Non-veridical perception in human amblyopia: perceptual evidence of neural changes in visual cortex

Abstract

Purpose: This study investigated the prevalence and nature of spatial misperceptions in human amblyopia. Methods: Thirty amblyopes with strabismus and/or anisometropia participated in the study. Subjects viewed sinusoidal gratings of various spatial frequencies, orientations and contrasts. Following inter-ocular comparison, subjects sketched the subjective appearance of those stimuli that had a non-veridical appearance. Results: Non-veridical visual perception was revealed in twenty amblyopes (∼67%). In some subjects, misperceptions were present despite normal contrast sensitivity. Distortions were present in both anisometropes and strabismics but were not linked to the depth of amblyopia in a simple way. Errors in perception became more severe at higher spatial frequencies and extended to spatial frequencies far below the contrast detection acuity limit, but very low spatial frequencies were perceived veridically. The prevalence and severity of misperceptions was frequently found to depend upon the orientation of the grating. All of the varied perceptual errors previously reported in the literature were observed in our sample of amblyopes, indicating that the wide range of distortions previously reported reflect genuine inter-subject differences. In spite of the seemingly heterogeneous nature of these perceptual distortions, almost all can be simulated by replacing the stimulus orientation with a combination of two oblique (relative to the stimulus) gratings. Conclusions: We propose that non-veridical perception in human amblyopia may have its origins in errors in the neural coding of orientation in visual cortex. Modeling shows that, due to the anatomical covariance of ocular dominance columns and orientation pinwheels, such errors in orientation coding are to be expected if the amblyopic eye's ocular dominance columns are reduced in size.