Abstract
Persons with amblyopia have poor or absent stereopsis. One hypothesis is that they have increased internal disparity noise due to either abnormal topographic representation of receptive fields and/or abnormal vergence noise, which results in depth deficits in amblyopia. To test this hypothesis, we developed a psychophysical procedure to measure the equivalent internal disparity noise by adding external disparity noise (Gaussian position noise) to random-Gabor-patch (RGP) stereograms, i.e., adding position jitter to paired Gabor patches. The two RGP stereograms (3 cpd) with external disparity noise were presented in two temporal intervals, one with mean crossed and the other with mean uncrossed disparity. The task was to detect which interval was closer. We tested 7 (contrast) x 6 (external noise) conditions. For each condition, two staircases, one 3-up-1-down and the other 3-down-1-up, were interleaved to measure the minimum and maximum disparity thresholds (Dmin and Dmax). We found that at small external disparity noise, Dmin remains constant independent of the external noise, while at large external disparity noise, Dmin is proportional to the external noise. However, Dmax remains constant at all external disparity noise levels. Based on the assumption that Dmin is proportional to the combination of internal and external noise, the equivalent internal disparity noise can be estimated by fitting the model to the Dmin data. Compared to normal controls, amblyopic observers had much larger equivalent internal disparity noise, which partially explains the poor depth perception in amblyopic vision. On the other hand, Dmax is independent of the external disparity noise in both normal and amblyopic vision, providing a reasonable explanation why Dmax seems ‘normal’ even though there is much larger internal disparity noise in amblyopia. In conclusion, internal disparity noise appears to play an important role in depth perception and should be considered when treating depth deficits in amblyopia.