2.3 Stimulation and recording parameters

All participants performed passive observation of the aforementioned visual stimuli. Within one recording run, either 96 Checkerboard patterns or 192 3D-Tunnels were presented to one subject, alternately with a uniform grey image (Fig 1A and 1B). An identical stimulation rate (1.0 Hz) was used in both conditions. Checkerboard stimulus consisted of black and white rectangles (4.5 x 4.0 cm) alternating 96 times with the grey page, which corresponded to full-contrast black and white squares (black field 15 lx; white field 101 lx).

The grey page luminance was about 43 lx. The 3D-Tunnel was non-stereoscopic but included perspective cues generated by the OpenGL graphic libraries [45] (Fig 1A and 1B). It represented a tunnel with stone-textured walls (stone dimension 1.25 cm² at the periphery to 0.15 cm² close to the center) in the form of a pipe with a constant circular cross-section. These different stimuli with a pattern contrast of about 50% display subtended 7°(w) × 5° (h) at the eye. Thus, both foveal and parafoveal retinal fields were stimulated. The luminance of the 3D-Tunnel evolved from 39 lx at the periphery to 74 lx close to the center. The presentation of each visual item (presentation time of 500 ms) was immediately followed by the presentation of a uniform grey image (also for 500 ms).