Perceptual Integration of Motion and Form in a Visual Search Task

Three visual search experiments were designed to investigate the processes involved in the efficient detection of motion - form conjunction targets. In experiment 1 the number of movement directions in the display were varied, and we tried to establish whether or not the target direction was predictable. Search was less efficient when items moved in multiple directions compared to just one direction; whether items moved in two, three, or four directions made relatively little difference. Pre-cuing of the target direction facilitated the search to a small, but non-negligible, extent; the facilitation was not due to better predictability of the display region that contained the target at the start of a trial.

Experiment 2 was designed to estimate the relative contributions of stationary and moving nontargets to the search rate. Search rates were primarily determined by the number of moving nontargets; stationary nontargets sharing the target form also exerted a significant effect, but this was only about half as strong as that of moving nontargets; stationary nontargets not sharing the target form had little influence.

In experiment 3 we examined the effects of movement speed and item size on search performance. Increasing the speed of the moving items (> 1.5 deg s⁻¹) facilitated target detection when the task required segregation of the moving from the stationary items; when no segregation was necessary, increasing the movement speed impaired performance. When the display items were ‘large’, motion speed had little effect on target detection; but when the items were ‘small’, search efficiency declined with item movement faster than 1.5 deg s⁻¹. A ‘parallel continuous processing’ account of motion form conjunction search is proposed.