M.Sc Thesis

M.Sc StudentPalkovich Zvi
SubjectStudying Stereopsis of Human Visual Areas Using Visual
Evoked Potentials
DepartmentDepartment of Biomedical Engineering
Supervisor ASSOCIATE PROF. Moshe Gur
Full Thesis textFull thesis text - English Version


Stereopsis is the process responsible for the perceptual reconstruction of the depth dimension when the brain combines 2D information from both eyes that differ in disparity. The resulting stereo-fused image is a unified, 3-dimensional percept. It is the added perception of the depth dimension that occurs constantly when seeing a solid image, fused from the images from your two eyes, as if you had one eye in the middle of your forehead.

How the brain computes this information is still disputed and widely researched. Earlier research shows two main pathways of the visual information in the brain - the Dorsal pathway and the Ventral pathway. These pathways differ in their anatomical structure and in aspects of the processing visual information, including stereoscopic information. A common tool for Stereopsis research is Random Dot Stereograms (RDS). When RDS are presented binocularly, the perception of stereo depth emerges. RDS stimuli can contain no monocular information, and depth is sensed only through Stereopsis.

The goal of this work was to identify and characterize the stereoscopic-related brain activity. We measured the Steady-State Visual Evoked Potentials (SSVEP) to study the activation of visual cortical areas in response to display of rhythmically changing various types of visual stimuli. The stimuli were presented by dynamic RDS, and VEPs were measured using an EEG system with high spatial resolution (64 electrodes). Using signal processing, and Independent Component Analysis (ICA) , dominant physiological artifacts like eye blinks were rejected  and  grand-average responses with high temporal resolution were calculated.

The results of the first experiment showed the differences in brain activity between watching stereoscopic and non stereoscopic stimuli, mainly manifested in the right parietal and left frontal areas of the brain. The second experiment showed how the perceptual content generated by contrast or by stereospsis evoked distinctly different responses, with unique temporal behavior. The third experiment revealed areas along the ventral pathway that are responsible to the perception of the fine details during stereoscopic structure perception. Spectral analysis of the data suggests that parietal areas take major part in the stereoscopic fusion process.

VEP features related to the stereoscopic depth were identified, some already known from prior studies and some new, in particular responses in areas in the temporal and parietal lobes during stereoscopic fusion, and in the frontal lobe when stereoscopic fusion  is impossible. The implications of these findings within the general scheme of human visual information processing are discussed.