Abnormal Function of the Parvocellular Visual System in Anisometropic Amblyopia

Abstract

To study the function of the parvocellular (P) and the magnocellular (M) visual systems with steady-state visual evoked potentials (VEPs) in anisometropic amblyopes. A matrix of isolated checks was superimposed on a steady background with different check sizes and temporal frequencies to form specific stimuli to preferentially activate the P or the M visual system. The amplitude of the VEP fundamental frequency was analyzed at the electrode Oz of 5 anisometropic amblyopes and 22 normal subjects. The normal subjects were tested at two visual acuity (VA) levels, 20/20 and 20/40, modified by lenses, to match with the VA levels of the fellow eyes and the amblyopic eyes of the amblyopes, respectively. No significant amplitude difference was found between the dominant eyes and nondominant eyes of the normal subjects for either P or M stimuli at both 20/20 and 20/40 VA levels (P>.05). No significant amplitude difference was found between the fellow eyes of the amblyopes and the dominant eyes of normals for either P or M stimuli at 20/20 VA level (P>.05). A significant amplitude difference was found between the amblyopic eyes and the nondominant eyes of the normals for P stimuli (P<.05) but not for M stimuli (P>.05) at 20/40 VA level. The amplitude of the VEP fundamental frequency was selectively reduced for P stimuli in anisometropic amblyopic eyes. This clinical electrophysiologic finding confirms that only the function of the P visual system is abnormal in anisometropic amblyopic eyes.