Gilchrist I D, Findlay J M, Heywood C A, 1996, "Converging evidence for two separate processes in perceptual grouping" Perception 25 ECVP Abstract Supplement
Converging evidence for two separate processes in perceptual grouping
I D Gilchrist, J M Findlay, C A Heywood
Using a visual search paradigm, Gilchrist, Humphreys, Riddoch, and Neumann (Journal of Experimental Psychology: Human Perception and Performance in press) demonstrated that grouping was mediated by a contrast-polarity-dependent surface-based process, and a contrast-independent edge-based process. A vertical pair of circles `popped out' amongst horizontal pairs of circles as distractor only if the items within each pair had the same contrast polarity. Alternatively, if the circles were replaced with squares that had collinear edges, the vertical pair `popped out' regardless of the contrast polarity of the squares.
Here we report new data in which the role of these two grouping processes in the control of saccadic eye-movements was investigated. If subjects are presented with two items concurrently in an eye-movement task they will often make a saccade to the midpoint between the two items---this has been called the `global effect'. This effect is assumed to reflect some underlying integration of the visual information during the programming of saccade landing position. Distractor items that either shared the same polarity or had collinear edges with the target were found to affect the landing position of saccades. Distractors that had neither collinear edges nor common polarity produced a smaller global effect but the landing position was still affected. This suggests that the integration processes that underlie the global effect are sensitive to both common edge and surface properties. However, some integration may occur as a result of the mere presence of activity in the visual field.