Disparity-Based Coding of Three-Dimensional Surface Orientation by Macaque Middle Temporal Neurons

Abstract

Gradients of binocular disparity across the visual field provide a potent cue to the three-dimensional (3-D) orientation of surfaces in a scene. Neurons selective for 3-D surface orientation defined by disparity gradients have recently been described in parietal cortex, but little is known about where and how this selectivity arises within the visual pathways. Because the middle temporal area (MT) has previously been implicated in depth perception, we tested whether MT neurons could signal the 3-D orientation (as parameterized by tilt and slant) of planar surfaces that were depicted by random-dot stereograms containing a linear gradient of horizontal disparities. We find that many MT neurons are tuned for 3-D surface orientation, and that tilt and slant generally have independent effects on MT responses. This separable coding of tilt and slant is reminiscent of the joint coding of variables in other areas (e.g., orientation and spatial frequency in V1). We show that tilt tuning remains unchanged when all coherent motion is removed from the visual stimuli, indicating that tilt selectivity is not a byproduct of 3-D velocity coding. Moreover, tilt tuning is typically insensitive to changes in the mean disparity (depth) of gradient stimuli, indicating that tilt tuning cannot be explained by conventional tuning for frontoparallel disparities. Finally, we explore the receptive field mechanisms underlying selectivity for 3-D surface orientation, and we show that tilt tuning arises through heterogeneous disparity tuning within the receptive fields of MT neurons. Our findings show that MT neurons carry high-level signals about 3-D surface structure, in addition to coding retinal image velocities.

Complex Cells in the Cat Striate Cortex Have Multiple Disparity Detectors in the Three-Dimensional Binocular Receptive Fields

“…This study investigated the organization of the binocular RFs of neurons in the early visual cortex, which are the lowest-level building blocks of depth-information processing in the brain (Maunsell and Van Essen, 1983;Ohzawa et al, 1990;Janssen et al, 1999;Taira et al, 2000;Connor, 2001, 2002;Prince et al, 2002a,b;Thomas et al, 2002;Nguyenkim and DeAngelis, 2003;Tanaka and Ohzawa, 2006). By analyzing the responses to dynamic 2D dichoptic random-dot stimuli whose patterns were uncorrelated between the two eyes, binocular interactions were examined for a pair of both X and Y positions in the RFs of single neurons.…”

Linking Neural Representation to Function in Stereoscopic Depth Perception: Roles of the Middle Temporal Area in Coarse versus Fine Disparity Discrimination

“…However, not all data are consistent with this simple view (see also Neri, 2005). Some MT neurons signal 3D surface tilt independent of absolute disparity (Nguyenkim and DeAngelis, 2003), which suggests that these neurons possess a form of relative disparity selectivity in response to linear gradients of disparity. In contrast, we do not see any clear relative disparity selectivity in MT using center-surround stimuli (Fig.…”

“…This highlights the point that relative disparity selectivity, in general, may be highly dependent on stimulus configuration. Tilt selectivity in MT was usually seen only at large slants (Nguyenkim and DeAngelis, 2003). Thus, this mechanism may be involved in representing the coarse 3D spatial layout of a scene, but may not be useful for fine depth or shape discrimination.…”

Why Is Spatial Stereoresolution So Low?

“…From this, Nienborg et al concluded (for reasons detailed in their paper) that V1 neurons provide piecewise frontal estimates of the depth map. Slant-and tilt-selective neurons have been observed in extrastriate cortex (Shikata et al, 1996;Nguyenkim and DeAngelis, 2003). If V1 neurons are in fact not selective for slant and tilt, as Nienborg and colleagues propose, how are higher-order neurons with such properties created?…”