Abstract
Purpose: :
Previously we reported that a severe loss of mf-VEP responses in the central visual field of human amblyopes is largely accounted for by the unsteady fixation of the amblyopic eye during the VEP experiments, which implied that the low-level cortical processing in amblyopic subjects may be relatively normal (Zhang et al, 2008, IOVS). In this study, therefore, we determined whether the spatial receptive-field (RF) properties of V1 and V2 neurons in amblyopic monkeys are different from those in normal monkeys.
Methods: :
Spatial contrast sensitivity functions were obtained in macaque monkeys and an amblyopia index was calculated to determine the severity of amblyopia in each subject. Microelectrode recording experiments were conducted in V1 and V2, using drifting sinusoidal gratings, to characterize the monocular spatial properties of individual neurons and their binocular signal interactions.
Results: :
The spatial frequency tuning of V1 and V2 neurons in the amblyopic monkeys was largely normal. However, the orientation tuning (assessed by calculating circular variance) of V2, but not V1, neurons was significantly different from that in normal monkeys. Both V1 and V2 neurons exhibited a high degree of binocular suppression, and the average magnitude of cortical binocular suppression was directly correlated with the depth of amblyopia in individual monkeys.
Conclusions: :
The present results suggest that during the critical period of development, impoverished signals from V1 to V2 in amblyopic monkeys due to severe binocular suppression appear to disrupt the normal maturation of the feed forward and intrinsic connections in V2, which in turn may alter the RF structures of V2 neurons. These anomalous alterations in the spatial RF structure of higher-order visual neurons may contribute to spatial deficits (distortion) in amblyopia.
Keywords: visual cortex • amblyopia • receptive fields