The purpose of the study was to determine whether amblyopes show detection loss for second-order spatial information, and if present, whether the loss is explained by the loss of first-order spatial input. We psychophysically determined detection thresholds for the amblyopic and non-amblyopic eyes of five adult amblyopes and the dominant eyes of three control observers. We found that four amblyopic eyes and two non-amblyopic eyes showed second-order loss relative to the control eyes. The second-order loss was greater than the first-order loss at the carrier spatial frequency (first-order input). The extra second-order loss indicates an early amplification of cortical neural loss that we speculate is due to deficient binocular input to second-order neurons.

Humans with amblyopia have a well-documented loss of sensitivity for first-order, or luminance defined, visual information. Recent studies show that they also display a specific loss of sensitivity for second-order, or contrast defined, visual information; a type of image structure encoded by neurons found predominantly in visual area A18/V2. In the present study, we investigate whether amblyopia disrupts the normal architecture of spatial interactions in V2 by determining the contrast detection threshold of a second-order target in the presence of second-order flanking stimuli. Adjacent flanks facilitated second-order detectability in normal observers. However, in marked contrast, they suppressed detection in each eye of the majority of amblyopic observers. Furthermore, strabismic observers with no loss of visual acuity show a similar pattern of detection suppression. We speculate that amblyopia results in predominantly inhibitory cortical interactions between second-order neurons.

Amblyopes show bilateral loss of sensitivity for second-order (contrast defined) stimuli that can be further suppressed by flanking second-order stimuli (whereas flanks facilitate sensitivity in normal observers). The suppressive flank effect in amblyopes might be explained by abnormal pooling of second-order contrast across visual space. In this study, we investigate whether amblyopes show abnormal second-order spatial summation by measuring contrast detection thresholds for 1c/deg modulations of random noise (stimuli 1-12 cycles) in amblyopic observers, strabismic observers with no visual acuity loss, and normal (control) observers. Non-control observers showed substantial bilateral loss of sensitivity relative to the control observers, as expected. However, all observers showed essentially equal second-order spatial summation: contrast detection threshold decreased with approximately the square root of the number of cycles, and then became independent of size at 6-8 cycles (similar asymptotes). We conclude that the pooling of second-order contrast across visual space is unaffected by amblyopia.

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