Contribution of middle temporal area to coarse depth discrimination: comparison of neuronal and psychophysical sensitivity.

Recent work suggests that the middle temporal (MT) area contributes to depth perception in addition to its well established roles in motion perception. To determine whether single MT neurons carry disparity signals with sufficient fidelity to account for depth perception, we have compared neuronal and psychophysical sensitivity to disparity while monkeys discriminated between two coarse disparities (near vs far) in the presence of noise. The strength of the visual stimulus was titrated around psychophysical threshold by varying the percentage of binocularly correlated dots in a random dot stereogram. We find that the average MT neuron has sensitivity equal to that of the monkey, as was reported previously for direction discrimination in MT. We further address some important factors that could bias the neuronal/psychophysical sensitivity comparison, including the possibility that monkeys reach a decision before the end of the stimulus presentation. Unlike the predictions of a simple model that uses Poisson spiking statistics, the sensitivity of many MT neurons has little dependence on the time interval over which spikes are counted to compute a neuronal threshold. Thus the response properties of many MT neurons appear to be adapted for rapid discrimination of depth, and we describe how temporal variations in both signal and noise contribute to this effect. We therefore predicted that psychophysical thresholds should exhibit little dependence on viewing duration in our task, and this was confirmed by additional behavioral experiments. Overall, our findings show that MT is well suited to provide sensory signals that form the basis for perceptual judgments of depth.