Tuesday, 17 January 2017

Deaf people use their auditory cortex to process visual rhythms

The brain can reorganize itself in the face of a traumatic injury or a sensory disability. For example, in deaf mammals, the auditory processing neurons of the brain may be rewired to handle other stimuli. But we haven’t been able to figure out if this reorganization is task-specific—will the circuits be recruited to do the same tasks?—or more general.

A recent study published in PNAS suggests that, in at least one case, these brain circuits are repurposed for a similar task. When deaf people were asked to interpret visual rhythms (represented by a flashing light), the same auditory processing regions used to listen to rhythms were activated.

This study used fMRI to look at the brain activation of both congenitally deaf subjects and those with normal hearing. While in the fMRI machine, all subjects were asked to discriminate between different rhythms of flashing lights. As a control, all subjects were also asked to look at a light that flashed with a regular, predictable pattern. Hearing subjects were then asked to discriminate between different auditory rhythms as well. As a control, these subjects were asked to listen to a similar noise occurring in a regular, consistent pattern.

The brain scans showed that, in deaf subjects, the auditory cortex was activated when looking at rhythmic patterns of flashing lights (as were parts of the visual processing system). By contrast, hearing subjects had activity in the same parts of the visual processing system, but the auditory cortex was not engaged. For hearing subjects, the auditory cortex was only activated when they completed the audible version of this task.

A more robust region-of-interest analysis looked very closely at each voxel (a small, 3D pixel). This found that visual rhythms induced significant activity in the highest processing levels of the auditory cortex for these deaf subjects. In fact, the deaf subjects' responses to the visual rhythms most closely matched the hearing subjects’ responses to the auditory rhythms. The authors zoomed in and looked at the peak activities in the auditory cortex of both groups of subjects. They found extremely close overlap in the peak activation for hearing and deaf subjects, even though the deaf subjects were watching the rhythms and the hearing subjects were listening to them.

To test if these results were consistent between deaf subjects, the authors also compared the brain scans of different deaf participants. They found that 80 percent of auditory cortex activation triggered by visual rhythms in deaf participants overlapped. This highly similar within-group activation was only seen in hearing subjects in response to the auditory stimuli. (In other words, it wasn't seen in the visual cortex).

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