Abstract
We discovered that stationary objects flashed periodically produce depth perception under ordinary viewing conditions. We believe that small involuntary eye movements (Ko, Snodderly & Poletti, 2016) induce apparent motions of magnitudes proportional to the flash periods (for a similar proposal, see Gosselin & Faghel-Soubeyrand, 2017) and that these apparent motions are interpreted by the brain as a form of parallax that results normally from microscopic head movements (Aytekin & Rucci, 2012). Here, we tested a somewhat counterintuitive prediction of this hypothesis: perceived depth should increase linearly with viewing distance. Eight observers were shown, on an Asus VG278HR at a refresh rate of 120 Hz, a stimulus made of 200 white discs distributed randomly on a black background spanning 10 x 10 cm. Each disc flashed with one of 13 periods evenly spread between 8.33 ms and 108.33 ms; the period was chosen to be inversely proportional to the value of a depth map at the disc location. The depth map represented the thick vertices of a cube. All observers reported seeing clearly this volumetric shape during the experiment. Participants viewed the stimulus binocularly, sitting comfortably in a chair at distances of 45, 70, 95, 120, 145 and 170 cm, three times. On every trial, they were asked to move their chair at a randomly selected viewing distance indicated on the computer monitor. Viewing distances were marked on the floor with stripes of photoluminescent tape. When ready, subjects pressed on the computer mouse button to initiate the presentation of the stimulus for 2 s. Finally, they were instructed to estimate the perceived depth in the stimulus by adjusting the length of a horizontal line drawn on the computer monitor with the computer mouse. As expected, we found a strong positive linear relationship between viewing distances and mean depth estimations (r=0.9585, p=0.0025).