Noise provides some new signals about the spatial vision of amblyopes

Amblyopia results in a loss of contrast sensitivity and position acuity. Here we report the results of experiments using noise to try to better understand the nature of the neural losses in amblyopia. In the first experiment, we used noise to derive the template or classification image used to detect a target and to discriminate its position. We found that some amblyopic observers show markedly abnormal templates for the position task and moderately abnormal classification images for the detection task; however, the abnormal template could not fully account for the loss of performance (efficiency). Reduced efficiency in the amblyopic visual system may reflect a poorly matched template, a high fraction of internal to external noise, or both. Comparison of the observers' performance with that of their template suggests that the amblyopes have a high fraction of internal (relative to external) noise. To analyze the internal noise further, we used a "double-pass" technique, in which observers performed the identical experiment twice. The amount of disagreement between the two experiments provides another estimate of the fraction of internal noise. Amblyopes show a much higher fraction of stimulus-dependent internal noise than do normal observers. We conclude that the loss of efficiency in amblyopia is attributable in part to a poorly matched template, but to a greater degree, to a high fraction of internal (relative to external) noise.