Sensory gain control at fixation
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One mechanism by which spatial attention affects visual perception is through the alteration of the signal-to-noise ratio for a particular stimulus. This is known as sensory gain control. Sensory gain effects can be measured electrophysiologically through changes in the amplitude of the P1 event related potential (ERP) component. Manipulating perceptual load by increasing or decreasing task difficulty can influence spatial attention and can therefore modulate the P1 component. Sensory gain effects are well characterized with peripheral attention, but have rarely been studied at fixation. The few studies that have been conducted that look at sensory gain for foveal stimuli have yielded conflicting results, and sensory gain with centrally presented extrafoveal stimuli has only been found in emotion studies. The present study manipulated attention allocation towards foveal and extrafoveal stimuli at fixation, using two levels of perceptual load for each stimulus size. ERPs were recorded in response to stimulus onset, and tested for differences in P1 and N1 amplitude across perceptual load conditions. Sensory gain effects, as indexed by an increase in P1 amplitude with an increase in perceptual load, were predicted for extrafoveal but not foveal stimuli. Changes in P1 amplitude were not found for either type of stimuli, suggesting that sensory gain effects either may not be present at fixation or are not susceptible to manipulation by perceptual load. The N1 component was expected to increase in amplitude for high-load stimuli, due to the N1 attention effect. However, the opposite result was found, suggesting that there is an additional effect of perceptual load on early visual processing, distinct from sensory gain control.