Visual search studies usually demand subjects to react with a key-press when a pre-specified target is found among a set of distracters. When the target-distracter similarity is high, RT's tend to increase with the amount of non-targets in the display (set size), as well as the error rate rises with the set-size. After parallel simple-feature analysis, or bottom-up processes, have taken place, a serial process is assumed to process the remaining information features until the target is found, under top-down, or attentional control (Wolfe, 1994, Psychonomic Bulletin and Review, 95: 15-48). An advantage of measuring saccades as a reaction on visual search, as it is done in the presented work, is the guarantee that the visual search processing took place on a stationary visual scene. A saccade is probably the fastest possible answer in tachoscopic visual search.

Classical visual search studies do usually require central fixation, but do not control this. In this study it is investigated if the planned reaction upon localisation of the target does influence these top-down processes. The rationale behind this study is that different goal-directed actions require different target information, and can influence top-down demands in visual search, if these processes subserve information processing for motor-purposes. Since a key press per se, as it usually measured to address visual search performance, does not require any target information, subjects had to react with a fast saccade towards the target, when it is found, or with a saccade-grasping sequence towards the target. Grasping requires prior knowledge of, for example, object orientation, in order to plan the movement successfully. A saccade alone does not require this information. Visual displays contained real 3D Plexiglas objects, that were lit from within, in a completely darkened room. 3 display sets were used, one with no distracters (just the target object), one with 4 distracters and a target, and one with eight distracters and a target, in a combined search task. Distracters differed from the target with respect to form (cylinder vs. bar) and colour (green vs. orange). For example, subjects had to saccade or saccade/grasp to a green bar (target) among green cylinders, orange cylinders and orange bars. Target forms and colours were completely randomised.

1. It is found that subjects were more likely to make an erroneous saccade to a distracter with the wrong shape, and correct colour (shape errors), when the saccade executed alone, and was not part of a saccade-grasp sequence. For saccadic colour errors, no such difference was found.
2. This implies that visual processing in a search task is influenced by the intended action, because form is a relevant feature for grasping, but not for a saccade alone. Form might be evaluated with higher quality when the reaction is a saccade/grasping movement.