Neurons that fire most when they receive input from both ears at the same time
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Extensive research has shown that the spatial cues extracted by the human brain are differences in the arrival time and the intensity, or force, of sound waves reaching the ears from a given spot. Differences arise because of the distance between the ears. When a sound comes from a point directly in front of us, the waves reach both ears at the same time and exert equal force on the receptive surfaces that relay information to the brain. But if a sound emanates from, say, left of center, the waves will reach the right ear slightly after the left. They will also be somewhat less intense at the right because, as they travel to the far ear, some fraction of the waves will be absorbed or deflected by the head.
The brain's use of disparities in timing and intensity becomes especially obvious when tones are delivered separately to each ear through a headset. Instead of perceiving two distinct signals, we hear one signal--a phantom--originating from somewhere inside or outside the head. If the stimuli fed to the ears are equally intense (equally loud) and are conveyed simultaneously, we perceive one sound arising from the middle of the head. If the volume is lowered in just one ear or if delivery to that ear is delayed, the source seems to move in the direction of the opposite ear.
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This much has long been known. What is less clear is how the brain manages to detect variances in timing and intensity and how it combines the resulting information into a unified spatial perception. My colleagues and I at the California Institute of Technology have been exploring this question for more than 25 years by studying the behavior and brain of the barn owl (Tyto alba). We have uncovered almost every step of the computational process in these animals. (The only other sensory vertebrate system that is as completely defined belongs to a fish.) We find that the owl brain combines aural signals relating to location not all at once but through an amazing series of steps. Information about timing and intensity is processed separately in parallel pathways that converge only late in those pathways. It is highly probable that humans and other mammals achieve binaural fusion in much the same manner.