving limb at all times, and entire reaching movements were recorded in addition to terminal errors. Experimental subjects were tested either with large-displacement prisms of the sort typically used in such experiments (20 diopters) or with small-displacement prisms (5 diopters) the properties of which went undetected in uninformed subjects. By using small displacements, it was possible to examine the process of visuomotor recalibration directly, free of contamination by "conscious" correction strategies. There were no differences in the terminal accuracies of the reaches made by subjects in any of the conditions. The availability of visual feedback allowed subjects to place their finger accurately on the target, despite the fact that in some cases their vision was displaced by as much as 11.4 degrees to the right. When the entire reach was examined, however, it was found that the amount of curvature in the path increased when large or small diopter prisms were unexpectedly introduced, with the subjects showing large deviations to the right. This rightward deviation was corrected in the final approach with a larger terminal correction. On some occasions, nonetheless, corrections were observed very early in the course of the reaching movement and appeared to be part of a natural process of trajectory fine-tuning. Uninformed subjects exposed to either large or small prismatic displacements also showed evidence of adaptation through an increased number of on-line corrections which compensated for a tendency to reach into the side of space opposite to the direction of the displacement (a "negative after-effect" in the path of the reach). Moreover, when questioned after the experiment, it became clear that uninformed subjects exposed to small prismatic displacements had apparently failed to detect any visual displacement whatsoever. Taken together, these results suggest that visuomotor recalibration can take place "automatically" without feedback from terminal errors and without the use of conscious strategies.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
PubMed1989

Focused search of semantic cases: the effects of question form and case status.

The present study was designed to identify and examine some of the variables that influence the focused search of semantic cases in question answering. Singer, Parbery, and Jakobson (1988) have previously reported that people can focus on the case interrogated by a question and can largely disregard irrelevant cases. In the present study, people learned facts, such as the pilot painted the garage with the roller, the spraygun, and the brush. One day later, they answered questions that focused on a particular case. For example, the question did the pilot paint with a spraygun? focuses on the instrument case. Experiment 1 revealed that people can focus on a particular case in response both to complete questions and to comparable word probes, such as "pilot spraygun." Therefore, the given-new structure of questions is not essential to focused search. Experiment 2 revealed that people have a difficult time ignoring the agent case, even when it is irrelevant to the question. This corroborates proposals that agent and action information are closely interrelated in the representation of a fact. These results help to delineate the phenomenon of the focused search of semantic cases.

Cues↗