Hemispheric preponderance in categorical and coordinate visual processes

Two experiments were conducted to address whether a left hemispheric bias w ould be observed for categorical processing in both 'what' and 'where' syst ems (experiment 1) while a reverse bias would characterize coordinate proce ssing whatever the systems (experiment 2). Young normal subjects were teste d using divided visual field tasks. The results of experiment 1 indicated t hat subjects made categorical judgments in both what and where systems fast er when stimuli are presented to the left hemisphere. The results of experi ment 2 showed a significant interaction between Visual field and difficulty of processing coordinate relationships. Indeed, a left-hemisphere advantag e was observed when the task required easy processing whereas a right-hemis phere advantage was noted for difficult distinctions either in location (wh ere system) or in lightness (what system). The left-hemisphere advantage we observed for categorization in both systems confirms the Kosslyn's hypothe sis (1989) for the where system and suggests that the same left-hemisphere advantage also exists for the what system. Concerning coordinate processing , our findings highlight the influence of processing difficulty on the hemi spheric lateralization and evidence a right hemispheric advantage for diffi cult coordinate processing and a left hemispheric advantage for easy coordi nate processing. The results are discussed in terms of possible link betwee n on the one hand difficulty and coordinate processing, and easiness and ca tegorization on the other hand. (C) 1999 Elsevier Science Ltd. All rights r eserved.