Amblyopia is usually defined as a deficit in optotype (Snellen) acuity with no detectable organic cause. We asked whether this visual abnormality is completely characterized by the deficit in optotype acuity, or whether it has distinct forms that are determined by the conditions associated with the acuity loss, such as strabismus or anisometropia. To decide this issue, we measured optotype acuity, Vernier acuity, grating acuity, contrast sensitivity, and binocular function in 427 adults with amblyopia or with risk factors for amblyopia and in a comparison group of 68 normal observers. Optotype acuity accounts for much of the variance in Vernier and grating acuity, and somewhat less of the variance in contrast sensitivity. Nevertheless, there are differences in the patterns of visual loss among the clinically defined categories, particularly between strabismic and anisometropic categories. We used factor analysis to create a succinct representation of our measurement space. This analysis revealed two main dimensions of variation in the visual performance of our abnormal sample, one related to the visual acuity measures (optotype, Vernier, and grating acuity) and the other related to the contrast sensitivity measures (Pelli-Robson and edge contrast sensitivity). Representing our data in this space reveals distinctive distributions of visual loss for different patient categories, and suggests that two consequences of the associated conditions--reduced resolution and loss of binocularity--determine the pattern of visual deficit. Non-binocular observers with mild-to-moderate acuity deficits have, on average, better monocular contrast sensitivity than do binocular observers with the same acuity loss. Despite their superior contrast sensitivity, non-binocular observers typically have poorer optotype acuity and Vernier acuity, at a given level of grating acuity, than those with residual binocular function.

Cortical visual neurons in the cat and monkey are inhibited by stimuli surrounding their receptive fields (surround suppression) or presented within their receptive fields (cross-orientation or overlay suppression). We show that human contrast sensitivity is similarly affected by two distinct suppression mechanisms. In agreement with the animal studies, human surround suppression is tightly tuned to the orientation and spatial frequency of the test, unlike overlay suppression. Using a double-masking paradigm, we also show that in humans, overlay suppression precedes surround suppression in the processing sequence. Surprisingly, we find that, unlike overlay suppression, surround suppression is only strong in the periphery (Ͼ1°eccentricity). This result argues for a new functional distinction between foveal and peripheral operations.

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Resolution thresholds for Landolt C's and for vernier targets remain the same whether the target is stationary or moving with horizontal or vertical velocities of up to 2.5 degrees for foveal presentations lasting 0.1 and 0.2 s. Oblique target motions are tolerated only up to 1 degree. Because visual pursuit is ruled out by randomization of direction of motion and by the short exposure, it is concluded that a stationary retinal image is not a prerequisite for good acuity.

MacLeod for useful conversations on thistopic.GeraldWestheimer wrote the original probit program for the yes-no case. This program served as a model for subsequent programs that employed different strategies to handle the 2AFC case and the simulations. We also want to acknowledge considerable assistance from Martha Teghtsoonian and an anonymous reviewer in the editing of the final version. A computer program for 2AFC probit analysis is available from the first author on written request. Another program, available from the secondauthor, works with several shapes of the psychometric function; it has several options for estimating confidence limits and an option for estimating the upper asymptote.

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