Temporal precision of interceptive action: differential effects of target size and speed

The duration of movements made to intercept moving targets decreases and movement speed increases when interception requires greater temporal precision. Changes in target size and target speed can have the same effect on required temporal precision, but the response to these changes differs: changes in target speed elicit larger changes in response speed. A possible explanation is that people attempt to strike the target in a central zone that does not vary much with variation in physical target size: the effective size of the target is relatively constant over changes in physical size. Three experiments are reported that test this idea. Participants performed two tasks: (1) strike a moving target with a bat moved perpendicular to the path of the target; (2) press on a force transducer when the target was in a location where it could be struck by the bat. Target speed was varied and target size held constant in experiment 1. Target speed and size were co-varied in experiment 2, keeping the required temporal precision constant. Target size was varied and target speed held constant in experiment 3 to give the same temporal precision as experiment 1. Duration of hitting movements decreased and maximum movement speed increased with increases in target speed and/or temporal precision requirements in all experiments. The effects were largest in experiment 1 and smallest in experiment 3. Analysis of a measure of effective target size (standard deviation of strike locations on the target) failed to support the hypothesis that performance differences could be explained in terms of effective size rather than actual physical size. In the pressing task, participants produced greater peak forces and shorter force pulses when the temporal precision required was greater, showing that the response to increasing temporal precision generalizes to different responses. It is concluded that target size and target speed have independent effects on performance.

The accuracy of interceptive action in time and space

Hitting a moving target demands that movement is both spatially and temporally accurate. Recent experiments have begun to reveal how performance of such actions depends on the spatial and temporal accuracy requirements of the task. The results suggest a simple strategy for achieving spatiotemporal accuracy using brief, high-speed movements.

Initiation of evasive manoeuvres during self-motion: a test of three hypotheses

To understand performance of evasive and interceptive actions it is important to know how people decide when to initiate a movement - initiating at the 'right' moment is often essential for successful performance. It has been proposed that initiation is triggered when a perceptually derived quantity reaches an invariant criterion value. Candidate quantities include time-to-collision (TTC), distance, and rate of image expansion ( ROE), all of which have received empirical support. We studied initiation of an evasive manoeuvre in a computer-simulated steering task in which the observer was required to steer through a stationary visual environment and avoid colliding with an obstacle in their path. The results could not be explained by hypotheses which propose that evasive manoeuvre initiation is based on a fixed criterion value of TTC or distance. The overall pattern was, however, consistent with the use of a criterion ROE value. This was further tested by analyses designed to directly evaluate whether the ROE value used to initiate the response was the same across experimental conditions. Only two of the six participants showed evidence for using the ROE strategy.

Effects of acoustic startle stimuli on interceptive action

In reaction time (RT) tasks, presentation of a startling acoustic stimulus (SAS) together with a visual imperative stimulus can dramatically reduce RT while leaving response execution unchanged. It has been suggested that a prepared motor response program is triggered early by the SAS but is not otherwise affected. Movements aimed at intercepting moving targets are usually considered to be similarly governed by a prepared program. This program is triggered when visual stimulus information about the time to arrival of the moving target reaches a specific criterion. We investigated whether a SAS could also trigger such a movement. Human experimental participants were trained to hit moving targets with movements of a specific duration. This permitted an estimate of when movement would begin (expected onset time). Startling and sub-startle threshold acoustic probe stimuli were delivered unexpectedly among control trials: 65, 85, 115 and 135 ms prior to expected onset (10:1 ratio of control to probe trials). Results showed that startling probe stimuli at 85 and 115 ms produced early response onsets but not those at 65 or 135 ms. Sub-threshold stimuli at 115 and 135 ms also produced early onsets. Startle probes led to an increased vigor in the response, but sub-threshold probes had no detectable effects. These data can be explained by a simple model in which preparatory, response-related activation builds up in the circuits responsible for generating motor commands in anticipation of the GO command. If early triggering by the acoustic probes is the mechanism underlying the findings, then the data support the hypothesis that rapid interceptions are governed by a motor program. © 2006 Published by Elsevier Ltd on behalf of IBRO.