P1-4: Neural Correlates of Fading Illusion Revealed in Responses of V1 Neurons

Visual stimuli without sharp edges fade gradually under visual fixation. This phenomenon is known as Troxler fading or fading illusion. Traditionally, this fading is explained by a hypothesis that physical stimulus is cancelled by a negative image generated in the visual pathway, because the negative image is perceived momentarily when the physical stimulus is removed suddenly. One prediction that may be tested neurophysiologically is that, in the faded or adapted state, visual neurons actually respond to a blank screen, as if they actually ‘see’ the negative image. To examine this prediction in cat V1, we measured the contrast response function of single neurons by presenting flashed grating stimuli of various contrasts (7 contrasts in straddle, ±25%, range and pedestal, within 0–50%, range; negative values for phase reversal) in a rapid succession. Data were analyzed using reverse correlation. The orientation and spatial frequency of gratings were fixed at the optimal value of each cell. We found that V1 neurons showed a shift of contrast response function during pedestal conditions to respond to a blank screen (i.e., 0% contrast stimulus). The amount of shift was consistent with the arithmetic mean of contrasts in each experimental condition. These results support the hypothesis that a negative image is generated during visual fading, and it behaves as if it were a real stimulus. Additionally, we examined phase retardation using periodic stimuli to investigate the temporal transition of contrast response functions. The time constant of phase retardation was comparable to that we observed perceptually.