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Abstract
Electrophysiological (EEG) studies of human perception have found that amplitude at posterior electrodes is more negative for symmetrical patterns compared to asymmetrical patterns. This negativity lasts for hundreds of milliseconds and it has been called sustained posterior negativity (SPN). Symmetry activates a network of visual areas, including the lateral occipital complex (LOC). The SPN is a response to presence of symmetry in the image. Given the sustained nature of this activation, in this study we tested the persistence of the SPN after stimulus offset. Two shapes were presented (for 0.5 s each) with a 1 s blank interval in between. We observed a sustained response after stimulus offset, irrespective of whether the task required processing of shape information. This supports the idea that the response to symmetry is generated by information in the image, independently of task, and that it is sustained over approximately one second post stimulus onset.
Highlights
Visual symmetry has an important role in the study of visual perception, and empirical work on perception of symmetry has a long history[1,2,3,4,5,6]
The International Society for Clinical Electrophysiology of Vision (ISCEV) defines three standard visual evoked potential (VEP) protocols based on the type of stimuli used: (a) Pattern-reversal, (b) Pattern onset/offset and (c) Flash[8]
For pattern reversal there is no distinction between onset and offset, and the typical VEP has an initial negative deflection (N70), a prominent positive peak (P100), and a later negative component (N155)
Summary
Visual symmetry has an important role in the study of visual perception, and empirical work on perception of symmetry has a long history[1,2,3,4,5,6]. FMRI studies have shown an extrastriate symmetry response when participants attended to www.nature.com/scientificreports colour rather than regularity[15]. We can test the effect of repeated presentation of symmetry, and compare the response to the second pattern based not just on the task but on the relationship with the first pattern.
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