Exploring the contribution of the magnocellular pathway in filling-in of artificial scotoma
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Perceptual illusions and disorders often provide insights into normal visual mechanisms. Artificial scotoma is a type of illusion where our visual system loses perception of a peripheral target on a dynamic noise background over several seconds. Several studies have shown that specific sensory manipulations like the size of target and background temporal variations affects the time taken for a target to fade. There is no unifying theory to account for the sensory factors that play a major role in determining the length of time taken to fade. The experiments described here explored the relation between sensory factors preferentially processed by the magnocellular pathway and the time taken to induce a scotoma. In addition to measuring time taken to fade, the duration of time a target stayed invisible was also measured. The two measures were recorded for conditions that either stimulate the magnocellular pathway or reduced its response relative to engaging the parvocellular pathway. The results indicate that altering the background characteristics by using different flicker rates, diffuse red light, and a background pulsed pedestal affected time to fade and the probability of fading. The spatial frequency within the target region affected the duration of fading. Time taken to fade seems to be modulated by background characteristics while the time taken to reappear seems to be more modulated by target region characteristics indicating the role of competition between figure and ground. The results provide strong support for the role of visual pathways and figure-ground segregation mechanisms in the perceptual filling-in of a scotoma.