Depth perception

Depth perception is so often thought to be equal to stereopsis or stereovision that I have written these few lines to clarify the concepts. Our perception of depth is based on a number of cues. In the following list only the most commonly used cues are mentioned. We use both binocular and monocular cues.

Binocular cues:
Stereopsis is a powerful source of information at near distances. Disparity of the details on the right and left image create the perception of distance. This "computation" takes some time, which one can experience when looking at stereo test pictures. For some time, one to several seconds, the picture is flat and then suddenly becomes three dimentional.

Half-occlusion is a powerful source of spatial information. Half-occlusion means that a surface is visible to only one eye. Such a situation is "interpreted" by the visual system that the surface is behind the surface that is visible to both eyes.

Accommodation means focusing the light rays with the lens so that a clear picture falls on the retina. The change in the lens curvature depends on contraction/relaxation of the ciliary muscles. They send out neural signals about their state of contraction. However, accommodation is used as the source of information on relative location only when other cues are not present.

Convergence of the eyes inwards when looking at objects at close distance creates information about distance. Like accommodation, corvergence is a weak source of determining location, useful only when other sources are not present. This happens for example if the target is an object in a glas chamber where there are no shadows.

Monocular cues:
Motion parallax is powerful source of information on depth. The farther away an object is the smaller does it seem to move when the observer moves his head sideways. Parallax gives an instant experience about relative distances. It functions at all distances and is therefore more important in occupational assessments where looking distances are from several meters to several kilometers.

Distance between parallel lines or Vernier acuity has remarkable acuity. While visual resolution in ordinary illuminations is 0.60min of visual angle vernier acuity may be 0.13 min. It is Vernier acuity that we use when lifting a box with a fork lift until its lower edge is just above the edge of the shelf.

Size of well known objects is a good source of information on distance. Perspective is the concept we use to analyse a scene knowing that parallel lines seem to converge towards the distance (you may notice from old paintings that perpective is a rather new concept).

Shades give the form to objects, reveal the location of light sources and thus are an important part of information needed for perceiving the view in three dimensions.

Details and colour change are useful cues. Surface details disappear as a function of distance from the viewer and colours fade and become bluish at far distance.

Occlusion of an object by another object is interpreted as distance difference, the one occluded is farther away.

These few examples may have demonstrated that our experience of space is based on numerous pieces of information that our brains can subtract from the constantly changing patterns falling on our retinas.