Final answer:
Stereopsis is the brain's ability to perceive 3D depth from 2D images, using both monocular and binocular cues. Binocular disparity is one such cue, where the brain processes slightly different images from each eye to create the perception of depth. This is used in 3-D movies and interpreted by the brain to represent spatial movement.
Step-by-step explanation:
The ability of the brain to understand the world in three dimensions (3D) from 2D images is known as stereopsis. This phenomenon relies on depth perception, which entails various cues to interpret spatial relationships. The two categories of depth cues are monocular, requiring only one eye, and binocular, requiring both eyes. Binocular disparity, a binocular cue, involves the slightly different views each eye has due to their separate positions on the face. When the brain processes these disparate images, it creates a sense of depth, enabling us to perceive the world in 3D.
In the context of 3-D movies, the screen projects a two-dimensional image that incorporates disparate information for each eye. 3-D glasses filter the projections so that each eye sees only one layer, producing the illusion of depth. The brain analyzes visual information from both eyes to perceive motion in space. For example, if an object appears to move leftward in one eye and rightward in the other, the brain interprets this as movement in the direction of the face. If an object moves in the same direction but at different rates in each eye, it suggests spatial movement within the environment.