Langley, K., Fleet, D.J., and Hibbard, P.
Stereopsis from contrast envelopes.
Vision Research, 39(14): 2313-2324, 1999
We report two experiments concerning the site of the principal
nonlinearity in second-order stereopsis. The first exploits the
asymmetry in perceiving transparency with second-order stimuli found
by Langley et al. (1998), i.e., the product of a positive-valued
contrast envelope and a mean-zero carrier grating can be seen
transparently only when the disparities are consistent with the
envelope appearing in front of the carrier. We measured the energy at
the envelope frequencies that must be added in order to negate this
asymmetry. We report that this amplitude can be predicted from the
envelope sidebands and not from the magnitude compressive pre-cortical
nonlinearities measured by other researchers. In the second
experiment, contrast threshold elevations were measured for the
discrimination of envelope disparities following adaptation to
sinusoidal gratings. It is reported that perception of the envelope's
depth was affected most when the adapting grating was similar (in
orientation and frequency) to the carrier, rather than to the contrast
envelope. These results suggest that the principal nonlinearity in
second-order stereopsis is cortical, occurring after orientation- and
frequency-selective linear filtering.
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