Blind people who use echoes to map their surroundings, akin to how bats or dolphins navigate, have an adapted brain region that allows them to 'see' with sound, a new study suggests.
The primary visual cortex is where the human brain normally interprets signals from the eyes, but that doesn't mean it can only process visual data. In some cases, especially when vision is deprived, this flexible system might also use the same organising principles to interpret sound.
By testing people expert in mouth-click-based echolocation, researchers have shown a pattern of brain activity in this group remarkably similar to that of sighted people when they're looking at light.
It's the first evidence that activity in the primary visual cortex can be flexibly ordered by how a sense is used, not necessarily the kind of sense being interpreted. And it clearly suggests that when this primary region is being under-stimulated, its system can be adopted by sound to map spatial locations.
"Although it is inarguable that primary sensory areas preferentially process input from one modality over others,"the authors write, "they nonetheless retain the ability to carry out at least some of the characteristic tasks when relevant information is provided through another sensory modality.
"This is pivotal in our interpretation of the functional organisation of the human brain."
This crossing of sensory inputs is not unheard of; our senses are sharing information all the time. In recent years, scientists have started to figure out that higher cortical areas, like the prefrontal cortex, are best understood in terms of the tasks they perform, not so much the specific senses they process.
So far, however, most evidence for this task-based structure has been confined to these higher-order sensory areas, not to the places where the sensory input is processed.
The first neuroimaging study of human echolocation, in 2011, shows the auditory and primary visual cortex are engaged when stimulated by sound. It also found that just the processing of those sounds activated only the visual cortex; since then, several more studies have implicated the visual cortex in echolocation.
Using brain imaging of expert echolocators in action, the new research tested whether the 'visual' cortex can map the spatial layout of sounds in the same way as visual input.
While laying in a functional magnetic resonance imaging scanner, five blind people, five blind echolocators, and five sighted people listened to recordings of a clicking sound bouncing off an object at different locations in the room, and guessed where it was coming from.
The findings show that stimulus maps for sound in blind echolocators are directly comparable to those for vision in sighted people. And the more skilled the echolocator, the more similar this map becomes.
What's more, these same maps don't appear in people who are blind but don't use echolocation; this suggests the brain activity is not simply a result of higher cortical regions producing mental imagery.
"If one were to argue that the results could be explained by a combination of neuroplastic changes due to blindness and imagery, then we would expect at least source sounds to map in our blind control participants," the authors write, "but there was no evidence for this."
The sample size is admittedly small, with only five expert echolocators, but we have to keep in mind the skill is rather niche. The first study conducted in 2011, for instance, had only two expert echolocators.
The authors think it's entirely possible that the primary 'visual' cortex in expert echolocators is functionally necessary for the perception of space through sound.
"An analogous neural map of space does not exist in primary auditory cortex, and so the map of space in primary 'visual' cortex might be the most suitable cortical site on which to map spatial location as conveyed through sound," they explain.
The findings were published in the Proceedings of the Royal Society B.