Stolte, MJK; (2013) The Role of Perceptual Load in Orientation Perception, Visual Masking and Contextual Integration. Doctoral thesis , UCL (University College London).

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Abstract

Previous research of the effects of perceptual load on visual processing demonstrates reduced perceptual sensitivity and reduced neural activity for task-irrelevant stimuli under high compared to low perceptual load. However, the precise underlying mechanisms for reduced processing remain unclear. The present thesis approaches this question by assessing the interactions between low level visual stimuli and their visual context under different levels of perceptual load. The results from Chapter 2 demonstrate that perceptual load modulates visual perception of orientation not only by reducing overall signal gain but also by broadening the orientation tuning profile. These findings suggest that perceptual load not only reduces the signal strength but also reduces the extent to which the signal is discriminated from noise; thus altering the strength of contextual interaction. Chapter 3 further demonstrates the role of perceptual load in contextual interactions by establishing the effects of load on the tilt-illusion, which is thought to rely purely on interactions among orientations. The results show that high perceptual load increases the impact of context (leading to greater tilt-illusion) for subthreshold context presentation, which precludes top-down suppression of the task-irrelevant context. Chapters 4 to 5 further establish reduced efficiency of separating signal from noise with higher perceptual load, found when signal and noise are presented successively (as in backward masking). The time course of perceptual load effects shows a distinct pattern of both, more effective and longer lasting masking under high compared to low load. This pattern evolves rapidly for pattern masking which indicates low-level integration and demonstrates an early locus for the effects of load. It also persists at later periods for metacontrast and object substitution masking where the stimulus and mask do not spatially overlap, indicating load effects at later processing stages. Together, the results provide mechanistic explanations for reduced perception under high perceptual load.

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