Abstract
Amblyopia is a developmental disorder known to result in abnormalities in the human visual cortex, although the extent of involvement of cortical areas beyond V1 remains unclear. We report here an updated analysis of an ongoing study (VSS 2003) of amblyopic visual cortex. This fMRI study of 12 adult amblyopes (6 esotropic strabismics, 6 anisometropes) and 6 controls directly compared the BOLD signals produced by monocular stimulation of each eye. Measures were made in three cortical regions-of-interest (ROIs) corresponding to extrafoveal V1 and V2 and the foveal representation at the occipital pole, individually defined using standard retinotopic mapping in the fellow (nonamblyopic) eye. Fixation stability was found to not differ significantly between subject groups, and correlated with signal magnitude only in the foveal ROI for strabismic eyes. Results showed that mean fMRI signal for amblyopic eyes was consistently lower than for fellow eyes in all ROIs, although a few amblyopes did not show this trend. Nevertheless, the interocular difference in extent of activation was significantly larger for both strabismics and anisometropes compared with controls. In addition, both subtypes showed significantly fewer voxels able to be driven by both eyes (significantly for V1 and V2 in anisometropes). Comparison of individual phase-encoded retinotopic maps obtained from amblyopic versus fellow eyes revealed less activation in the occipital pole corresponding to foveal stimuli, consistent with known psychophysical losses. Finally, a few amblyopes demonstrated the novel finding of more activity in parietal and temporal cortex from the amblyopic than the fellow eye. These results confirm that the amblyopic eye is deficient in driving visual cortex compared with both the fellow eye and control eyes, and demonstrate that both strabismic and anisometropic amblyopes have abnormal cortical representations of foveal stimuli.
Supported by grant I P20 RR15574-01 from NIH/NCRR to JM.