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Abstract:
Theories of cue combination suggest the possibility of
constructing visual stimuli that evoke different patterns of
neural activity in sensory areas of the brain, but that cannot be
distinguished by any behavioral measure of perception. Such
stimuli, if they exist, would be interesting for two reasons.
First, one could know that none of the differences between the
stimuli survive past the computations used to build the percepts.
Second, it can be difficult to distinguish stimulus-driven
components of measured neural activity from top-down components
(such as those due to the interestingness of the stimuli).
Changing the stimulus without changing the percept could be
exploited to measure the stimulus-driven activity. Here we
describe stimuli in which vertical and horizontal disparities
trade during the construction of percepts of slanted surfaces,
yielding stimulus equivalence classes. Equivalence class
membership changed after a change of vergence eye posture alone,
without changes to the retinal images. A formal correspondence
can be drawn between these ``perceptual metamers'' and more
familiar ``sensory metamers'' such as color metamers.
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