Stereopsis

As illustrated by the number and variety of topics covered in this encyclopedia, the human brain is a highly complex system. One of the true wonders is that it performs these feats simultaneously, and in most cases, without conscious effort. One such ability is that of stereoscopic depth perception. As described below, stereopsis is based on binocular disparity, one of many sources of depth information available in the environment. Other depth cues, such as perspective, shading, texture gradients, and occlusion, also provide clues as to the relative distances of objects in the environment, but none of these comes close to the quality and precision of depth percepts provided by stereopsis. The subsequent sections will review some of the defining features of stereopsis in humans and animals and its associated neural mechanisms.

Stereopsis is a cue to depth based on the fact that we have two eyes, which are laterally separated (in humans by about 6.5 cm). This positional difference in the two eyes results in each eye receiving a slightly different image of the world. So the image of one object will fall on slightly different or disparate retinal locations. This difference in location is referred to as binocular disparity and is the key information used by the stereoscopic system. Imagine that you have two cameras positioned side by side that take a picture of the same scene. While the resulting images will be very similar, there will be subtle differences, as illustrated in Figure 1.

Figure 1 The two eyes illustrated here are fixating object F, as seen from above

In Figure 1 the observer fixates object F and another object M is positioned closer to the observer. If we trace the lines of sight from the objects to the back of the eye, we can see this positional difference (as illustrated at the bottom of the figure). Note that this binocular disparity information is generated in the same manner for objects in front of fixation (crossed) and beyond fixation (uncrossed). That is, the geometry of these arrangements is the same, but the position of the retinal images relative to the fixation point is reversed.

An important aspect of stereoscopic depth perception is that retinal disparity is generated between the object or point fixated and another object in the scene. Thus the perceived depth is relative to where one is looking, and if a third object is introduced at the same distance as the fixation point it will have zero disparity. In turn, zero binocular disparity indicates that an object lies on the plane of fixation. The set of points that are equidistant with a given fixation location define the horopter (e.g., the dashed line and point P in Figure 1).

The preceding description defines the theoretical horopter; however, when observers are asked [Page 720]to set stimuli to be equidistant with fixation, the shape of the horopter is not circular and instead is somewhat flattened. This is known as the empirical horopter. As retinal disparity is increased, there is a corresponding increase in perceived relative depth. Interestingly, over a large range of retinal disparities, the disparate images appear single, or fused. This region has been referred to as Panum's fusional area. Beyond this range, the images are no longer fused but appear double or diplopic.

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