Hemispheric preponderance in categorical and coordinate visual processes

Two experiments were conducted to address whether a left hemispheric bias would be observed for categorical processing in both 'what' and 'where' systems (experiment 1) while a reverse bias would characterize coordinate processing whatever the systems (experiment 2). Young normal subjects were tested using divided visual field tasks. The results of experiment 1 indicated that subjects made categorical judgments in both what and where systems faster when stimuli are presented to the left hemisphere. The results of experiment 2 showed a significant interaction between visual field and difficulty of processing coordinate relationships. Indeed, a left-hemisphere advantage was observed when the task required easy processing whereas a right-hemisphere advantage was noted for difficult distinctions either in location (where system) or in lightness (what system). The left-hemisphere advantage we observed for categorization in both systems confirms the Kosslyn's hypothesis (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 hemispheric lateralization and evidence a right hemispheric advantage for difficult coordinate processing and a left hemispheric advantage for easy coordinate processing. The results are discussed in terms of possible link between on the one hand difficulty and coordinate processing, and easiness and categorization on the other hand.