Where is it? Deriving stimulus location from the responses of visual neurons

Hyperacuity, the ability to determine the location of an object with accuracy much finer than an individual photoreceptor, is one of the most striking capabilities of the visual system. The simplest and best known form of hyperacuity is the vernier task: to discriminate the horizontal offset of two identical bar stimuli. I recorded the visual responses of neurons in the cat lateral geniculate nucleus (LGN) to determine how well single-cell responses could be used to perform the vernier task. First, I examined the ability of a single cell to make a spatial discrimination based solely on the number of spikes evoked by a briefly flashed bar stimulus. I found that the performance derived from single-cell responses was worse than the psychophysical acuity previously reported. Second, single-cell performance does not improve significantly when I used different methods of translating a single-trial neuronal response to a 2-alternative forced-choice (2AFC) decision in vernier task. Finally, I investigated how to integrate the responses from multiple neurons to improve neuronal acuity. The level of psychophysical acuity could be reached either with single neurons fulfilling certain requirements (which would require additional information from other neurons) or with the pooled responses from a few (3 to 6) neurons. Comparison of these two scenarios with psychophysical results suggests that a vernier discrimination would be most simply achieved by pooling the responses of a few LGN neurons, all located on the same side of the bar stimulus.