Illusory conjunctions and the cerebral hemispheres

JOSE MORAIS 0 1 2 3 0 Free University of Brussels , Brussels, Belgium 1 REGINE KOLINSKY National Funds for Scientific Research , Brussels , Belgium and Free University of Brussels , Brussels, Belgium 2 YANIK MIOSSEC University of Litle III , Villeneuve d'Ascq, France 3 Copyright 1993 Psychonomic Society, Inc Two visual half-field experiments tested Moscovitch's (1979) proposition that cerebral asymmetry does not concern the earliest perceptual stages but only later processing. Subjects were briefly shown displays that included one (Experiment 1) or two (Experiment 2) types of forms differing 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' scores). In Experiment 1, displays included either rectangles or triangles. In the first case, the target was a cross; in the second case, it was a Star of David. A hemifield x size interaction was observed both on 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 simultaneously, 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 discussed. The results suggest that early preattentive processes of feature extraction are not lateralized, whereas some integrative mechanisms, such as Treisman's (1988) focal attention, may operate differently in the two hemispheres. - as a function of processing stage. The second, even more important, difficulty is that the characterization of early processing has for a long time been impeded by method ologicallimitations-namely, that early stages cannot be directly accessed, either by introspection or by instrumen tal responses (Fodor, 1983; Marcel, 1983; Treisman, 1986a). Yet some indirect ways of studying early pro cessing have been proposed and developed in the last 10 years. The present study attempted to test Moscovitch's hypothesis by using such an indirect approach-namely, the illusory conjunction phenomenon-and by contrast ing it with more traditional measures of performance, which probably reflect later processing stages. The illusory conjunction (ICs) phenomenon was first observed by Treisman and Schmidt (1982). When a dis play consisting of blue As and red Bs is briefly flashed with masking before and after presentation, observers quite often may report to have seen a blue B or a red A. Those errors in which the features, but not their combi nation, are correct occur more frequently than do simple feature misperceptions (e.g., a green A or a blue D). ICs were actually predicted by Anne Treisman's theory of ob ject perception, which involves two stages of processing that is, a stage of feature extraction followed by a stage of feature integration (Treisman, 1988; Treisman & Gelade, 1980). Simple features (such as color, segments, orien tation, etc.) are supposed to be extracted in parallel at the first processing stage. Then, at the second stage, the fea tures would be serially integrated into multidimensional objects. Co-occurrence in time and place of the already detected features would make them "glue" together. The early stage is assumed to be preattentive or automatic, whereas the second, which implies a serial scanning of each discontinuity detected in the visual field, would re quire focal attention. ICs would be observed when task conditions selectively prevent attention from correctly in tegrating the previously extracted features (Treisman & Paterson, 1984; Treisman & Schmidt, 1982). ICs reflect relatively early processing of feature extrac tion, followed by some "automatic" (and incorrect), by-default binding. How early these operations are in per ceptual processing is a matter for discussion (cf. Houck & Hoffman, 1986; Kolinsky, 1988, 1989). Yet accord ing to Treisman' s (l986b) theory, ICs arise without the intervention of focal attention, since by definition it is pre cisely attentional serial scanning that allows correct fea ture integration. In other words, correct integration and ICs share the same perceptual processes except for what concerns the mechanism of focal attention. Correct in tegration results from the action of this mechanism, whereas ICs arise when focal attention is prevented from acting efficiently. Under this view, ICs undoubtedly reflect more primitive, less complex mechanisms than does the correct perception of objects. According to Moscovitch's (1979) hypothesis, one may thus expect ICs of the display features to occur evenly in the two hemifields, despite the occurrence of a later ality effect for correct perceptions. Such a pattern of results would indicate that what is lateralized is the mech anism of integration, perhaps the focal attention mecha nism proposed by Treisman (1986b). The basic idea of the present study is to inspect both ICs and correct perceptions for the occurrence of laterality effects. To obtain unequivocal evidence on the laterality issue, we used two different materials that, according to previous evidence, might lead to opposite laterality ef fects for correct identification. Thus, the aim of the present study was to assess whether or not ICs and correct per ceptions show the same laterality pattern in a situation in which correct perception of one type of material may be presumed to rely mainly on left hemisphere processing (directly accessed by the right visual field) and correct perception of the other material may be presumed to rely mainly on right hemisphere processing (directly accessed by the left visual field). To obtain laterality effects on (at least) correct per ceptions, we manipulated the size of simple geometrical forms. We assumed that, in principle, the larger the form, the greater the involvement of the right hemisphere com pared with the left. This size manipulation was applied to two different kinds of forms: isosceles triangles point ing either to the top or to the bottom and rectangles laid either on their large or on their narrow side. These two types of displays may lead one to illusorily perceive either a Star of David or a cross, respectively (see examples in Figures Ia, lb, lc and la', lb', l c'). Sergent (1982) ob tained the predicted change in laterality effects for forms subtending visual angles similar to the ones we used (see Van Kleeck, 1989). Although size effects may possibly be interpreted in the context of the spatial frequency ap proach of laterality differences (Sergent, 1982), we wish to state that testing this notion was not a purpose of the present study. Before the present work, only one published study, to the best of our knowledge, dealt with the question of hemifield diffe (...truncated)