Visual perceptual processing in unilateral spatial neglect The case of visual illusions

This chapter reviews the empirical evidence concerning one specific aspect of visual perception in patients with unilateral spatial neglect (USN), namely the case of visual illusions (figures delimited by subjective contours, illusions of length and position). Studying the processing of visual illusions in neglect patients is of interest in three main respects. First, the observation that processing of illusions is spared vs. defective in such patients elucidates the role of spatial awareness (putatively defective in neglect) in the analysis of illusory configurations. In addition, illusory effects may be assessed both explicitly and implicitly, testing the view that unilateral spatial neglect is a disorder of spatial awareness. Second, a particular type of illusions, namely illusions of length and position, mimic some aspects of neglect: the expansion of the right side of a line, and the compression of its left side, induced by appropriately arranged fins, provide an analogue of the anisometric distortion of space representation in left neglect, that may bring about a rightward ipsilesional error in line bisection, and a leftward contralesional overextension. These similarities can help in elucidating possible mechanisms of some aspects of the USN. Third, anatomo-clinical correlation studies in brain-damaged patients provide evidence concerning the localization of brain regions concerned with the processing of visual illusory stimuli.