You might have heard the remarkable story of Daniel Kish, the blind man who can ride a bicycle using a technique that he calls "flash sonar." Kish's method involves a series of verbal clicks that bounce back and tell him the location of objects in his path. The technique is also known as human echolocation, and it works just the same as the sonar that bats use to fly through the woods in the dark. Some people navigate using only the natural reverberations of sound off of objects around them, while others, like Kish, actively generate sounds that they can then interpret as they are reflected back. This method is surprisingly accurate, enabling expert practitioners to perceive not only the size and distance of objects, but also their shape and texture. It can be completely smooth and intuitive, as in the case of another famous echolocator, "bat man" Brian Borowski who as a child learned to ride a bicycle despite being blind since birth.
One of the most interesting features of echolocation is that it activates the visual cortex of the brain in experienced echolocators. The primary visual cortex (PVC) is located in the occipital lobe at the back of the head, which is why hitting your head backwards against an object often makes you see stars. The PVC is shown with red and yellow colors in the brain figure at the top of this page. The brain's primary auditory cortex, responsible for processing sounds, is nowhere near this area -- it's in the temporal lobe on the side of the brain, and in the sulcus (groove) between the temporal and parietal lobes. I mentioned the temporal lobe's speech-processing areas in a recent post about learning a language. The PVC is quite distinct, and has neurons that are used to visualize objects rather than to understand noises; it is not activated in blind individuals who do not use echolocation.
Kish describes the experience of echolocating as a form of seeing:
It’s flashes. You do get a continuous sort of vision, the way you might if you used flashes to light up a darkened scene. It comes into clarity and focus with every flash, a kind of three-dimensional fuzzy geometry. It is in 3D, it has a 3D perspective, and it is a sense of space and spatial relationships. You have a depth of structure, and you have position and dimension. You also have a pretty strong sense of density and texture, that are sort of like the color, if you will, of flash sonar.
The visual experience of echolocation also has been likened to seeing the world in black and white. This is much more than the ability to navigate; Kish reports that he can actually draw a scene from his perceptions of it. In fact, here's a video of one of Kish's students describing an abstract sculpture to a forensic artist who then sketches it. The type of click used by the echolocator also seems to matter: Expert echolocators produce a specific narrow "beam" of sound that is easier to interpret on its return.
Although some research suggests that the earlier echolocation is learned the more accurate it is, the neural location of this ability in the PVC is true both for echolocators who never had sight, and for those who had normal vision at one time in their lives but later lost it. Sighted people can also be trained to echolocate, and their use of the technique seems to produce results that are similar in kind but not in degree to those achieved by blind people. Another recent study with 5 expert echolocators shows that the "mapping" of objects onto the PVC is virtually the same regardless of whether an image is produced by echolocation or by sight. This supports echolocators' reports that their visual experience is similar to that produced by the eyes, even though the specific sensing method used is different.
I find echolocation fascinating from a Two-Minds perspective because it shows how visual representations of things in the world around us are cognitive representations (Narrative System) and not purely perceptual ones. We often think of narratives as products of language and logic. But visual perceptions occur in the brain's cortex (the PVC) just as other narratives do, they can be fooled by contextual cues or violation of expectations (think about optical illusions), and they are in some sense constructed realities that help us to make sense of the world around us.
Vision is so central to our experience that we think about the glass on the table in front of us as being "really there," but our experience is based on a photon emitted from the book hitting our retinas, generating a neural impulse, and then being interpreted by the PVC. An expert echolocator could achieve the same type of seeing with a pattern of auditory clicks, which then travel to the PVC via the ears instead of the eyes, and generate the same narrative result. Even what we see is, in some sense, simply a story that we are telling ourselves about our perceptions of reality. Echolocation tells the same story in a different language, but the experience of the story itself may be largely the same.
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