The precision of single neuron responses in cortical area V1 during stereoscopic depth judgments

The performance of single neurons in cortical area V1 of alert macaque monk eys was compared against the animals' psychophysical performance during a b inocular disparity discrimination task. Performance was assessed with stimu li that consisted of a patch of dynamic random dots, whose disparity varied from trial to trial, surrounded by an annulus of similar dots at a fixed d isparity. On each trial, the animals indicated whether the depth of the cen tral patch was in front of or behind the annulus. For each disparity of the center patch, neural performance was assessed by calculating the probabili ty that the response of the neuron was greater or less than the response wh en the center disparity was the same as that of the annulus. Initially the animals performed the task simultaneously with the neural recording. Howeve r, the range of disparities used, which was appropriate for the neuronal re cording, may have affected performance, because the thresholds were substan tially lower (2.6x) when the psychophysical measurements were repeated late r. Average neuronal thresholds were similar to 4x poorer than these behavio ral thresholds, although the best neurons were marginally better than the a nimals' behavior. Thus, the well known precision of relative depth judgment s can be supported with signals from a small number of V1 neurons. Interfer ence with the relative depth information in the stimulus profoundly affecte d behavioral thresholds, which were similar to 10x poorer when the surround was absent or contained binocularly uncorrelated dots. In this case, singl e V1 neurons consistently outperform the observer: presumably here, psychop hysical thresholds are limited by other factors (such as uncertainty about vergence eye position).