The integration of parallel and serial processing mechanisms in visual search: evidence from eye movement recording.

We examined timing and scanning paths of eye movements during a visual search task, in which subjects had to detect, as quickly as possible, the presence or absence of a target among distractors [Q-like element among O stimuli (QvsO) and vice-versa (OvsQ)]. According to an influential theory [Treisman, A. & Gelade, G. (1980) Cognitive Psychol, 12, 97--136; Treisman, A. & Sato, S. (1990) J. Exp. Psychol. Hum. Percept. Perform., 16, 459--478], only tasks yielding nonflat search functions (OvsQ) involve focal attention. Alternative models propose that all kinds of visual search are resolved by a biased competitive process, working in parallel across the visual field. Data show that QvsO and OvsQ tasks are characterized by quantitative rather than by qualitative differences in search strategy. No differences between the two tasks were found regarding either the percentage of saccades foveating single stimulus items or the timing of the button response with respect to the onset of the last foveation saccade within a trial. Furthermore, the number of saccades made during search predicted very accurately the time required to accomplish the task and fixation times were independent of the number of stimulus items. On the basis of our results there is no reason to postulate the occurrence of shifts of visuospatial attention, other than those associated with the executions of saccadic eye movements, which are driven by a parallel feature analysis of the visual scene, in both types of search tasks. A time-limited competitive model for attentive target identification, in which both parallel (competitive) and serial (attentive) processing mechanisms are integrated, can account for these findings, providing a unified conceptual framework for all kinds of visual search.