Neuronal responses to first- and second-order motion in the superior temporal sulcus of the alert macaque

Purpose. Neurons in and around extrastriate cortical area MT provide behaviorally relevant signals about visual motion generated by luminance cues ("first-order" motion). Some neurons in this region also respond to motion generated by cues other than luminance ("second-order" motion). We wished to know if these neurons provide behaviorally relevant signals about second-order motion. Methods. We recorded from single neurons in and around area MT while the monkey attended to a fixation spot. We measured directional tuning and contrast response to sets of stimuli whose motion was generated by different cues. Two of the stimuli used luminance (first-order) cues to generate motion, while the other two stimuli used contrast or flicker (second-order) cues to generate motion. Results. Nearly all neurons gave vigorous, direction-selective responses to first-order motion. Responses to second-order motion varied widely. Most neurons gave weaker responses and were less selective for the direction of second-order targets, but a subset of neurons showed reliable, direction-selective responses to second-order motion. We measured contrast responses in the preferred and null direction for each class of motion, and calculated sensitivity to each of these targets. Neurons generally had low thresholds for first-order motion (0.005 - 0.1). Over half of the neurons were unable to reliably signal the direction of second-order motion at any contrast. The remaining neurons had thresholds that spanned a range of contrasts roughly 10 times higher than those for first-order motion. Conclusions. We find a small population of neurons in or around area MT that could provide a behaviorally relevant signal about second-order motion. The most sensitive of these cells had thresholds roughly similar to those of human observers viewing the same stimuli, and in general, the relative thresholds for first- and second-order motion were similar for cells and human observers.