Binocular Disparity

Binocular disparity is a depth cue based on differences in relative positions of the retinal images of objects in the two eyes. There are several ways to describe the positions of objects in regards to binocular disparity. Corresponding points are those in which if the left and right retinas were superimposed would produce coinciding points. Noncorresponding points are just the contrary; points on the retinas would not coincide if superimposed. When an observers focuses on an object, a horopter is established. A horopter is a term used to describe an imaginary surface from which objects would produce retinal images at corresponding points. Objects that do not fall on the horopter are noncorresponding points. There are three types of binocular disparity. Crossed disparity is exhibited when an object is closer than the horopter and uncrossed disparity is exhibited when the object is farther than the horopter. Whereas zero disparity is exhibited when the retinal image of an object falls on corresponding points for both eyes. Stereopsis is
the depth information provided by binocular disparity, the different retinal images in the two eyes. According to Marr and Poggio, correspondence matching between the two retinal images is conducted through two assumptions by the visual system; that each one feature in the retinal image will match only one other feature in the other retinal image and that visual scenes tend to comprise of smooth and continuous surfaces instead of any abrupt changes. Correspondence matching does precede objection recognition, in which the visual system matches parts of the retinal images based on simple properties. In regards to neural processing, the brain utilizes disparity information in order to produce stereopsis and in order to analyze motion in depth. There are also binocular cells that respond best to stimulation of their receptive fields in both eyes simultaneously. Binocular cells are found in V2, V3, MT, and to some degree also in V1. The first auditory mechanism that supports localization is the interaural level difference (ILD).
This refers to the difference in the sound level reaching the two ears. The ILD is best for perceiving the azimuth of sounds at high frequency but not low frequency sounds. Interaural time difference (ITD) is the second binaural cue. It refers to the difference between the times that a sound reaches the two ears. When the distance of the sound to each ear is the same then there is no difference in time however when the sound is to a side of the individual then there will be a difference in time. In regards to neural processing, the medial superior olive (MSO) brain structure that is in the brain stem contains neurons that detect specific ITDs and consequently showing the azimuth of sound sources. This similar feature also exists in barn owls in which a different structure in the brain stem contains neurons that detect for specific
ITDs. There are many ways in which the auditory and visual systems are similar and different. Take for example, how the neurons in the MSO are coincidence detectors, meaning that they fire only if signals from the two cochlear nuclei arrive simultaneously. This is similar to how binocular cells for the visual system function; they respond when stimulation is received in both eyes simultaneously. Additionally, both these types of neurons have specifications that cause them to respond to different stimuli within their own system. Different binocular cells respond to different types of disparities and other auditory neurons respond only to more complex tones. As for location cues, the visual location cues are directly represented on the retina. However, auditory location cues must be first calculated through other cues and therefore not directly represented in the receptor cells. The auditory system requires more extensive precortical processing, meaning that more synapses occur prior to the thalamus.