ABNORMAL LONG-RANGE SPATIAL INTERACTIONS IN AMBLYOPIA

Neural interactions between widely separated stimuli were explored wit h psychophysical and visual evoked potential (VEP) measures in normal and amblyopic observers. Contrast detection thresholds were measured p sychophysically for small foveally viewed Gabor patches presented, in isolation and in the presence of similar, but laterally displaced flan ks. The amplitude and phase of; VEPs elicited by similar targets were also measured. The presence of neural interaction between the target a nd flank responses was assessed by comparing the unflanked threshold t o the flanked threshold in the psychophysical experiments and by compa ring the response predicted by the algebraic sum of test and flank res ponses to that measured when test and flanks were presented simultaneo usly. In normal observers simultaneous presentation of test and flank targets produces a VEP response that is up to a factor of two larger t han the linear prediction (facilitation). Psychophysical threshold is also facilitated by a comparable factor. Facilitation was found mainly for configurations in which local (carrier) and global (patch) orient ations resulted in collinearity, independent of global orientation (me ridian). Amblyopic observers showed several deviations from the normal pattern. The facilitation for the collinear configurations was either markedly lower than normal or was replaced by inhibition. The normal pattern of spatial interaction may facilitate the grouping of collinea r line segments into smooth curves. In contrast, abnormal long-range s patial interactions may underlie the grouping disorders and perceptual distortions found in amblyopia. (C) 1997 Elsevier Science Ltd.