Localization (Sensory)

The ability of animals and humans to determine the origin of a sensory input.

One of the highly developed abilities that humans and other animals possess is the ability to determine where a sensory input originates.

The capacity to localize a sound, for example, depends on two general mechanisms. The first is relevant for low frequency (i.e., low pitch) sounds and involves the fact that sound coming from a given source arrives at our ears at slightly different times. The second mechanisms applies to high frequency (i.e., high pitch) sounds; if such a sound comes from one side, one ear hears it more loudly than the other and we can detect location based on differences in the loudness of the sound at each ear.

Low frequency sounds that come from the noisemaking source will enter the nearer ear first; these sound waves will then bend around our head and arrive at the far ear a short time later. If the sound is almost directly in front of us, the sound arrives at one ear an extremely short time ahead of its arrival at the other ear. Humans can detect differences of perhaps 10 millionths of a second in arrival time. If the sound comes from the side, the difference in time of arrival at the two ears is longer. In either case, our brain executes quick computations to inform us about the location of the sound. Other animals, like nocturnal owls, have shown greater sensitivity to differences in time of arrival.

The second mechanism involves intensity differences in sound waves traveling to the ears. High frequency sound waves do not bend around the head like low frequency waves. Instead, high frequency sound waves tend to reflect off the surface of the head. As a consequence, a sound coming from one side of the head will show greater intensity in one ear; that is, it will be slightly louder in one ear. The brain uses this intensity difference to tell us where a sound originates.

In general, we locate sounds below about 1500 Hz (i.e., 1500 cycles per second) by analyzing differences in time of arrival at each ear; above 1500 Hz, we use intensity differences. Sounds that are right around 1500 Hz are hardest to localize. Further, we are likely to confuse sounds that are directly in front of us, above us, and behind us because their positions are such that we cannot use time of arrival and intensity differences.

Finally, sometimes we ignore the cues for sound localization if logic tells us that the sound should be coming from another direction. For example, when we listen to somebody on a stage, we may hear the sounds they produce from a loudspeaker that is above us. Nonetheless, we localize the sound as coming from the person on the stage because it seems more logical. Psychologists refer to this phenomenon as "visual capture."