What if your brain knew something but couldn’t tell you? New research suggests that this is exactly what may be behind two rather curious conditions.
Most of us are familiar with people who are tune deaf – these are the people who not only cannot sing in tune but are also unaware of that fact. Individuals with severe forms of this condition, known as amusia, are unable to detect whether particular notes within a melody are out of tune or out of key. Many are also unable to recognise melodies without lyrics or to hold a tune in their heads, even if they have just heard it. These difficulties arise despite normal hearing and also a fairly normal ability to hear the difference between isolated tones. The defect lies in connecting this sensory input with some implicit knowledge of musical structure and contours. Amusia thus falls into a class of conditions known as agnosias, which are characterised by the lack of knowledge of some, often very specific, category of object.
Another, equally curious, example of this class of condition is prosopagnosia – the lack of knowledge of faces. People with severe prosopagnosia may be completely unable to recognise the faces of famous people, friends, loved ones, even their own faces. As with amusia, this reflects a high-level deficit – people with prosopagnosia have normal vision and the ability to distinguish specific facial features, gender, even facial emotions. Both conditions thus seem to reflect the inability to link incoming sensory information (a person’s face or a specific note) with stored, implicit knowledge about that category (the person’s identity or a specific melody or general rules of melodic stucture).
At least, that is how the defects manifest at a behavioural level. It had been predicted that this defect would also be apparent in the normally highly selective responses of brain areas that are specialised for processing music or faces. Yet recent experiments suggest that the underlying defect lies not in the responses of these specialised areas, which are still highly selective, but in how these responses are communicated to higher brain centres.
In one telling experiment into the neural basis of amusia, subjects were connected to an electroencephalography (EEG) machine to measure the electrical responses of different brain regions. In control subjects, notes that are out of tune or out of key induce a specific, easily detected EEG response after about 200ms. When the experiment was done with amusia subjects, exactly the same response was found. Their brains were just as sensitive to discordant notes as those of controls. However, a later response, around 600ms, was almost completely absent. This later response likely represents the communication of the detection of a discordant note to higher brain areas, which are responsible for the conscious awareness of that event.
This conclusion is strongly supported by the results of functional and structural neuroimaging experiments. In people with congenital amusia, frontal areas are more weakly coupled to posterior auditory areas. These findings thus suggest that the brains of people with amusia can detect discordant notes just fine – the people are simply not aware of it. Their brain knows but their mind does not.
Very similar effects have been observed in neuroimaging experiments of people with prosopagnosia. Normally, the activity of a brain area in response to a specific stimulus (such as a particular face) will decrease with repeated presentations, but will increase again in response to a new example from the same category (a new face). If the brains of prosopagnosics are really unable to discriminate between different faces then the increase in response to a new face should be absent. In fact, the “face areas” of prosopagnosics are still quite sensitive to differences in facial identity. What is different is that these responses are not communicated to areas in the frontal and parietal lobes, where conscious awareness is triggered.
These results are consistent with studies which use galvanic skin responses to detect emotional responses. These studies have shown that even though prosopagnosics may not consciously be able to distinguish a loved one’s face from a stranger’s, they still experience a specific autonomic response when shown a loved one’s face. Again, structural neuroimaging supports the notion that the reason for this failure in communication lies in a structural disconnection – a reduction in the nerve bundles linking these areas.
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.
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters co-editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe, where he edits the Sunday Ideas section. He can be reached at garethideas AT gmail.com