The effect of metacontrast masking in perception of symmetry in peripheral and foveal vision

Type-B masking is a technique widely used for preventing conscious access to visual information by presenting non-overlapping stimuli and masks at brief SOAs (20-100ms). Facilitation effects are observed at shorter SOAs (0-10ms). This phenomenon is thought to rely on feedback mechanisms interfering with feedforward processing of visual information. The CPD model hypothesises that peripheral vision is dominated by feedforward processing: therefore feedback-dependent visual phenomena is less effective in the periphery. This thesis tests this hypothesis through a series of experiments using symmetry detection tasks with simple and complex stimuli (disks, polygons, abstract shapes; Experiments 1-3). The results reveal a U-shaped curve, typical of type-B masking, in fovea and parafovea, while in periphery a monotonic type-A masking function was observed. Symmetry detection was tested as a tool to isolate and study feedback-dependent processing, as it would allow for the use of more complex visual stimuli that require global integration of information, while minimizing the confound of target-mask integration. This was confirmed by both behavioural and EEG data (Experiment 4).