Dissociating neural activity associated with the subjective phenomenology of monocular stereopsis: an EEG study

Summary

The vivid sense of depth and solidity that defines stereopsis is usually attributed to the brain's processing of binocular disparity. However, the same impression can occur when viewing a picture with one eye through a small hole. By measuring EEG brain activity while people looked at images of 2D and 3D shapes under different viewing conditions, a specific pattern of neural activity was identified that accompanied this qualitative depth experience. Only the monocular aperture condition, which produces the strongest depth impression, showed elevated gamma-band synchronization in the parietal cortex when comparing 3D to 2D shapes. This suggests that the subjective experience of stereopsis involves neural processes distinct from those that compute binocular disparity.

Study at a glance

Abstract

The subjective phenomenology associated with stereopsis, of solid tangible objects separated by a palpable negative space, is conventionally thought to be a by-product of the derivation of depth from binocular disparity. However, the same qualitative impression has been reported in the absence of disparity, e.g., when viewing pictorial images monocularly through an aperture. Here we aimed to explore if we could identify dissociable neural activity associated with the qualitative impression of stereopsis, in the absence of the processing of binocular disparities. We measured EEG activity while subjects viewed pictorial (non-stereoscopic) images of 2D and 3D geometric forms under four different viewing conditions (Binocular, Monocular, Binocular aperture, Monocular aperture). EEG activity was analysed by oscillatory source localization (beamformer technique) to examine power change in occipital and parietal regions across viewing and stimulus conditions in targeted frequency bands (alpha: 8-13Hz & gamma: 60-90Hz). We observed expected event-related gamma synchronization and alpha desynchronization in occipital cortex and predominant gamma synchronization in parietal cortex across viewing and stimulus conditions. However, only the viewing condition predicted to generate the strongest impression of stereopsis (monocular aperture) revealed significantly elevated gamma synchronization within the parietal cortex for the critical contrasts (3D vs. 2D form). These findings suggest dissociable neural processes specific to the qualitative impression of stereopsis as distinguished from disparity processing.