Spatial hearing refers to listeners' perception of the locations of sound sources, the environment that contains them, and their own orientation within that environment. There are many different facets to spatial hearing, including sound localization (can you point to a sound source with your eyes closed?), segregation (can you tell that two sounds have different locations?), perception of moving sounds or one's own motion through space, multisensory perception (does seeing a talker influence where you hear them?), and reverberation perception (can you tell, just from listening, what kind of room you are standing in?).

Spatial hearing relies on several different features of sounds, or "acoustical cues," that inform listeners about auditory space. These include binaural (two-ear) cues: sound arrives earlier (by a few millionths of a second), and also with greater intensity, at the ear nearer to a sound source than the far ear. The brain has special mechanisms for detecting and computing these "interaural" differences, and using them to determine the left-to-right direction of a sound source. In fact, the timing differences are so small that this system requires the fastest connections in the entire brain, by far!

Other important cues for auditory space include the effects of the outer ear, or pinna. The intricate folds of the pinna act to filter the sound, enhancing some frequencies while reducing others. The frequency pattern of that filtering process is unique to each possible sound direction, and to the specific shape of each listener's ear. Thus, each of us has a sonic "fingerprint" that is particularly important for localizing sounds in elevation (up versus down), where binaural cues are not as informative. That fingerprint, or "head-related transfer function" can be recorded and used to process sounds to mimic locations in 3-D space, a technique that is often used in computer games and virtual reality.

Some animals have specialized systems for spatial hearing. The Barn Owl, for example, has feathers on its face that act like our own pinna. In the barn owl, however, the orientation of each "ear" is different, so that binaural cues can be used for localizing sounds in elevation as well as side-to-side. Because the specialized brain systems for binaural hearing are far more accurate than for pinna cues, the Barn Owl can accurately localize the sounds of its prey in both dimensions, even in complete darkness. Other animals, such as bats and dolphins, use echolocation to localize sounds in the environment. These animals emit loud souds such as clicks or whistles, which then reflect off of objects. The brains of echolocating animals are specialized to pick up these reflections (echoes, really) and use them to avoid obstacles and capture prey.

Next post: Why is spatial hearing important?