Human echolocation is the ability of humans to detect objects in their environment by sensing echoes from those objects, by actively creating sounds: for example, by tapping their canes, lightly stomping their foot, snapping their fingers, or making clicking noises with their mouths. People trained to orient by echolocation can interpret the sound waves reflected by nearby objects, accurately identifying their location and size.
Background
The term "echolocation" was coined by zoologist Donald Griffin in 1944; however, reports of blind humans being able to locate silent objects date back to 1749. Human echolocation has been known and formally studied since at least the 1950s. In earlier times, human echolocation was sometimes described as "facial vision" or "obstacle sense," as it was believed that the proximity of nearby objects caused pressure changes on the skin. Only in the 1940s did a series of experiments performed in the Cornell Psychological Laboratory show that sound and hearing, rather than pressure changes on the skin, were the mechanisms driving this ability. The field of human and animal echolocation was surveyed in book form as early as 1959. See also White, et al. (1970)
Many blind individuals passively use natural environmental echoes to sense details about their environment; however, others actively produce mouth clicks and are able to gauge information about their environment using the echoes from those clicks. Both passive and active echolocation help blind individuals sense their environments.
Those who can see their environments often do not readily perceive echoes from nearby objects, due to an echo suppression phenomenon brought on by the precedence effect. However, with training, sighted individuals with normal hearing can learn to avoid obstacles using only sound, showing that echolocation is a general human ability.
Lore Thaler led researchers at Durham University to determine if they could teach echolocation to people. Over a ten-week period, they taught it to 12 blind people and 14 others who were not blind.
Mechanics
Vision and hearing are akin in that each interprets detections of reflected waves of energy. Vision processes light waves that travel from their source, bounce off surfaces throughout the environment and enter the eyes. Similarly, the auditory system processes sound waves as they travel from their source, bounce off surfaces and enter the ears. Both neural systems can extract a great deal of information about the environment by interpreting the complex patterns of reflected energy that their sense organs receive. In the case of sound these waves of reflected energy are referred to as echoes.
Echoes and other sounds can convey spatial data that are comparable in many respects to those conveyed by light. A blind traveler using echoes can perceive very complex, detailed, and specific features of the world from distances far beyond the reach of the longest cane or arm. Echoes make information available about the nature and arrangement of objects and environmental features such as overhangs, walls, doorways and recesses, poles, ascending curbs and steps, planter boxes, pedestrians, fire hydrants, parked or moving vehicles, trees and other foliage, and much more. Echoes can give detailed information about location (where objects are), dimension (how big they are and their general shape), and density (how solid they are). Location is generally broken down into distance from the observer and direction (left/right, front/back, high/low). Dimension refers to the object's height (tall or short) and breadth (wide or narrow).
By understanding the interrelationships of these qualities, much can be perceived about the nature of an object or multiple objects. For example, an object that is tall and narrow may be recognized quickly as a pole. An object that is tall and narrow near the bottom while broad near the top would be a tree. Something that is tall and very broad registers as a wall or building. Something that is broad and tall in the middle, while being shorter at either end may be identified as a parked car. An object that is low and broad may be a planter, retaining wall, or curb. And finally, something that starts out close and very low but recedes into the distance as it gets higher is a set of steps. Density refers to the solidity of the object (solid/sparse, hard/soft). Awareness of density adds richness and complexity to one's available information. For instance, an object that is low and solid may be recognized as a table, while something low and sparse sounds like a bush; but an object that is tall and broad and very sparse is probably a fence.