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Chandani Kulatunga-Moruzi and Digby Elliott

The present study was designed to investigate attentional processes and performance asymmetries in goal-directed aiming in individuals with Down syndrome (DS; n = 6 in each group). Using the right and left hands, young adults with and without DS completed rapid aiming movements to small targets in ipsilateral and contralateral space. On some trials, a visual distractor was present. As attention and action were assumed to be coupled, the impact of distractors on reaction time (RT) and movement kinematics was examined. The performance of individuals with DS was quantitatively and qualitatively different from nonaffected participants, suggesting that participants in the two groups used different strategies to complete the task. Individuals with DS exhibited movement time (MT) interference when a distractor was present. This finding is consistent with an action-centered framework of attention.

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Timothy N. Welsh and Digby Elliott

Previous research has indicated that individuals with Down syndrome (DS) have difficulties processing auditory movement information relative to their peers with undifferentiated developmental disabilities. The present study was conducted to assess whether a model of atypical cerebral specialization could explain these findings. Thirteen adults with Down syndrome (8 men, 5 women), 14 adults with undifferentiated developmental disabilities (7 men, 7 women), and 14 adults without disabilities (8 men, 6 women) performed rapid aiming movements to targets under three conditions: a visual cue at the target location, a visual cue remote from the target location, or a verbal cue. Results revealed that, while the reaction times did not differ between the two groups with disabilities across conditions, the participants with DS, unlike their peers, had significantly longer movement times in the verbal than in two visual conditions. These results are consistent with the model of biological dissociation.

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Digby Elliott and Daniel J. Weeks

Discussed in this paper is the application of a neurobehavioral, functional systems approach to the understanding of verbal-motor integration difficulties experienced by persons with Down syndrome. In initial work, noninvasive neuropsychological techniques were used to examine both the similarities and differences in cerebral organization and perceptual-motor behavior between persons with Down syndrome and control subjects of the same chronological and/or mental age. This group-difference research led to the development of a specific model of brain-behavior relations in persons with Down syndrome. The main feature of the model is the neuroanatomical disconnection of the brain areas responsible for speech perception and movement organization. The basic tenets of the model are described, and efforts to test and refine it are discussed. This approach exemplifies how general neurobehavioral rules and principles can be harnessed to understand the exceptions to those rules often encountered with special populations.

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David A. Le Clair and Digby Elliott

This study examined the extent to which individuals with Down syndrome benefit from visual and verbal advance information about a manual aiming movement. Adults with Down syndrome as well as control subjects with and without mental handicaps performed 10.5-cm manual aiming movements with their preferred hand. On each trial subjects were cued about the specific movement either visually or verbally. On different trial blocks, the cue provided either 50% or 80% certainty. Nonhandicapped control subjects initiated and completed their manual aiming movements more quickly than subjects with mental handicaps. As well, individuals with Down syndrome were found to be slower and more variable in reaction time than participants in the other mentally handicapped group when valid information was provided verbally but not when the cue was provided visually. These results are consistent with the proposal that atypical hemispheric lateralization for speech perception associated with Down syndrome disrupts communication between functional systems responsible for processing of verbal language and organizing movement (Elliott & Weeks, 1993b).

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Digby Elliott, Susan Gray and Daniel J. Weeks

The present study was designed to determine whether the verbal-motor performance deficits sometimes exhibited by Down syndrome persons interfere with their capacity to acquire a novel motor task. Mentally handicapped adults with and without Down syndrome, as well as nonhandicapped adults, practiced a verbally cued three-element movement sequence. When the verbal cue was terminated during retention, Down syndrome subjects made no more errors and performed the motor sequence just as rapidly as did the other mentally handicapped adults. However, Down syndrome subjects took longer to organize and initiate their movements. Both mentally handicapped groups performed more poorly than nonhandicapped subjects. The results provide partial support for the notion that Down syndrome persons have difficulty organizing limb movements on the basis of verbal instruction.

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Luc Tremblay, Timothy N. Welsh and Digby Elliott

Although proponents of the motor schema theory hold that a decrease in the reliance on afferent information will occur with practice in consort with the development of motor programs, supporters of the specificity of practice hypothesis suggest that a performer's reliance on the available sources of afferent information during acquisition increases with the amount of practice. To reconcile these competing positions, four groups of 9 participants aimed to targets under either a constant or a variable practice schedule, with or without vision of the effector. After modest (15 trials) and moderate (150 trials) practice, participants were tested in both their own and in the alternate vision condition. Results indicate mat the utilization of online kinesthetic information was enhanced through practice regardless of the availability of vision during acquisition. This was especially true for the groups practicing under a variable practice schedule.

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Cheryl M. Glazebrook, Digby Elliott and James Lyons

We examined the planning and control of goal-directed aiming movements in young adults with autism. Participants performed rapid manual aiming movements to one of two targets. We manipulated the difficulty of the planning and control process by varying both target size and amplitude of the movements. Consistent with previous research, participants with autism took longer to prepare and execute movements, particularly when the index of difficulty was high. Although there were no group differences for accuracy, participants with autism exhibited more temporal and spatial variability over the initial phase of the movement even though mean peak accelerations and velocities were lower than for control participants. Our results suggest that although persons with autism have difficulty specifying muscular force, they compensate for this initial variability during limb deceleration. Perhaps persons with autism have learned to keep initial impulses low to minimize the spatial variability that needs to be corrected for during the online control phase of the movement.

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Steve Hansen, Digby Elliott and Michael A. Khan

The utility of ellipsoids for quantifying central tendency and variability throughout the trajectory of goal-directed movements is described. Aiming movements were measured over 2 days of practice and under full-vision and no-vision conditions. A three-dimensional optoelectronic system measured the movements. Individual ellipsoid locations, dimensions, and volumes were derived from the average location and the spatial variability of the effector’s trajectory at proportional temporal periods throughout the movement. Changes in ellipsoid volume over time illustrate the evolution in motor control that occurred with practice and the processes associated with visual control. This technique has the potential to extend our understanding of limb control and can be applied to practical problems such as equipment design and evaluation of movement rehabilitation.

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Digby Elliott, Kathryn L. Ricker and James Lyons

Fifteen participants practiced a two-target sequential aiming movement with either full vision of the movement environment, vision during flight, or vision while in contact with the first target. After 100 acquisition trials, participants performed a retention test in their own condition and then were transferred to each of the other two vision conditions. Both performance and kinematic data indicated that rather than becoming less dependent on visual information with practice, subjects learned to adjust their movement trajectories to use the visual information available in their particular vision condition. Although transfer to a degraded vision condition disrupted performance, when vision was augmented participants quickly adjusted their aiming trajectories to use the added information. The findings suggest that at least part of learning involves the development of rapid and efficient procedures for processing afferent information, including visual response-produced feedback.

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Simon Bennett, Derek Ashford and Digby Elliott

The aim of this research was to examine the temporal limits of binocular and monocular integration useful for one-handed catching. Participants performed 20 one-handed catching trials in 12 conditions (N = 240) defined according to the type of viewing (binocular, monocular) and the manipulation of the visual sample (continuous, intermittent). Catching performance deteriorated significantly when there was no visual information available for 80 ms between 20-ms visual samples provided simultaneously to each eye. However, there was no change in catching performance when continuous monocular vision was available for 80 ms between the binocular samples. Performance under monocular viewing decreased significantly when intermittent samples were separated by less than 20 ms. These results confirm that the equivocality in previously reported temporal limits of binocular integration is due to the different arrangement of the intermittent vision conditions and hence the information available between intermittent samples.