Lorna S. Jakobson

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

SummaryPast 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.

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.

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 targets 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.

The Role of Surface Information in Object Recognition: Studies of a Visual Form Agnosic and Normal Subjects

Three experiments were conducted to explore the role of colour and other surface properties in object recognition. The effects of manipulating the availability of surface-based information on object naming in a patient with visual form agnosia and in two age-matched control subjects were examined in experiment 1. The objects were presented under seven different viewing conditions ranging from a full view of the actual objects to line drawings of those same objects. The presence of colour and other surface properties aided the recognition of natural objects such as fruits and vegetables in both the patient and the control subjects. Experiment 2 was focused on four of the critical viewing conditions used in experiment 1 but with a large sample of normal subjects. As in experiment 1, it was found that surface properties, particularly colour, aided the naming of natural objects. The presence of colour did not facilitate the naming of manufactured objects. Experiment 3 was focused on possible ways by which colour could assist in the recognition of natural objects and it was found that object naming was facilitated only if the objects were presented in their usual colour. The results of the experiments show that colour does improve recognition for some types of objects and that the improvement occurs at a high level of visual analysis.

Music training and rate of presentation as mediators of text and song recall.

The present research addresses whether music training acts as a mediator of the recall of spoken and sung lyrics and whether presentation rate is the essential variable, rather than the inclusion of melody. In Experiment 1, 78 undergraduates, half with music training and half without, heard spoken or sung lyrics. Recall for sung lyrics was superior to that for spoken lyrics for both groups. In Experiments 2 and 3, presentation rate was manipulated so that the durations of the spoken and the sung materials were equal. With presentation rate equated, there was no advantage for sung over spoken lyrics. In all the experiments, those participants with music training outperformed those without training in all the conditions. The results suggest that music training leads to enhanced memory for verbal material. Previous findings of melody’s aiding text recall may be attributed to presentation rate.

The nature and limits of orientation and pattern processing supporting visuomotor control in a visual form agnosic

We have previously reported that a patient (DF) with visual form agnosia shows accurate guidance of hand and finger movements with respect to the size, orientation, and shape of the objects to which her movements are directed. Despite this, she is unable to indicate any knowledge about these object properties. In the present study, we investigated the extent to which DF is able to use visual shape or pattern to guide her hand movements. In the first experiment, we found that when presented with a stimulus aperture cut in the shape of the letter T, DF was able to guide a T-shaped form into it on about half of the trials, across a range of different stimulus orientations. On the remaining trials, her responses were almost always perpendicular to the correct Orientation. Thus, the visual information guiding the rotation of DFs hand appears to be limited to a single orientation. In other words, the visuomotor transformations mediating her hand rotation appear to be unable to combine the orientations of the stem and the top of the T, although they are sensitive to the orientation of the element(s) that comprise the T. In a second experiment, we examined her ability to use different sources of visual information to guide her hand rotation. In this experiment, DF was required to guide the leading edge of a hand-held card onto a rectangular target positioned at dHerent orientations on a flat surface. Here the orientation of her hand was determined primarily by the predominant orientation of the luminance edge elements present in the stimulus, rather than by information about orientation that was conveyed by nonluminance boundaries. Little evidence was found for an ability to use contour boundaries defined by Gestalt principles of grouping (good continuation or similarity) or nonaccidental image properties (colinearity) to guide her movements. We have argued elsewhere that the dorsal visual pathway from occipital to parietal cortex may underlie these preserved visuomotor skills in DF. If so, the limitations in her ability to use different kinds of pattern information to guide her hand rotation suggest that such information may need to be transmitted from the ventral visual stream to these parietal areas to enable the full range of prehensive acts in the intact individual.

Evaluation of spatial working memory function in children and adults with fetal alcohol spectrum disorders: a functional magnetic resonance imaging study.

Magnetic resonance imaging (MRI) and functional MRI studies involving n-back spatial working memory (WM) tasks were conducted in adults and children with Fetal Alcohol Spectrum Disorders (FASD), and in age- and sex-matched controls. FMRI experiments demonstrated consistent activations in regions of the brain associated with working memory. Children with FASD displayed greater inferior-middle frontal lobe activity, while greater superior frontal and parietal lobe activity was observed in controls. Control children also showed an overall increase in frontal lobe activity with increasing task difficulty, while children with FASD showed decreased activity. FASD adults demonstrated less functional brain activity overall, but greater inferior-middle frontal lobe activity during the simpler tasks, relative to controls. Control adults demonstrated greater inferior frontal activity with increasing task difficulty, while this pattern was not consistently observed in FASD adults. All four groups showed increasing activity with increases in task difficulty in the parietal and frontal regions at more superior slice levels. The results suggest impairment in spatial working memory in those with FASD that does not improve with age, and that fMRI may be useful in evaluation of brain function in these individuals.

Trajectories of reaches to prismatically-displaced targets: evidence for “automatic” visuomotor recalibration

SummaryThe present study examined the kinematics of unrestricted reaches to prismaticallydisplaced 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° 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 finetuning. 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 aftereffect” 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. In fact, making subjects aware of the distortion by providing them with explicit information about the prisms led to reduced levels of adaptation. These “informed” subjects showed more smoothly generated reaches during prism exposure, while post-exposure reaches showed less evidence of a negative aftereffect. In fact, postexposure reaches of subjects informed of the presence of the 5 diopter prismatic displacement were indistinguishable from reaches of control subjects.

Neuropsychological and neurobehavioral functioning in Duchenne muscular dystrophy: A review

Duchenne muscular dystrophy (DMD) is a genetic condition affecting predominantly boys that is characterized by fatal muscle weakness. While there is no cure, recent therapeutic advances have extended the lifespan of those with DMD considerably. Although the physiological basis of muscle pathology is well-documented, less is known regarding the cognitive, behavioral, and psychosocial functioning of those afflicted. Several lines of evidence point to central nervous system involvement as an organic feature of DMD, challenging our view of the disorder as strictly neuromuscular. This report provides a review of the literature on neuropsychological and neurobehavioral functioning in DMD. Recent research identifying associations with DMD and neuropsychiatric disorders is also discussed. Lastly, the review presents implications of findings related to nonmotor aspects of DMD for improving the quality of life in those affected. While the literature is often contradictory in nature, this review highlights some key findings for consideration by clinicians, educators and parents when developing therapeutic interventions for this population.

Visual and visuomotor performance in dyslexic children

The present study was designed to compare the performance of nine dyslexic boys and nine age- and IQ-matched controls on tasks which presumably tap visual functions dependent on the subcortical magnocellular (M) pathway (flicker sensitivity) and the cortical dorsal stream (stereoacuity, structure-from-motion, visuomotor control). Increasing evidence suggests that dyslexics experience impairments in M-system functioning. In keeping with previous work supporting this conclusion, dyslexic subjects in the present study were found to have reduced sensitivity to flicker relative to controls. Given that the M system provides the predominant input to the dorsal stream, it was expected that reduced functioning of the M system in dyslexics would result in disruptions of functions related to this cortical visual pathway. Indeed, dyslexic subjects in the present study were found to be less efficient at recognizing structure-from-motion and less accurate at grasping objects precisely. They also showed a mild impairment in stereoacuity. These results, then, lend some support to the hypothesis that dyslexic individuals should show deficiencies on tasks dependent on dorsal stream processing of visual information.