Temporal constraints on the grouping of contour segments into spatially extended objects

The speed of contour integration was investigated in a task that can be sol ved by grouping contour segments into elongated curves. Subjects had to det ect a continuous curve, which could be intersected by one or two other curv es. At locations where these curves came in close proximity, the assignment of contour segments to the different curves could be based on collinearity . Reaction times exhibited a strong dependence on (I) the presence of inter sections among curves; and (2) the context provided by the stimulus set fro m which individual stimuli were selected. Reaction times were shortest when grouping of contour segments depended on information at a single location in the visual field. In this condition, responses to stimuli containing an intersection were faster than responses to stimuli that did not. When respo nses were determined by information at spatially separate locations, respon ses were delayed, and every intersection increased the reaction time consid erably. This result contrasts with earlier investigations which have sugges ted that contour integration on the basis of collinearity is performed pre- attentively but is in accordance with studies on curve tracing. We propose that the assignment of contour segments to equally coherent curves, a proce ss which may be called figure-figure segregation, is a function of object-b ased attention. Moreover, the protracted reaction times for some of the sti muli indicate that spread of attention within an object costs time. This im plies that object recognition is not always as fast as is sometimes assumed . (C) 1999 Elsevier Science Ltd. All rights reserved.