Spatial interactions in simple and combined-feature visual search

To isolate the mechanisms responsible for spatial interactions in visual search, we investigated the effects of inter-element distance and positional jittering in both simple (\ in/s) and combined-feature (inverted V in Vs) search tasks in which the observer had to find a target in a background of similar nontargets. Thresholds, defined as the stimulus duration for 75% correct, were measured for 'present' and 'absent' target conditions as a function of background numerosity (ranging from 4 to 64 background elements), independently for four inter-element distances and three positional jittering conditions. Results show: (1) both simple and combined-feature search involve a parallel, capacity limited process, (2) thresholds for parallel search of simple features are directly related to inter-element distance whereas this has little effect on thresholds in combined-feature search, and (3) positional jittering has a direct effect on thresholds in combined-feature search and an inverse effect in simple-feature search. These results indicate that two different mechanisms of spatial interactions are involved in parallel search. The activation of each of the two mechanisms depends on the stimulus used. In parallel search for simple and dissimilar features, the underlying mechanism is a short-range one, based on lateral inhibition, whereas the parallel search for combined and similar features is based on a nontarget grouping mechanism which relies on facilitatory interactions between regular elements.