The Temporal Responses of Neurons in The Primary Visual Cortex to Transient Stimuli

Neurons in primary visual cortex (VI) have periodical responses to smoothly drifting gratings, but only have transient responses to the static stimuli that evoke drastic responses at the initial tens of milliseconds. This hints that the information of stimuli could be processed primarily in this initial period. Study on neuronal responses during this period is critical for understanding the characters of neuronal responses to the static stimuli. We investigated the temporal response time courses of V1 neurons to six stimulus durations (5, 10, 20, 30, 40, and 50 ms) of static gratings. Responses to the static stimuli were denoted by the time course curves which were the contours of the Pen-Stimulus Time Histograms (PSTHs) with a resolution of 1 ms bin. Along with the prolongation of the stimulus duration, PSTH curves to different stimulus durations reflected the evolution of responses to the static stimuli. All the data were collected from the anesthetized cat V1 with extracellular unit recording. VI neurons showed wave-like response curves that grew up after a short latency to the stimulus onset and then dropped off gradually due to the stimulus offset. When stimulus duration prolonged, the time and width of the main peak (the first peak, the largest one, of PSTH response curve) increased and saturated at 30 ms stimulus duration that we tested. Magnitudes of most main peaks were almost equal except that the response peak to the 5 ms stimulus duration was significantly lower than those to the others. There were also offset response peaks which were evoked by the stimulus offset, but were smaller than the main peaks and contained less information about visual stimuli. The magnitude of the offset responses increased with the stimulus duration, so it could be regarded as the aftereffects of the stimulus offset. Statistically, the response durations (represented by 2 x half peak width) of VI neurons to different stimulus durations were not shorter than 39 ms (even the stimulus duration was as short as 5 or 10 ms), which might be the physiological basis of the visual persistence (the duration of perception for a visual stimulus is longer than the physical presentation) at the primary visual cortex level. On the other hand, the time differences of the main and offset peaks were longer than the corresponding stimulus durations, suggesting that the offset responses were delayed by the onset responses. This may be a kind of mechanism to ensure that the information processed by V1 neurons is not disrupted by the offset responses or the onset responses of another stimulus. Furthermore, the similarity of the minimal response duration (39 ms) and the minimal time peak (36 ms) between the main peak and offset peak suggest that the minimal time (response duration) necessary for VI neurons to process information is at least about 35 similar to 40 ms even for the stimuli presented for a time shorter than 30 ms. These results illustrate that with this very short period of time VI neurons can code the main characters of visual stimuli and transmit the enough information about the stimuli to the high visual cortex. The 35 similar to 40 ms may be the essential time for VI neurons to process visual information.