The role of perceptual load in orientation tuning

Abstract

Much previous research has demonstrated that perceptual processing depends on the level of perceptual load involved. Tasks of high perceptual load result in reduced perception of any task-irrelevant stimuli accompanied by reduced visual cortex response. However, it remains unclear whether these effects are mediated by an effect of perceptual load on the strength of neural signals or on the level of noise in the neural coding. We report a study investigating this question in the case of orientation perception. We varied the level of perceptual load in a letter search task presented in a circle surrounding fixation and assessed the effect on orientation discrimination for a vertically oriented Gabor patch presented inside the letter circle (either slightly above or below fixation). The Gabor patch was embedded in bandpass-filtered orientation noise. By varying the mean frequency of the orientation noise we were able to construct orientation tuning curves. The results revealed significant effects of perceptual load on both the amplitude and bandwidth parameters of the tuning curves. High perceptual load during letter search significantly increased the amplitude (i.e. increased the contrast threshold) and led to a considerable increase in the bandwidth, indicating broadened orientation tuning. These results suggest that depleting perceptual resources (when attention is engaged in a high load task) not only reduces the overall percept signal strength but also results in increased noise, thus compromising both the strength and precision of perceptual representations. The results are discussed in relation to previous research on the effects of attention on neural tuning.

Wellcome Trust (NL).