Describing and analyzing error for one-dimensional performance tasks is fairly straightforward, but suggestions for describing and analyzing error for two-dimensional performance tasks (e.g., marksmanship) are quite problematic. Specifically, imposing an arbitrary axis onto the two-dimensional work space, along which traditional one-dimensional measures can be computed and analyzed, yields measures of accuracy, bias, and consistency that are entirely dependent upon the choice of axis. The present contribution offers new measures and methods for describing and analyzing data from two-dimensional performances. Unlike the resu1ts from previous suggestions, the approaches described herein yield results that are completely independent of the axes used to quantify the individual two-dimensional trials. These new approaches are strongly related to well-established methods for describing and analyzing error for one-dimensional tasks.

The end-state comfort effect has been observed in recent studies of grip selection in adults. The present study investigated whether young children also exhibit sensitivity to end-state comfort. The task was to pick up an overturned cup from a table, turn the cup right side up, and pour water into it. Two age groups (N = 20 per group) were studied: preschool children (2-3 years old), and kindergarten students (5-6 years old). Each child performed three videotaped trials of the task. Only 11 of the 40 children exhibited the end-state comfort effect, and there were no differences between age groups. Results revealed the emergence of five different performance patterns, none of which were consistent with sensitivity to end-state comfort. The findings have implications for the advance planning of manual control in young children.

The prediction emanating from memory drum theory (Henry & Rogers, 1960') that simple reaction time (SRT) increases as a response becomes more complex (i.e., increases in number of movement parts) was investigated. Experiments 1 (N = 20) and 3 (N = 16) indicated that SRT was longer for responses consisting of two and three parts than it was for a one-part response and this may be interpreted as support for the prediction. Failing to support the prediction, however, was the finding that SRT was essentially the same for responses consisting of two and three parts. This may not be too damaging to the theory because it could simply be reflecting an upper limit in terms of numbers of parts or response duration for causing an increase in SRT. Experiments 2 (N = 20) and 3 revealed an SRT effect between two responses that were supposed to be equal in complexity. At first, this finding appeared to be contrary to the prediction, but it may be interpreted as support for it because one of the responses defined as having one movement part could actually have had two

The question of whether changes seen in simple reaction time (SRT) as a function of response complexity (i.e., number of movement parts) should be considered as differences in the time needed to centrally program a motor response was addressed. Using a large-scale tapping response, 14 subjects contacted from one to five targets positioned in a straight line, while a second group of 14 subjects executed 90 degrees changes in direction in striking the targets. Results revealed that mean SRT and mean premotor time increased linearly as the number of movement parts increased, regardless of whether changes in movement direction had to be programmed, with the greatest increase occurring between one-, and two-part responses. Increases in motor time were not sufficient to account for the sizeable SRT effect. These findings support the position of increased central programming time for more complex responses, and also help establish some of the boundaries of the complexity effect.

Two experiments examined the interaction of vision and articular proprioception in simple one-hand catching. In Experiment 1 (N = 18) skilled baseball and softball players used the left and right hands to catch slowly moving tennis balls, while Experiment 2 (N = 16) used novice catchers as subjects. In half the trials, sight of the catching hand was prevented by placing a screen alongside the subjects' face. Results of Experiment 1 revealed that the screen caused minimal disruption of the positioning phase of the catch, with moderate disruption of the grasping phase. However, for the unskilled subjects of Experiment 2, the screen caused considerable disruption of positioning. The data provide only minimal support for Smyth and Marriott' (1982) contention that limb position is inadequately specified by articular proprioception. It is argued that skill level serves as a mediator in the ability to use proprioception for limb positioning, but vision appears necessary to control the precise temporal organization of the grasp phase of one-hand catching.