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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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Timothy N. Welsh and Michele Zbinden

The “proximity-to-hand” effect refers to the finding that distractors between the home position and the target cause more interference in a selective reaching movement than distractors farther from the home position. Based largely on the proximity-to-hand effect, Tipper, Lortie, and Baylis (1992) proposed that attention is distributed in an action-centered framework such that the interference caused by a specific stimulus depends on the action. The current experiments sought to determine if there is an attentional preference for stimuli closer to home or for stimuli that activate more efficiently executed actions regardless of the location. Results supported the latter hypothesis in that the greatest interference was observed when the distractor activated an action with a lower index of difficulty than the target, even though that distractor was farther from home than the target. These findings indicate that the action context mediates the influence that nontarget stimuli have on the processing of target responses.

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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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Heather F. Neyedli and Timothy N. Welsh

Previous research has shown in a motor decision task involving a target (yielding a reward) and overlapping penalty area (yielding a loss), people initially aim closer to the penalty area than optimal. This risky strategy may be adopted to increase target hits, thereby increasing net worth. The purpose of the current study was to determine whether the starting net worth level (either 5,000 or 0 points) affected the influence of task experience on endpoint selection. It was hypothesized the 5,000-point group should adopt a less risky strategy and aim further from the penalty area than those with 0 points. Net worth affected participants’ initial endpoint where the 5,000-point group aimed further from the penalty circle, and closer to the optimal endpoint, than the 0-point group. The 0-point group adapted their endpoint over the course session to aim closer to the optimal endpoint whereas no such change was seen in the 5,000-point group. The results show that changing the participants’ reference point through initial net worth can affect the optimality of participants’ endpoint and how endpoints change with experience.

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Steve Hansen, Bridget Sheahan, Melinda Wu, James Lyons, Timothy N. Welsh, and Digby Elliott

Adults with Down syndrome (DS), an undifferentiated developmental delay (UnD) and no developmental delay practiced a manual target aiming task either with or without on-line visual feedback. Following acquisition, participants performed a retention test involving the same sensory condition available during practice, followed by a transfer test under the other sensory condition. Although the participants with UnD were highly dependent on visual feedback for movement accuracy, participants with DS relied more on either kinesthetic feedback or feedforward control for spatial precision. Participants in all three groups improved their movement times with practice. This improvement was associated both with an increase in peak velocity and a reduction in the time required to decelerate their aiming movements. Contrary to our expectations, improvements in performance were not specific to the sensory conditions available during practice.

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Digby Elliott, Timothy N. Welsh, James Lyons, Steve Hansen, and Melinda Wu

Williams syndrome (WS) is a genetic disorder that causes general cognitive and developmental delays. Compared to persons with Down syndrome (DS) at the same developmental level, individuals with WS generally exhibit superior expressive language abilities, but have difficultly with tasks that require the visual control of movement. Recently it has been suggested that this latter problem reflects a deficit in dorsal stream function. In the present study, this hypothesis was investigated by examining the kinematics of rapid aiming movements. The performance of the participants with WS (n = 4) was compared to the performance of participants with DS (n = 8), with undifferentiated developmental delays (n = 8), and from the general population (n = 8). In partial support of the dorsal deficit hypothesis, the results suggest that, compared to people from the other groups, the participants with WS had difficulty in preparing their movements on the basis of the visual and other information available to them. This was particularly evident in their inability to properly scale movement velocities to the amplitude of the movements and in the number of discrete corrections made during movement execution.