BIOSONAR-seeing with sound

                        BIOSONAR: seeing with sound

Echolocation, also called biosonar, is the biological sonar used by several animals such as dolphins, shrews, most bats, and whales. They use their own emitted sounds, along with their auditory systems to perceive and localize objects in their environment. Pulses of emitted sound reflect from objects in the animal’s path and may be interpreted by the auditory system. This ability to localize objects with echoes was termed echolocation.

Human echolocation

 

It is the ability of humans to sense objects in their environment by hearing echoes from those objects. Blind and blindfolded sighted human subjects were in fact able to learn to use echolocation to detect objects in their environement.They actively create sounds ,such as tapping their canes or by making clicking noises with their mouths. Some blind people have described the phenomenon not as a learned method of navigation, but as an inherent and intuitive extra sense. For example, a blind person could walk past a line of trees and feel a “pressure “at their sides as they passed each tree. The cause of this would be the echo of the sound of their footsteps; however they may not consciously be aware of this mechanism, only that the phenomenon exists and can often be relied upon to detect obstacle.

Facial vision

Before it was known to be based on localization of echoes, human echolocation was sometimes described as “facial vision”. It is described as, ability to judge the nearness of bodies by the action of the air against the face.

Two major sets of theories evolved regarding the nature of this sense. One set constituted the tactile or skin sense theory by Diderot in 1749, that the blind were sometimes able to sense, through the skin of their face, some systematic change in subtle properties of nature that alerted them to the presence of objects in their vicinity.

A second set of theories comprised the audition theory which implicated auditory processing as responsible for the perception of objects. These fell in to 2 main classes -the pressure theory which stated that tympanic membrane was sensitive to subtle changes in air pressure caused by the presence of objects, and the auditory theory which asserted that the auditory system can perceive subtle variations in sound waves as they bounce off objects. Thus the term facial vision is little understood phenomenon implying that sensory mechanisms in the face provided some pseudo visual perception of space.

Later the investigators clearly established a definitive relationship between the presence of perceptible sound and the ability to detect obstacles, and they confirmed that no such relationship exists, involving tactile sensation. It was concluded that auditory perception is necessary and sufficient for the detection of obstacles. The perception of  obstacles without vision depends on a rise in the pitch of sounds as they are reflected or echoed from approaching surfaces, and this rise in pitch is only perceptible with frequencies around 10 kHz and above. Since these 3 reports, terms that refer to the perception of echoes -echo detection, echolocation and echo ranging have come in to common use in references to the non visual perception of obstacles by humans.

Mechanism

Vision and audition are close cousins in that both can process reflected waves of energy. Vision processes photons (waves of light) as they travel from their source, bounce off surfaces throughout the environment, and enter the eyes. Similarly the auditory system can process waves of sound as they travel from their source, bounce off surfaces and enter the ears. Both systems can extract a great deal of information about the environment by interpreting the complex patterns of reflected energy that they receive.

3 components must be present for the perception of echoes to take place-sound (an incident wave), a surface or surfaces to reflect sound, and an observer with auditory perception. The quality at which echoes are perceived depends upon characteristics of each of these 3 components and the spatial relationship and interactions among them.

With echoes, a blind traveler can perceive very complex, detailed and specific information 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, steps, parked or moving vehicles, trees and other foliage and much more. Echoes can give detailed information about location, dimension, and density. By understanding the quality the nature of the object can be detected.

Echolocation in Bats

 

Bats use echolocation to orient themselves and to locate objects; their auditory systems are adapted for this purpose. This specialization is evident from the inner ear up to the highest levels of information processing in the auditory cortex. Bats have specialized inner ears which allow them selves to hear sounds in the ultrasonic range. The basilar membrane within the cochlea contains specialized neurons for echo information processing. These neurons are specifically tuned to different frequency of returning echoes. Neurons in the inferior colliculus are highly sensitive to time differences, respond even to weak stimuli. Auditory cortex is quite large in comparison with other mammals. The cortex contains series of maps of auditory information, organized systematically based on sound frequency and amplitude. Neurons in these areas respond only to a specific combination of frequency and timing known as combination-sensitive neurons.

Notable individuals who employ echolocation

James Holman

Daniel Kish

Ben Underwood

Dr.Lawrence Scadden