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Erwin H. Wong, Dennis M. Levi, Brendan T. Barrett, I. Pacey; Non-linear transformation of sinusoidal gratings in amblyopia. Journal of Vision 2002;2(7):135. doi: 10.1167/2.7.135.
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PURPOSE: Both amblyopic and fellow non-amblyopic eyes show reduced sensitivity for static second-order stimuli (contrast modulation (envelope) of a sinusoidal grating (carrier)) (Wong, Levi & McGraw, 2001). In the present study we investigated whether that result was confounded by amblyopes having an abnormal early non-linearity to luminance. That is, an abnormal non-linearity to sinusoidal gratings could produce luminance artifacts at the envelope spatial frequency that might be detected in addition to the second-order (non-luminance defined) structure.
METHODS: We investigated this by having amblyopic and normal adult subjects view sinusoidal gratings (prior study carriers, contrast 5 – 90%) in which we varied the width of light and dark bars, with the provision that mean luminance was maintained. We psychophysically determined the point of subjective equality between light and dark bars for each eye of the amblyopes and the dominant eye of the control subjects.
RESULTS: All eyes perceived light bars to be wider than dark bars when bars were of equal width. This perception occurred for all sinusoidal gratings and corresponded to a non-linearity that weights luminance decrements more than luminance increments by a factor of 1.01 – 1.21. The weighting factors for both amblyopic and non-amblyopic eyes were not larger than that for the control eyes. Therefore, with respect to transformation of luminance, the second-order filter stage in amblyopia appears to receive normal first-order input.
CONCLUSION: We conclude that the loss of second-order sensitivity in our prior study was not confounded by amblyopes perception of a luminance (first-order) artifact at the envelope spatial frequency.
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