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Abstract
Global context can dramatically influence local visual perception. This phenomenon is well-documented for monocular features, e.g., the Kanizsa triangle. It has been demonstrated for binocular matching: the disambiguation of the Wallpaper Illusion via the luminance of the background. For monocular features, there is evidence that global context can influence neuronal responses as early as V1. However, for binocular matching, the activity in this area of the visual cortex is thought to represent local processing, suggesting that the influence of global context may occur at later stages of cortical processing. Here we sought to test if binocular matching is influenced by contextual effects in V1, using fMRI to measure brain activity while participants viewed perceptually ambiguous “wallpaper” stereograms whose depth was disambiguated by the luminance of the surrounding region. We localized voxels in V1 corresponding to the ambiguous region of the pattern, i.e., where the signal received from the eyes was not predictive of depth, and despite the ambiguity of the input signal, using multi-voxel pattern analysis we were able to reliably decode perceived (near/far) depth from the activity of these voxels. These findings indicate that stereoscopic related neural activity is influenced by global context as early as V1.
Highlights
Visual perception is influenced by spatial context
When we considered the aggregate activity of voxels within area V1, we found that we were unable to reliably decode depth from fMRI activity evoked by ambiguous stimuli (Fig. 7)
It has long been understood that the visual system must first match the features of the image between the retinae (“solve the correspondence problem”)
Summary
Visual perception is influenced by spatial context. For instance, two objects of equal luminance can appear to have different levels of brightness if they are observed in different contexts (Fig. 1a). By manipulating the background luminance of the grating, it is possible to constrain the perceptual interpretation of the stimulus from its ambiguous state so that one perceptual interpretation is chosen (Anderson & Nakayama, 1994; Fig. 1c) This is thought to occur as a result of the visual system seeking to preserve contrast polarity during the matching of monocular inputs. While this matching constraint is only present at the left and right edges of the stereogram, the entire pattern is perceived as consistently near or far. This suggests a neural mechanism sensitive to extensive portions of the viewed scene
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