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

Abstract

Neurons selective for binocular disparity form the neural substrate for stereoscopic depth perception and are found in several areas of primate visual cortex. Presumably, multiple representations of disparity exist to serve different functions, but the specific contributions of different visual areas to depth perception remain poorly understood. We examine this issue by comparing the contributions of the middle temporal (MT) area to performance of two depth discrimination tasks: a "coarse" task that involves discrimination between absolute disparities in the presence of noise, and a "fine" task that involves discrimination of very small differences in relative disparity between two stimuli in the absence of noise. In the fine task, we find that electrical microstimulation of MT does not affect perceptual decisions, although many individual MT neurons have sufficient sensitivity to account for behavioral performance. In contrast, microstimulation at the same recording sites does bias depth percepts in the coarse task. We hypothesized that these results may be explained by the fact that MT neurons do not represent relative disparity signals that are thought to be essential for the fine task. This hypothesis was supported by single-unit recordings that show that MT neurons signal absolute, but not relative, disparities in a stimulus configuration similar to that used in the fine task. This work establishes a link between the neural representation of disparity in MT and the functional contributions of this area to depth perception.

Neural Activity in Cortical Area V4 Underlies Fine Disparity Discrimination

“…The mean eccentricity of stimulus presentation was 7.3°(6.7°for Monkey R; 8.3°for Monkey K). These thresholds were comparable to those measured in previous studies using similar visual stimuli (Prince et al, 2000;Uka and DeAngelis, 2006). Our subjects performed near the limit of stereoacuity.…”

“…N/P ratios in fine disparity discrimination for V4 neurons are higher than those reported for V1 and MT neurons (Prince et al, 2000;Uka and DeAngelis, 2006). This does not necessarily indicate that V4 neurons are less sensitive than V1 and MT neurons because different N/P ratios might have resulted from the difference in methodology.…”

“…The primary visual cortex (V1) and the middle temporal area (MT) are unlikely to play a direct role in fine stereoscopic depth perception because neurons in these areas do not signal the relative disparity between adjacent features (Cumming and Parker, 1999;Uka and DeAngelis, 2006). Microstimulation and inactivation have not provided a causal link between neural activity in MT and discrimination of fine disparity (Uka and DeAngelis, 2006;Chowdhury and DeAngelis, 2008).…”

“…Microstimulation and inactivation have not provided a causal link between neural activity in MT and discrimination of fine disparity (Uka and DeAngelis, 2006;Chowdhury and DeAngelis, 2008). In contrast, the inferior temporal cortex (IT) is related to fine disparity discrimination because trial-by-trial fluctuations of neuronal responses are correlated with perceptual decisions about subthreshold disparity (Uka et al, 2005).…”

The neural basis of depth perception from motion parallax

“…However, a causal test of area MT's role in perceiving depth from motion parallax has not yet been performed, and such a test (e.g. based on electrical microstimulation or reversible inactivation, [40,42]) will be important to establish that MT is needed to perceive depth from motion parallax.…”