Can We Really See the World in Three Dimensions? An Exploratory Study with Finger Alignment Task and Ponzo Illusion

Stereopsis is the perception of depth when two 2-dimensional (2D) images of an object from slightly different angles align with corresponding retinal positions in both eyes. However, when we consider the blind spot (BS), a new set of challenges arises. In binocular vision, one eye's higher visual areas compensate for the other's non-receptive region. However, our brain only receives information from one image patch in the BS region, and another visual patch does not contribute to constructing a comprehensive 3-dimensional (3D) representation of the scene. This suggests that the brain adeptly manages this dimensional transition. We aimed to determine the extent to which we perceive 3D across our entire field of view (FoV). We employed the Ponzo illusion, a stimulus relying on visual depth cues (pictorial depth cues), and an eye-tracking technique to assess participants' accurate fixation and eye movement patterns. The participants attempted to align their index fingers before a chart paper to create a Horopter-like setup, supporting the hypothesis about our capacity for stereoscopic perception. The results of our exploratory pilot study with two specific tasks revealed that our ability to perceive depth diminishes beyond approximately 8$^{\circ}$ of retinal eccentricity (in the horizontal meridian) in both halves of the visual field centered around the fovea. This limitation is not apparent in our day-to-day experience.