Binocular and oculomotor cues

The pictorial cues we have discussed could all be used as well by one-eyed people as by those with normal vision. Depth perception also depends on oculomotor cues, based on perceiving contractions of the muscles around the eyes. One such cue is convergence, which refers to the fact that the eyes turn inwards to focus on an object to a greater extent with a very close object than with one that is further away. Another oculomotor cue is accommodation, which refers to the variation in optical power produced by a thickening of the lens of the eye when focusing on a close object.

Depth perception also depends on binocular cues, which are only available when both eyes are used. Stereopsis involves binocular cues. It is stereoscopic vision depending on the differences in the images projected on the retinas of the two eyes. Convergence, accommodation, and stereopsis are only effective in facilitating depth perception over relatively short distances.

There has been some controversy about the usefulness of convergence as a cue to distance. The findings have tended to be negative when real objects are used, but more promising findings have been obtained with use of the "wallpaper illusion" (Logvinenko & Belopolskii, 1994). In the wallpaper illusion, there is underestimation of the apparent distance of a repetitive pattern when the fixation point is shifted towards the observer, and overestimation when the fixation point moves away from the observer. It has generally been assumed that convergence of the eyes explains the wallpaper illusion, but Logvinenko and Belopolskii cast doubt on that assumption. It is possible to perceive two illusory patterns at two different apparent distances at once, which would be impossible if the phenomenon depended entirely on convergence. In addition, participants can move their gaze around (and so change convergence) without any loss of the illusion. Such findings led Logvinenko and Belopolskii (1994, p. 216) to conclude as follows:

In view of the fact that the wallpaper illusion is commonly assumed to be the main evidence for convergence as a cue to distance, we conclude that convergence does not supply sufficient information for the perception of distance.

Accommodation is also of limited use. Its potential value as a depth cue is limited to the region of space immediately in front of you. However, distance judgements based on accommodation are rather inaccurate even when the object is at close range (e.g., Kunnapas, 1968).

The importance of stereopsis was shown clearly by Wheatstone (1838), who is generally regarded as the inventor of the stereoscope. What happens in a stereoscope is that separate pictures or drawings are presented to an observer in such a way that each eye receives essentially the information it would receive if the object or objects depicted were actually presented. The simulation of the disparity in the images presented to the two eyes produces a strong depth effect.

One might think that stereopsis is a straight-forward phenomenon. In fact, it has proved very hard to work out in detail how two separate images turn into a single percept. In general terms, we must somehow establish correspondences between the information presented to one eye and that presented to the other eye. At one time, it was believed that the forms or objects presented to one eye were recognised independently, and that they were then fused into a single percept. However, this does not seem likely. Crucial evidence was obtained by Julesz (1971), who made use of random-dot stereograms. Each member of such a stereogram seems to consist of a random mixture of black and white dots, i.e., neither member seems to contain a recognisable form. However, when the stereogram is viewed in a stereoscope, an object (e.g., a square) can be clearly seen.

If stereopsis does not result from a matching of the forms from each image, how does it happen? Part of the answer was obtained by Frisby and Mayhew (1976). They made use of a process known as filtering to remove certain spatial frequencies (these are determined by the closeness together of alternating dark and light bars; see Chapter 4). Stereopsis remained when only high spatial frequencies (fine details) were removed from both halves of a stereogram, or when only low spatial frequencies (coarse, blurred structures) were removed from both. However, when high spatial frequencies were removed from one half and low spatial frequencies from the other, stereopsis was lost, and only one half of the stereogram could be seen at any one time. Thus, some overlap of spatial frequencies between the two halves of a stereogram is necessary for stereopsis.

Marr and Poggio (1976) proposed three rules that might be useful in matching up information from the two eyes:

• Compatibility constraint: elements from the input to each eye are matched with each other only if they are compatible (e.g., having the same colour; edges having the same orientation).

• Uniqueness constraint: each element in one image is allowed to match with only one element in the other image.

• Continuity constraint: matches between two points or elements are preferred where the disparities between the two images are similar to the disparities between nearby matches on the same surface.

These three constraints were incorporated into a theory that was able to produce appropriate solutions to random-dot stereograms (Marr & Poggio, 1976). However, Frisby (1986) pointed out that the continuity constraint is the least adequate. For example, if an object slants steeply away from the observer, then nearby matching points will not have very similar disparities. As a result, there may be a failure to match corresponding points with each other.

Mayhew and Frisby (1981) argued that stereopsis is very much bound up with the elaboration of descriptions in the raw primal sketch (see Chapter 4). This contrasts with the view of Marr and Poggio (1976), which is that stereopsis is rather separate from other aspects of visual processing. May hew and Frisby (1981) put forward a figural continuity constraint: most erroneous possible matches can be eliminated by considering the pattern of light-intensity changes in the area close to that of the potential match.

The emphasis in most theories of stereopsis has been on the basic visual processes involved. However, cognitive factors can be important. A case in point is Gregory's "hollow face" illusion (Figure 2.8). In this illusion, observers looking at a hollow mask of a face from a distance of a few feet report seeing a normal face (see also Chapter 3). Stereoscopic information is ignored in favour of expectations about human faces based on previous experience.

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Responses

  • ermias
    What depth cue could be classified as a binocular cue and an oculomotor cue?
    10 months ago
  • kristin
    Are binocular cues the same as oculomotor cues?
    6 months ago
  • Johanna
    Are binocular cues the same as occulomoter cues?
    4 months ago
  • petra
    What is convergence in terms on occulomotor depth cues?
    4 months ago

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