Both prosopagnosia and amusia can be caused by injuries or lesions to very specific areas of the cerebral cortex. As such, they dramatically illustrate the specialisation of cortical areas for very particular functions.. In fact, different types of amusia, specifically affecting timbre or melody or rhythm perception can be induced by lesions to different brain regions.
These kinds of highly selective neurological conditions are well known, thanks to the writings of Oliver Sacks and others. What is less widely appreciated is that they also have congenital forms, which are not associated with any kind of brain lesion. In fact, both conditions are surprisingly common – the prevalence of congenital prosopagnosia is estimated at 2.5 percent and that of congenital amusia at 4 percent of the population. They are also highly familial, with very high rates among first-degree relatives of affected people.
The mutations that cause these conditions most likely affect the connective wiring of various parts of the brain. No particular genes have yet been identified, but whatever their normal functions, their disruption seems to leave sensory processing areas unable to communicate the results of their computations to higher areas associated with conscious awareness. Genetic variation can thus affect, very literally, the way people’s brains are wired, influencing not just how people perceive various categories of objects but also, fundamentally, how they think about them.
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