ILLUSORY CONJUNCTIONS AND THE CEREBRAL HEMISPHERES

Two visual half-field experiments tested Moscovitch's (1979) propositi on that cerebral asymmetry does not concern the earliest perceptual st ages but only later processing. Subjects were briefly shown displays t hat included one (Experiment 1) or two (Experiment 2) types of forme d iffering in size and which, according to previous evidence, might lead to opposite laterality effects. Laterality effects were assessed for correct detections and for illusory conjunctions, both in terms of raw detection scores and in terms of perceptual discriminability (d' scor es). In Experiment 1, displays included either rectangles or triangles . In the first case, the target was a cross; in the second case, it wa s a Star. of David. A hemifield x size interaction was observed both o n correct detections and on associated discriminability. Yet, no such interaction was obtained for illusory conjunctions or for associated d ' scores. In Experiment 2, the two types of forms were presented simul taneously, with the small ones either inside or outside the large ones . No laterality effects were observed. Some implications of these data for both hemispheric asymmetry and feature integration issues are dis cussed. The results suggest that early preattentive processes of featu re extraction are not lateralized, whereas some integrative mechanisms , such as Treisman's (1988) focal attention, may operate differently i n the two hemispheres.