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François Bonnetblanc

Fitts’s law states that movement time (MT) is related to precision (index of difficulty, ID). The introduction of a new effector in a Fitts’s task induces an increase of the MT/ID slopes (Langolf et al., 1976). It is unclear, however, whether kinetics or pure coordinative constraints based on the introduction of new degrees of freedom (at the joint or muscular level) are responsible for this effect. To determine the influence of body kinetics on the MT/ID slope when pointing beyond reach, we compared pointing movements executed within (control) or beyond reach, and in this latter case with (loaded) and without (distant) an additional mass applied to the trunk. Eight subjects were required to point as fast and as accurately as possible to a target (width: 0.5, 1.0, or 2.5 cm; amplitude: 30 cm). The MT/ID slope increased when pointing beyond reach (control versus distant or loaded). This slope did not increase with an additional load applied to the trunk (distant versus loaded). Therefore, we conclude that the MT/ID slope is more likely a function of the number of degrees of freedom introduced in the task rather than a function of the kinetics constraints.

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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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Stacey L. Gorniak, Marcos Duarte and Mark L. Latash

We explored possible effects of negative covariation among finger forces in multifinger accurate force production tasks on the classical Fitts’s speed-accuracy trade-off. Healthy subjects performed cyclic force changes between pairs of targets “as quickly and accurately as possible.” Tasks with two force amplitudes and six ratios of force amplitude to target size were performed by each of the four fingers of the right hand and four finger combinations. There was a close to linear relation between movement time and the log-transformed ratio of target amplitude to target size across all finger combinations. There was a close to linear relation between standard deviation of force amplitude and movement time. There were no differences between the performance of either of the two “radial” fingers (index and middle) and the multifinger tasks. The “ulnar” fingers (little and ring) showed higher indices of variability and longer movement times as compared with both “radial” fingers and multifinger combinations. We conclude that potential effects of the negative covariation and also of the task-sharing across a set of fingers are counterbalanced by an increase in individual finger force variability in multifinger tasks as compared with single-finger tasks. The results speak in favor of a feed-forward model of multifinger synergies. They corroborate a hypothesis that multifinger synergies are created not to improve overall accuracy, but to allow the system larger flexibility, for example to deal with unexpected perturbations and concomitant tasks.

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Patricia S. Pohl and Carolee J. Winstein

The purpose of ihis study was to compare the effects of a single practice session on performance strategies used by young and older adults to decrease movement time (MT) while maintaining accuracy of an aiming task. Five young and 5 older adult males practiced until each accumulated 500 target hits in each of two complexity conditions as defined by Fitts’s law. Participants decreased MT with practice; however, older adults had longer MTs than the young, particularly in the high-complexity condition. With practice in the low-complexity condition, participants decreased absolute acceleration and deceleration times but maintained the relative amount of MT devoted to temporal phases. In contrast, with practice in the high-complexity condition, participants decreased absolute deceleration and dwell time and changed the temporal structure. Results suggest that older adults can decrease MT with practice and that the performance strategies adopted to speed performance are more a function of task complexity than age.

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Marlowe Pecora, Luc Tremblay and Matthew Heath

, J. , & Fischer , M.H. ( 2006 ). Moving farther but faster: An exception to Fitts’s law . Psychological Science, 17, 794 – 798 . PubMed ID: 16984297 doi:10.1111/j.1467-9280.2006.01784.x 10.1111/j.1467-9280.2006.01784.x Ariely , D. ( 2001 ). Seeing sets: Representation by statistical

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Aaron England, Timothy Brusseau, Ryan Burns, Dirk Koester, Maria Newton, Matthew Thiese and Benjamin Chase

physical development ( Choudhury et al., 2007 ). Choudhury et al. ( 2007 ) found that performance on a visually guided pointing motor task conformed to Fitts’s law for both execution and imagery conditions in adolescents and adults, indicating that both adolescents and adults were able to generate motor