Echo-location in humans

We can echo-locate but it is only possible to master well if blind. This is because, to be well done, echolocation uses parts of the visual cortex. A few years ago Thaler et al published the details (see citation below). Here is their description of this natural ability.

The enormous potential of this ‘natural’ echolocation ability is realized in a segment of the blind population that has learned to sense silent objects in the environment simply by generating clicks with their tongues and mouths and then listening to the returning echoes. The echolocation click produced by such individuals tends to be short (approximately 10 ms) and spectrally broad. Clicks can be produced in various ways, but it has been suggested that the palatal click, produced by quickly moving the tongue backwards and downwards from the palatal region directly behind the teeth, is best for natural human echolocation. For the skilled echolocator, the returning echoes can potentially provide a great deal of information regarding the position, distance, size, shape and texture of objects. ”

They found that their blind echo-locating subjects (early-blind and late-blind) used visual areas of cortex in processing echo information. When they were presented with recordings of clicks with and without the resulting echoes, they found activity in the calcarine cortex when there were echoes but not in echo free recordings. But there was no difference in the activity of the auditory cortex in hearing the two recordings. There was also activity of other visual areas when listening to echoes reflected by moving objects. They conclude that blind echo-locating experts use brain regions typically used for vision rather than auditory areas to process the echoes into a perception of objects in space.

The calcarine cortex has other names: primary visual cortex, striate cortex, V1 visual area. It is the area that first receives information from the eye (via the thalamus). It contains a point-to-point map of the retina. V1 is known for several visual abilities: the identification of simple forms such as lines with orientations and lengths, aiming the eyes (saccades) towards interesting clues in the peripheral vision, and participating in forming images even with the eyes closed. This is the sort of processing area that can be taken over to process echoes into a spatial image when vision is not able to use it.

It is likely that our senses are all (to some extent) building 3D models of our surroundings and they would all contribute to our working model of the world. Particularly what we see, what we hear and what we feel, all seem to be part of one reality, not three. This must mean that the models from each sense are fitted together somewhere, or that the models of each sense fed information into each other, or, of course, both. In the end though, the visual model seems, in our case, to be the more influential part of our working model.

The mechanisms for finding discontinuities in light and finding their linear orientation and length, would not be that much different from finding discontinuities in echoes and finding their linear orientation and length. Fitting this sort of information into a perceptual model would use mechanisms that are used for visual lines and objects in sighted people. But is there evidence of this coordination of perception?

Buckingham et al (see citation below) have looked at this and found “that echolocation is not just a functional tool to help visually-impaired individuals navigate their environment, but actually has the potential to be a complete sensory replacement for vision.” There is an illusion where the size of an object affects the perceived weight. With boxes weighing exacting the same but of different sizes – the smaller boxes feel heavier than the larger ones. This illusion was used to show the size information which is usually visual can be replaced by echolocation information without changing the illusion.

Here is their abstract: “Certain blind individuals have learned to interpret the echoes of self-generated sounds to perceive the structure of objects in their environment. The current work examined how far the influence of this unique form of sensory substitution extends by testing whether echolocation-induced representations of object size could influence weight perception. A small group of echolocation experts made tongue clicks or finger snaps toward cubes of varying sizes and weights before lifting them. These echolocators experienced a robust size-weight illusion. This experiment provides the first demonstration of a sensory substitution technique whereby the substituted sense influences the conscious.”

Why don’t sighted people echo-locate. I do not believe it has been shown that we don’t. If we do it is not rendered consciously or used in preference to visual data. But there is no reason to assume that it is not there in the background helping to form a perceptual working model. For example, if an echo based edge coincided with an optical edge in the V1 area, it could give additional information about the nature of the edge.

I also think it may be that in order to simplify auditory perception, our brains suppress the low level echoes of any sound we make. We would be aware of the sound we made but much less aware of echoes of that sound. The auditory cortex would then be unable to echo-locate and the visual cortex would be busy with vision (and perhaps some echoes and other sounds) producing a visual model. In this case, we would not consciously hear our echoes and we would not directly consciously ‘see’ them either, although we might be processing them as additions to visual input.
ResearchBlogging.org

Thaler, L., Arnott, S., & Goodale, M. (2011). Neural Correlates of Natural Human Echolocation in Early and Late Blind Echolocation Experts PLoS ONE, 6 (5) DOI: 10.1371/journal.pone.0020162

Buckingham, G., Milne, J., Byrne, C., & Goodale, M. (2014). The Size-Weight Illusion Induced Through Human Echolocation Psychological Science DOI: 10.1177/0956797614561267

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