Action-related intentional effects in a visual search task

The aim of the current study was to test if visual features relevant to the task are processed more efficiently at an early, presumably parallel, level, compared to irrelevant target features. Subjects either had to grasp or point at a target. It was hypothesised that since orientation, in contrast to colour, is relevant to the action of grasping, enhancement of orientation-discrimination performance should be selective. The visual search task required searching for a conjunction of a particular colour and orientation. Subjects viewed stimuli on a screen while their gaze was tracked to determine the targeting of the first saccade. Target - distractor discriminability and set size were manipulated. In experiment 1, the difficulty of colour and orientation search was matched so that subjects would make 50% errors in feature search. In experiment 2, the colour contrast of target and distractors was decreased. There were two levels of set size of the search display. Enhanced orientation discrimination (relative to colour) was found for the condition in which subjects grasped the target compared to the condition in which they pointed towards the target. The action effect was most prominent in the small set-size, high-discriminability condition, and weakest in the large set-size, low-discriminability condition, with intermediate effects for the other two conditions. Action intention selectively enhances the processing of a behaviourally relevant feature. Signal detection modeling indicates that our results can be explained on the basis of an enhancement of an early and possibly parallel stage of feature processing