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 sear ch task, in which subjects had to detect, as quickly as possible, the prese nce or absence of a target among distractors [Q-like element among O stimul i (QvsO) and vice-versa (OvsQ)]. According to an influential theory [Treism an, A. & Gelade, G. (1980) Cognitive Psychol, 12, 97-136; Treisman, A. & Sa to, S. (1990) J. Exp. Psychol. Hum. Percept. Perform., 16, 459-478], only t asks yielding nonflat search functions (OvsQ) involve focal attention. Alte rnative models propose that all kinds of visual search are resolved by a bi ased competitive process, working in parallel across the visual field. Data show that QvsO and OvsQ tasks are characterized by quantitative rather tha n 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 n umber of saccades made during search predicted very accurately the time req uired to accomplish the task and fixation times were independent of the num ber of stimulus items. On the basis of our results there is no reason to po stulate the occurrence of shifts of visuospatial attention, other than thos e associated with the executions of saccadic eye movements, which are drive n by a parallel feature analysis of the visual scene, in both types of sear ch tasks. A time-limited competitive model for attentive target identificat ion, in which both parallel (competitive) and serial (attentive) processing mechanisms are integrated, can account for these findings, providing a uni fied conceptual framework for all kinds of visual search.