Area MT Encodes Three-Dimensional Motion

Abstract

We use visual information to determine our dynamic relationship with other objects in a three-dimensional (3D) world. Despite decades of work on visual motion processing, it remains unclear how 3D directions-trajectories that include motion toward or away from the observer-are represented and processed in visual cortex. Area MT is heavily implicated in processing visual motion and depth, yet previous work has found little evidence for 3D direction sensitivity per se. Here we use a rich ensemble of binocular motion stimuli to reveal that most neurons in area MT of the anesthetized macaque encode 3D motion information. This tuning for 3D motion arises from multiple mechanisms, including different motion preferences in the two eyes and a nonlinear interaction of these signals when both eyes are stimulated. Using a novel method for functional binocular alignment, we were able to rule out contributions of static disparity tuning to the 3D motion tuning we observed. We propose that a primary function of MT is to encode 3D motion, critical for judging the movement of objects in dynamic real-world environments.

A Model of Binocular Motion Integration in MT Neurons

“…Applying their protocol will require incorporating sensitivity to changing disparity into our model framework. Disparity studies may provide additional constraints on sources of opponent inhibition, as disparity-tuned motion-opponent suppression has been described in studies of transparent motion in MT (Qian and Andersen, 1994;Bradley et al, 1995). Opponent suppression may be mediated by reciprocal interactions between disparity channels in MT or from interactions between disparity-tuned V1 subunit inputs to MT, as proposed in models of IOVD processing (Sabatini and Solari, 2004).…”

“…1A), we chose a simple and robust model of direction selectivity based on the motion energy model (Adelson and Bergen, 1985). Our V1 computation consists of the following steps for each direction channel: (1) convolving the stimulus, a sequence of images over time, with two spacetime oriented Gabor filters in quadrature; (2) squaring and summing the outputs to produce a directional motion energy signal; and (3) half-wave rectifying this opponent motion energy signal to produce a non-negative response.…”

“…It is also less sensitive to spatial phase, being more complex-cell-like (Adelson and Bergen, 1985), thereby providing a more stable signal. The square root is taken to keep the amplitude of the self-normalization term on the same order as the numerator.…”

“…Tailby et al (2010) found a striking reduction in pattern motion sensitivity in virtually all cells they recorded when the sinusoidal components of a plaid pattern (Adelson and Movshon, 1982) were presented to different eyes. Czuba et al (2014) used a similar dichoptic grating protocol to test sensitivity to 3D motion, or motion-in-depth (MID) and found MT neurons that responded well to MID, including many cells tuned for opposite directions in each eye, as previously reported (Zeki, 1974;Albright et al, 1984; but see Maunsell and Van Essen, 1983). Another recent study of MT used a stimulus protocol that isolated motion and disparity cues and found that dichoptic motion signals alone can produce responses tuned for MID (Sanada and DeAngelis, 2014), underscoring the importance of understanding binocular integration of motion in MT.…”

Binocular Mechanisms of 3D Motion Processing

“…Two independent and complementary studies recently examined this issue. Czuba et al (2014) found that the majority of neurons in MT (70%) encode information about 3D direction of motion on the basis of binocular cues. Approximately one-third of those (17% of the total) preferred motion trajectories directly toward or away from the observer (note that this is actually a neural overrepresentation because, at most reasonable viewing distances, the distance between the eyes subtends far less than 17% of the horizontal visual field).…”