Vernier acuity in single neurons of monkey inferior temporal cortex

Humans can discriminate small offsets between nearly collinear lines, a phenomenon known as vernier acuity. However, the underlying neural correlates have not been investigated. To investigate this issue, we asked whether monkeys experience visual acuity like humans and which brain regions form the underlying neural basis. We created stimuli containing a square frame with a disk that could be moved along a horizontal or vertical line, in the presence of a horizontal or vertical bar at the center. If vernier acuity is present in behavioural or neural responses, we should observe greater sensitivity to small changes in the horizontal position of the disk when a nearby bar is oriented vertically rather than horizontally. Conversely, there should be greater sensitivity to changes in vertical position when the nearby bar is oriented horizontally but not vertically. We tested these predictions on monkeys performing a same-different task, as well as using neural responses recorded from their inferior temporal cortex, while they passively viewed the same stimuli. In Experiment 1, we tested 3 monkeys trained to perform a same-different task. Here, all three animals showed higher sensitivity to position changes in the vernier conditions compared to the non-vernier conditions. In Experiment 2, we tested these predictions using wireless brain recordings from the inferior temporal cortex, a region critical for object recognition, while monkeys viewed these stimuli in a fixation task. Here too, we observed greater neural dissimilarity between the stimuli in the vernier condition compared to the non-vernier condition. Interestingly, this effect arose late in the neural response, suggesting that this effect arises through computation and is not simply inherited from the early visual areas. Taken together, our results show that monkeys, like humans, experience vernier acuity and this effect is likely driven by single neurons in the inferior temporal cortex.