Contrast and phase processing in amblyopia

Contrast-coding was investigated in amblyopic, fellow non-amblyopic and control eyes. Using a contrast-matching paradigm similar to Georgeson and Sullivan [J. Physiol. 252, 627-656 (1975)], amblyopic eyes were found to have a high frequency contrast-coding deficit not only at threshold but also at suprathreshold levels up to at least 0.45 contrast. The results do not support the claim of Hess and Bradley [Nature 287, 463-464 (1980)] and Hess, Bradley and Piotrowski [Proc. R. Soc. Lond. B217, 309-330 (1983)] that amblyopes have normal or near-to-normal suprathreshold contrast-coding. By taking this contrast-coding deficit into account we demonstrate that previously reported poor phase discrimination [Lawden, Hess and Campbell, Vision Res. 22, 1005-1016 (1982); Pass and Levi, Invest. Ophthal. Visual Sci. 23, 780-786 (1984)] in amblyopia may, in part, reflect a more basic deficit in contrast-coding across spatial frequencies. The possible involvement of contrast processing mechanisms in phase discrimination is discussed.