Video-rate and continuous visual stimuli do not produce equivalent response timings in visual cortical neurons

Video cathode ray tube (CRT) technology has proven to be extremely valuable for performing research in the visual system. However, the image on a CRT monitor is not constant, but consists of a series of brief pulses. This has implications for any study that explores the responses of neurons in the visual system on short time scales. In particular, there is no unambiguous time point at which a visual stimulus presented via CRT may be said to have ended. Recordings from single units in visual cortical area VI of an awake primate demonstrate that, when studying changes in response timing on the order of 10 ms or less, stimuli delivered at video frame rates do not duplicate the effects seen with stimuli that have continuous functions of luminance versus time. Additionally, there does not seem to be any clear method of comparing the results obtained with video-rate stimuli with results obtained with continuous-time stimuli that holds for all conditions. These effects are especially critical when exploring the time course of the neuronal responses to the ending of a visual stimulus (off-response). Our findings cast doubt upon the recently reported result that off-responses have consistently shorter latencies than on-responses.