Revisiting hemispheric asymmetry : perceptual representation and experiential modulation

Abstract

There are two important hypotheses that are central to the field of hemispheric asymmetry, namely the analytic/holistic dichotomy between the two cerebral hemispheres, and a perceptual bias to the left side of the stimuli or space (i.e., the left side bias effect) as a product of right hemisphere (RH) processing. The analytic/holistic dichotomy posits that the RH is more holistic in information processing whereas the left hemisphere (LH) is more analytic. Here this hypothesis was tested in Experiment 1 (face recognition) and Experiment 2 (Chinese character recognition) using the divided visual field paradigm; holistic processing was assessed through the composite task, which is commonly used in perceptual expertise research. The results of Experiment 1 showed that the RH and LH can be comparably holistic in recognizing unfamiliar faces, as opposed to the traditional analytic/holistic dichotomy hypothesis. In Experiment 2, it was found that while a standard holistic processing pattern emerged only in the left visual field/RH but not in the right visual field/LH in novice Chinese readers, neither visual field showed the holistic processing pattern in expert Chinese readers. These results demonstrated that hemispheric asymmetry in analytic/holistic processing, as assessed through the composite paradigm, may depend on the nature of the stimuli and can be modulated by expertise.

As for the left side bias (LSB) effect, it has been widely believed to reflect the dominance of RH processing. Nevertheless, it remains unclear whether reading direction can also account for the bias effect. Previous studies comparing readers of languages read from left to right with those read from right to left (e.g., French vs. Hebrew) have obtained inconsistent results, possibly due to potential confounding factors related to differences in culture or other characteristics of the languages instead of reading direction. Although Chinese is typically read from left to right, in contrast to other languages, it can also be read from right to left. Thus, Chinese provides a unique opportunity for a within-culture examination of reading direction effects. Chinese participants performed a perceptual judgment task (with both face and Chinese character stimuli) in Experiment 3 and two visuospatial attention tasks (the greyscales and the line bisection task) in Experiment 4 once before and once after reading direction priming, in which they read Chinese passages either from left to right or from right to left for about 20 minutes. After reading from right to left, participants showed significantly reduced LSB in Chinese character perceptual judgments but not in the other three tasks. This effect suggests that the modulation of reading direction on LSB effects may be stimulus specific. It also suggests that different forms of LSB may involve different underlying mechanisms.

In short, this thesis revisits two fundamental hypotheses related to hemispheric asymmetry. It demonstrates how our perceptual experience interacts with hemispheric processing differences in shaping our behavior.