Contextual influences in the peripheral retina of patients with macular degeneration.

Macular degeneration (MD) is the leading cause of low vision in the elderly population worldwide. In case of complete bilateral loss of central vision, MD patients start to show a preferred retinal region for fixation (PRL). Previous literature has reported functional changes that are connected with the emergence of the PRL. In this paper, we question whether the PRL undergoes a use-dependent cortical reorganization that alters the range of spatial lateral interactions between low-level filters. We asked whether there is a modulation of the excitatory/inhibitory lateral interactions or whether contextual influences are well accounted for by the same law that describes the integration response in normal viewers. In a group of 13 MD patients and 7 age-matched controls, we probed contextual influences by measuring the contrast threshold for a vertical target Gabor, flanked by two collinear high-contrast Gabors. Contextual influences of the collinear flankers were indicated by the changes in contrast threshold obtained at different target-to-flanker distances (λs) relative to the baseline orthogonal condition. Results showed that MDs had higher thresholds in the baseline condition and functional impairment in the identification tasks. Moreover, at the shortest λ, we found facilitatory rather than inhibitory contextual influence. No difference was found between the PRL and a symmetrical retinal position (non-PRL). By pulling together data from MD and controls we showed that in the periphery this inversion occurs when the target threshold approach the flankers' contrast (about 1:3 ratio) and that for patients it does occur in both the PRL and a symmetrical retinal position (non-PRL). We conclude that contrary to previous interpretations, this modulation doesn't seem to reflect use-dependent cortical reorganization but rather, it might result from a reduction of contrast gain for the target that promotes target-flankers grouping.