Final answer:
Vision converts horizontal relative disparity into absolute depth primarily through the binocular depth cue of retinal disparity, in combination with vergence eye movements. Monocular cues and motion parallax contribute to depth perception but do not directly convert disparity into depth.
Step-by-step explanation:
Vision converts horizontal relative disparity into absolute depth by using retinal disparity, which is a binocular depth cue. Retinal disparity refers to the slight differences in the two visual fields that each eye perceives due to their different positions on the head. Because the two eyes view the world from slightly different angles, an object will fall on slightly different spots on the two retinae. The brain computes these differences to perceive depth. This process involves vergence eye movements, during which the eyes move together to focus on an object, using the degree of convergence to judge the object's distance. Monocular cues, while important for depth perception, contribute to the perception of depth using information from one eye only, such as linear perspective or size constancy. Motion parallax is also a monocular cue indicating depth when we move our heads or bodies, but it does not convert relative disparity into depth. Hence, absolute depth perception is primarily derived from the binocular cue of retinal disparity in conjunction with vergence movements.