A Functional Link between MT Neurons and Depth Perception Based on Motion Parallax

Abstract

As an observer translates, objects lying at different distances from the observer have differential image motion on the retina (motion parallax). It is well established psychophysically that humans perceive depth rather precisely from motion parallax and that extraretinal signals may be used to correctly perceive the sign of depth (near vs far) when binocular and pictorial depth cues are absent or weak. However, the neural basis for this capacity remains poorly understood. We have shown previously that neurons in the macaque middle temporal (MT) area combine retinal image motion with smooth eye movement command signals to signal depth sign from motion parallax. However, those studies were performed in animals that were required simply to track a visual target, thus precluding direct comparisons between neural activity and behavior. Here, we examine the activity of MT neurons in rhesus monkeys that were trained to discriminate depth sign based on motion parallax, in the absence of binocular disparity and pictorial depth cues. We find that the most sensitive MT neurons approach behavioral sensitivity, whereas the average neuron is twofold to threefold less sensitive than the animal. We also find that MT responses are predictive of perceptual decisions (independent of the visual stimulus), consistent with a role for MT in providing sensory signals for this behavior. Our findings suggest that, in addition to its established roles in processing stereoscopic depth, area MT is well suited to contribute to perception of depth based on motion parallax.

The neural basis of depth perception from motion parallax

“…The average choice probability for MT neurons was comparable with that found previously in studies of coarse motion discrimination [46,50], but smaller than that measured in a coarse depth-discrimination task based on binocular disparity cues [51]. The latter difference may stem from MT neurons having lower sensitivity to depth variations based on motion parallax cues than to depth variations based on binocular disparity cues [6].…”

“…To further understand the functional links between MT responses and depth perception from motion parallax, it is necessary to examine neural activity in animals that are trained to report their depth percepts. Kim et al [6] trained animals to judge whether a random-dot stimulus, in which depth was only defined by motion parallax, appeared to be nearer or farther than the fixation target. The experimental approach was largely similar to that used by Nadler et al [35], except that the proportion of dots that appeared at a near or far depth was variable and this 'depth coherence' parameter was used to titrate the difficulty of the task.…”

“…This suggests that behavioural performance could be accounted for by pooling the responses of relatively small populations of MT neurons. In addition, Kim et al [6] examined whether the activity of MT neurons was predictive of perceptual decisions regarding depth by computing choice probabilities [46,47]. Choice probability quantifies the trial-by-trial correlation between responses of a single neuron and the animal's choices, and generally reflects both correlated noise among neurons as well as the weights that are applied to neural responses in decoding [48].…”

“…The finding of significant choice probabilities is consistent with the possibility that neurons provide evidence used to perform the task, but it could also reflect top-down signals related to featural attention or decision-making (see [47,49] for more extensive discussions of the interpretation of choice probabilities). In the depthdiscrimination task based on motion parallax, Kim et al [6] found that responses of many MT neurons were weakly predictive of the animals' choices. The average choice probability for MT neurons was comparable with that found previously in studies of coarse motion discrimination [46,50], but smaller than that measured in a coarse depth-discrimination task based on binocular disparity cues [51].…”

“…For humans and non-human primates, depth perception based on binocular disparity cues generally outperforms that based on motion parallax, both in precision [6,7] and accuracy [8,9]. For many species with laterally placed eyes, such as birds and rodents, the visual fields of the two eyes have much less binocular overlap than for primates.…”