Three-dimensional motion aftereffects reveal distinct direction-selective mechanisms for binocular processing of motion through depth

Abstract

Motion aftereffects are historically considered evidence for neuronal populations tuned to specific directions of motion. Despite a wealth of motion aftereffect studies investigating 2D (frontoparallel) motion mechanisms, there is a remarkable dearth of psychophysical evidence for neuronal populations selective for the direction of motion through depth (i.e., tuned to 3D motion). We compared the effects of prolonged viewing of unidirectional motion under dichoptic and monocular conditions and found large 3D motion aftereffects that could not be explained by simple inheritance of 2D monocular aftereffects. These results (1) demonstrate the existence of neurons tuned to 3D motion as distinct from monocular 2D mechanisms, (2) show that distinct 3D direction selectivity arises from both interocular velocity differences and changing disparities over time, and (3) provide a straightforward psychophysical tool for further probing 3D motion mechanisms.

Figure 1

Screen capture of the basic stimulus. In the actual experiments, the right and left halves were split between two monitors and viewed through a mirror stereoscope. Nonetheless, free-fusing will give a reasonable impression of the experimental percepts. We found that the 1/f texture in the center and surround greatly facilitated fusion and held stable vergence (which subjects could monitor with the horizontal and vertical nonius lines at fixation).

Figure 2

Movie 1. A (2D) sampling of various coherence levels as used in our method of constant stimuli. Each panel represents a single monocular half-image across a range of increasing rightward motion coherence levels (5, 20, 50, 80, and 95%). The reader should readily appreciate a continuum of motion strength.

Figure 3

Movie 2. Depiction of the 3D motion-through-depth stimulus during a series of top-up adaptation and test presentations. The right panel shows a faithful rendition of the stereoscopic stimuli used (fusible stereopair), and the left panel shows a perspective view of the same stimulus sequence.

Figure 4

Movie 3. Same as Figure 3 but showing the frontoparallel motion stimulus.

Figure 5

(A) A schematic of the basic experimental paradigm; subjects adapted to equal and opposite motion in the two eyes (producing a 3D motion percept of dots moving either toward or away from the observer), and then judged the perceived direction of motion in depth of a test stimulus with a coherence that varied from trial to trial. (B) The psychometric functions (parametrically combined across observers; see Methods section) mapping the coherence of the test stimulus (x-axis) to the percent of trials judged as “toward” the observer (y-axis). The green curve is a “toward” adapter, the red curve is an “away” adapter, and the black curve is a reference curve collected without any adaptation. Gray error bars show bootstrapped 95% confidence intervals. The abscissa corresponding to the 0.5 ordinate on each curve represents the point of subjective equality for each condition, and bootstrapped 95% confidence intervals are shown by the black horizontal bars. Clearly, a substantial 3D MAE is present.

Figure 6

(A) Schematic of and (B) data from the frontoparallel motion condition. The aftereffect is much smaller than when the adaptation stimulus moved though depth, and the magnitude is also consistent with what has been reported previously for similar experiments (see text for references).

Figure 7

(A) Schematic of and (B) data from the monocular motion condition (monocular adaptation, monocular test presented to the same eye). The magnitude of the MAE is no different (statistically) than for the frontoparallel condition shown in Figure 6.

Figure 8

Data and psychometric functions from the (A) 3D adapt, monocular test condition (3D-mono) and (B) the interocular transfer condition (IOT). Under 3D adaptation conditions, one would expect the MAE resulting from monocular adaptation in the tested eye to be partially canceled by the interocular transfer of the (opposing) adaptation in the untested eye. The magnitude of the 3D-mono MAE is smaller than either 2D or 3D MAEs, confirming this expectation. (B) The data resulting from monocular adaptation in one eye and testing in the other eye (i.e., a direct measurement of the interocular transfer). Note that the reversal in the direction of the shift for the “toward” and “away” curves is as expected.

Figure 9

Movie 4. Same format as the previous stimulus movies (Figures 3 and 4), but showing the CD-isolating stimulus.

Figure 10

Movie 5. Same as the previous figure but showing the anti-correlated stimulus we used to bias the observers toward using the IOVD cue. Note the relative contrast of dots presented to the left and right eyes in the fusible stereopair (right side).

Figure 11

Movie 6. Same as the previous figure but showing the planar wedge stimulus configuration used in Experiment 2.

Figure 12

(A) Essentially, a replication of the main data from Experiment 1 (i.e., Figure 5(B)). The close agreement between the experiments indicates that the specific geometry of the stimulus was of little importance. (B) The data from the IOVD-biased adaptation stimulus; crucially, it is nearly identical to the standard 3D MAE. (C) The data from the CD-isolating adaptation stimulus. Despite generation of a clear depth percept during adaptation, this stimulus produced a surprisingly weak MAE.

Figure 13

Bar graphs depicting MAE magnitudes from individual observers (first 3 columns) for all 8 motion conditions as well as the combined data shown in the previous figures (last column). The first row shows MAE magnitudes with bootstrapped 95% confidence intervals. The second row shows the same data normalized to each observer’s 3D MAE magnitude.

Figure 14

Parametric plot summarizing the psychometric functions across all conditions in both experiments. Specifically, the steepness of the psychometric function (threshold sensitivity = α⁻¹) is plotted on the y-axis as a function of the MAE magnitude (β) on the x-axis. The solid and dashed contours show bootstrapped 68% and 95% confidence intervals across all subjects. The most striking observation is that adaptation containing IOVDs produced similar large MAEs (3D, 3D-planar, and IOVD), while adaptation lacking IOVDs (CD and all frontoparallel conditions) yielded comparatively small MAEs.