Cross-modal activation of visual cortex during depth perception using auditory substitution of vision

Abstract

Previous neuroimaging studies identified multimodal brain areas in the visual cortex that are specialized in the processing of specific information, such as visual-haptic recognition of objects. Here we test whether visual brain areas are involved in depth perception when auditory substitution of vision is used. Seven early blind subjects (EB) and nine blindfolded sighted volunteers (BS) were trained to use a prosthesis substituting vision with audition (PSVA) to recognize two-dimensional figures. They were also taught some pictorial monocular depth cues during an object distance estimation task with the prosthesis in a real three-dimensional environment. Using positron emission tomography, regional cerebral blood flow was assessed during exploration of virtual 3D images with the prosthesis while focusing either on 2D features (target search) or on depth (target distance comparison). Results in sighted subjects showed activation in visual association areas for both the target search task, involving the occipito-parietal cortex, and the depth perception condition, which activated occipito-parietal and occipito-temporal areas. This indicates that some brain areas of the visual cortex are relatively multimodal and may be recruited for depth processing via another sense than vision. By contrast in EB subjects the activation patterns during both target search and 3D perception were quite similar and restricted to the dorsal visual stream. The absence of any specific brain activation for depth perception in the EB subjects underlined the crucial role of previous visual experience to get a visual-like depth perception with the PSVA.