Laboratory of Neuropsychology, National Institute of Health
Early visual system neuronal responses: A precise start with an imprecise finish
We examined the information content of responses of single neurons in the primary visual cortex of the fixating monkey while presenting a variety of stimuli that varied both in contrast and form (e.g. size, orientation). We calculated the information carried by three classes of neural code: (1) the spike count, (2) the precise times of spikes relative to stimulus onset and (3) precisely timed spike patterns where the times were expressed relative to the times of other spikes. All three codes carried stimulus related information. The information available from the spike count (meanSEM 0.460.022 bits, code 1) and the information from the time of the first spike in the response (0.490.023 bits, code 2) both carried approximately three times the information available from precisely timed patterns (0.14 0.013 bits, code 3). The time of the first spike of the response (response latency) and spike count codes showed a high degree of independence with nearly half (independence index=0.46) of the potential information from response latency being independent of the information from response strength. Spike count carried information almost entirely about the stimulus form whereas the information carried by response latency was almost entirely about stimulus contrast. The transmitted information of progressively later spikes of the response was progressively less than the information available from the first spike of the response (1st spike = 0.490.023, 2nd spike = 0.420.017, 3rd spike = 0.390.016 bits) and progressively more redundant with the information available from spike count (independence index, 1st spike = 0.46, 2nd spike = 0.29, 3rd spike = 0.16). The information from precise spike patterns was entirely redundant with the information from spike count (independence index = 0.0). These results indicate that the precise time (1 ms accuracy) at which a visually elicited response occurs relative to stimulus onset conveys substantial information, but thereafter the information is carried by spikes with lower (> 20 ms) temporal precision.