Binaural fusion

Binaural fusion is a auditory perceptual process that involves the "fusion" or integration of different auditory information presented binaurally, or to each ear. In humans, this process is essential in understanding speech as one ear may pick up more information about the speech stimuli than the other. Each ear sends the signal it receives up the Central Auditory Nervous System (CANS) where it is fused as one complete "auditory picture" at the level of the brainstem.

This process is also used by both humans and animals to determine sound localilization, the direction from which sounds are heard, wherein the brain compares information from each ear and then translates the differences into a unified perception of the point in space from which a sound originates. The spatial cues include differences in the arrival time and the intensity, or force, of sound waves reaching the ears from a specific point in space. These cues are processed in parallel as a series of distinct steps, eventually converging as the perception of a single sound. The difference in arrival time of sounds between the ears is used to determine the direction of a sound source in the horizontal plane.

The auditory system derives the pitch of complex tones from the tone's harmonics. The processing of pitch in the auditory cortex, via binaural fusion, enables locations to be determined.

Binaural Fusion in (C)APD testing
Many tests have been created to try and diagnose (C)APD (Central Auditory Processing Disorder) by assessing the CANS' binaural fusion ability. These tests have been shown to be moderately sensitive for diagnosing brainstem lesions.