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Abstract: Motion aftereffects (MAE) are generally considered evidence for neuronal
populations tuned to specific directions of motion. Despite some early reports, there is scant
physiological or psychophysical evidence for neurons in visual cortex selective for the direction
of motion through depth (i.e., tuned to 3D motion) (Sakano et al., 2005). By comparing
adaptation under dichoptic and monocular conditions, we found large 3D MAEs that could not
be explained by a straightforward combination of monocular 2D MAEs.
Subjects viewed random dot stereograms containing corresponding dots moving in opposite
horizontal directions in the two eyes, thus producing 3D motion percepts. Following adaptation
to this 3D motion (100 sec, towards or away), subjects performed a series of direction
discrimination trials composed of a 4 sec top-up adaptation and 1 sec test stimulus. On each trial,
the test stimulus contained a variable proportion of signal dots moving through depth, and the
remainder of the dots followed random walks through depth (e.g. Hiris, Blake 1992). On each
trial, subjects reported the perceived global motion direction (towards or away).
Prolonged viewing of unidirectional 3D motion biased subsequent percepts of noisy 3D motion
test stimuli in the direction opposite adaptation. The contribution of 2D monocular adaptation
was dissociated by measuring monocular MAEs after identical 3D adaptation. Monocular MAEs
were ~4x weaker than the corresponding 3D MAE. This suggests that the effects of adapting to
3D motion cannot simply be accounted for by the combination of monocular 2D aftereffects;
mechanisms tuned to 3D motion must be involved. The underlying locus of adaptation was
explored by extending the motion adaptation paradigm to stimuli that isolate binocular 3D
motion cues (velocity-based (IOVD) cue by binocular contrast anti-correlation; disparity-based
(CD) cue by dynamic random dot stereogram).
Psychophysical results were supported by a matched series of human fMRI experiments which
revealed a decreased MT+ response to test stimuli moving in the adapted direction compared to
unadapted directions. Stimuli presented during event-related fMRI sessions were identical to the
psychophysical stimuli. Monocular contributions were evaluated by sequential presentation of
the left- and right-eye test stimuli. Attention was equated throughout scanning sessions by
requiring subjects to detect brief dot color changes that were uncorrelated with the stimulus
percept.
These results provide clear evidence for the existence of cell populations tuned to 3D direction,
and support a model of 3D motion computation based on interocular velocity differences.
Disclosures: T.B. Czuba, None; L.K. Cormack, None; A.C. Huk, None; B. Rokers, None.
Poster
558. Visual Motion: Neural Mechanisms
Location: South Hall A
Time: Tuesday, October 20, 2009, 8:00 am - 12:00 noon
Program#/Poster#: 558.25/CC42