Sex Related Effect of Unilateral Brain Lesions on the Perception of the Mueller-Lyer Illusion

Abstract

The aim of this study was to investigate the effect of unilateral brain lesions on Mueller-Lyer (M-L) illusion in the two sexes. Patients with left hemisphere (LH) and right hemisphere (RH) damage and control subjects participated in the experiment. They inspected series of M-L patterns in which the shaft with out-going fins was gradually shortened until it induced a perception opposite to the original illusion, that is, the shaft with out-going fins appeared to be shorter than the shaft with in-going fins. The subjects' task was to decide, on each trial, whether the variable shaft was longer or shorter than the other one. The point where the judgements changed from one category to the other was established using the Spearman distribution method for determining psychophysical thresholds, and was considered the measure of the strength of the illusion. The higher the value of the threshold, the stronger the illusion. Our results showed sex-related hemispheric asymmetry in subjects' susceptibility to the M-L illusion, i.e., both LH and RH lesions in females, but only RH lesions in males resulted in an increase of the strength of illusion. Moreover, males with LH lesion as well as controls partially corrected the illusory perception with practice, while both LH and RH damaged females and RH damaged males did not show this learning effect.