The effect of awareness on hemispheric asymmetries in object-based processing

Abstract

Research has suggested that object-based processing is lateralized more to the left than right hemisphere. Independently, Breitmeyer and colleagues have observed that object-based processing varies with awareness. The present study integrated these findings by investigating whether hemispheric lateralization for object-based processing changes as a function of awareness. While participants fixated a centrally located cross, two stimuli were consecutively presented in the same location to either the left or right visual field. Both the prime and target stimuli could be one of four color/shape combinations (blue/square, blue/diamond, green/square, green/diamond) creating three congruency conditions (all-congruent, part-congruent, all-incongruent). To manipulate the participants' awareness of the prime, the stimulus onset asynchrony (SOA) was either 50ms (unaware) or 250ms (aware). SOA was blocked, while congruency was randomized within each block. Reaction times were measured and a main effect of congruency was found, indicating interference by the priming stimulus as expected. Following the methodology developed by Breitmeyer and colleagues, expected part-incongruent RTs were estimated by averaging the observed RTs for the all-congruent and all-incongruent conditions. It was predicted that if the two features of the targets (form and color) were being processed as objects (i.e. conjoined), the RTs for the part-congruent pairs would show more interference than a neutral prime. The RTs for the observed part-congruent condition should then be significantly larger than the expected part-congruent RTs. Analyses were conducted comparing the expected and observed part-incongruent RTs at the two SOAs for the left and right hemispheres. A significant difference was found when comparing the expected and observed part-incongruent RTs for the 250ms SOA when stimuli were presented directly to the left hemisphere. This result is consistent with previous research showing that object-based processing is associated more with left hemisphere function than right, but suggests that lateralization occurs only when objects enter awareness.