Functional organization of motion and disparity sensitivity in human visual cortex

Vision with two eyes makes perception of weak visual contrast signals easier and, due to the lateral separation of the eyes, allows for the triangulation of depth relationships. While binocular summation of contrast signals affords the observer increased sensitivity, binocular summation of spatial cues related to changes in depth is associated with decreased sensitivity to the corresponding retinal image displacements. Perceptual models of contrast and motion-in-depth sensitivity have explained this divergence in sensitivity by proposing that probabilistic neural noise limits summing and differencing operations on small signals. Because these models do not scale well for highly suprathreshold visual signals typical of the natural environment, we approached the question of how dynamic binocular image differences are coded using direct neural measurements. Here we use Steady-State Visual Evoked Potentials in human participants to show that inter-ocular differences in retinal image motion that produce elevated perceptual thresholds generate strongly suppressed evoked response amplitudes compared to motion that is matched between the two eyes. This suppression is strongly dependent on the availability of well-defined spatial references in the image and is highly immature in 5-month-old infants. Because the suppression is of equal strength for horizontal and vertical directions of motion, it is not specific to the perception of motion in depth. Relational image cues play a critical role in early to intermediate perceptual processing stages, and these results suggest that a succession of spatial and inter-ocular differencing operations condition the visual signal representation, prior to the extraction of motion-in-depth.\n\nSignificance StatementThe present work underscores the importance of relational spatial cues in both the motion and disparity domains for binocular visual coding. Relative motion and relative disparity cues not only support fine-grain displacement sensitivity but strongly influence suprathreshold responsiveness. Extraction of these cues supports a powerful binocular interaction within the motion pathway that is suppressive in nature and poorly developed in infants. This suppressive interaction is present for both horizontal and vertical directions of motion and is thus not specific to motion-in-depth, as previously believed, but is rather hypothesized to be a pre-processing step, with motion-in-depth being computed at a later or separate stage.