Contour interaction in amblyopia: scale selection

It has been known for some time that visual acuity in amblyopia is higher f or single letters than for letters in a row (termed crowding). Early work s howed that this could not be accounted for on the basis of the destructive interaction of adjacent contours (termed contour interaction), which was sh own to be, in resolution units, normal in amblyopia. We have re-examined th is issue using a letter stimulus that is modulated about a mean light level . This allows an examination of the effects of contrast polarity and spatia l filtering within the contour interaction paradigm. We show that the major ity of strabismic amblyopes that we investigated exhibit an anomalous conto ur interaction that, in some cases, was dependent on the contrast polarity of the flanking stimuli. Furthermore, we show that while amblyopes do selec t the optimum scale of analysis for unflanked stimuli, they do not select t he optimum scale of analysis for flanked stimuli. For reasons that may have to do with their poorer shape discrimination, they select a non-optimal sc ale to process flanked stimuli. (C) 2001 Elsevier Science Ltd. All rights r eserved.