Variation in visual egocentre location measured using a monocular and binocular sighting task

Directional judgements are generally referenced to a single point relative to the viewer, known as the visual egocentre, which is presumed to be situated midway between the two eymmes. However, its precise location and variability across individuals with normal vision remain unclear. This study investigated the variability in the egocentre location using both monocular and binocular sighting tasks in a representative sample (N = 26) of adults with normal vision. Using a custom-built sighting apparatus, observers were required to rotate the orientation of a rod in the horizontal plane until it pointed directly at either their right eye, left eye, or themselves during binocular viewing. Measurements were taken along the horizontal azimuth for a range of eccentricities spanning ±52.5 deg., relative to the centre of the head. The mean point of intersection of the rod's axis extensions at each eccentricity was used to derive estimates of the monocular visual directions and the visual egocentre location for each individual. Surprisingly, observers exhibited considerable variability in their monocular estimates (74.22 ± 9.59 mm), which were typically much larger than their inter-pupillary distance (IPD) (62.04 ± 2.76 mm). For binocular viewing, the group egocentre was located, on average, 0.41 (SD = 11.13) mm to the right of the median plane of the head and 24.63 (SD = 13.85) mm behind the corneal plane, with marked individual differences in its location. The results suggest that human directional judgements of space are made relative to an individualised virtual cyclopean retina.