Eye scanning of multi-element displays: II. Saccade planning.

The properties of saccadic eye movements were studied in a task that required observers to scan through a display consisting of a set of discrete objects. The saccades forming the scanpath showed very high accuracy with almost no undershoot provided no distractor item was located within a critical region around the saccade target. When a distractor item was located in the critical region, saccade accuracy was impaired. This result suggests that the intrinsic spatial selection process for saccadic scanning movements is of low resolution. The initial saccade following display onset showed different properties. The instructions required the observer to look first at a clearly distinct target at the top left of the display. However, the first saccade frequently showed 'oculomotor capture' and was directed either to a different item in the display, or to a midway location between items. The fixation following such erroneous first saccades was generally very short and showed a substantial drift towards the instructed location.