Perceptual size distortion: Expansion of left hemispace

Abstract

Errors of perceptual judgement are systematically made by normal observers during visual line bisection tasks. In this pattern of response, termed pseudoneglect, participants tend to bisect lines slightly to the left of their true centre. One explanation of this effect is that attentional differences between left and right hemispheres cause the leftward extent of a line to be misrepresented and appear longer than it actually is. Recent findings have also shown that patients with hemispatial neglect perceive objects in right hemispace as larger than physically identical objects in left hemispace. Presuming that this type of size estimation task involves the same attentional resources as line bisection, it is predicted that observable differences should be obtained in measurement of perceived size of objects presented in left and right hemispace in normal observers. Sensitivity in detection of object size was measured by presenting a circle simultaneously with an ellipse varying in horizontal or vertical dimension only. In one experimental manipulation participants were asked to indicate which of the two objects was wider in horizontal direction, and in another to report which object was taller in the vertical direction. A psychometric function for discrimination of a circle versus an oval was obtained for each condition for comparisons across hemifields. These revealed a consistent bias of size judgements of horizontal extent but not of vertical extent. For horizontal orientation judgements, the point of subjective equality for objects presented in the left hemispace decreased. This finding suggests that in much the same way as normal observers misperceive true centre in line bisection tasks, perception of space is distorted; left hemispace is enlargened relative to right hemispace and therefore the left stimulus is overestimated when left and right stimuli are identical. The findings are discussed in relation to attentional and perceptual accounts of visual bias.