Tone deafness (or amusia) is an impairment in the ability to discriminate changes in the pitch of a melody. The condition, which was first described in 1878, affects about 4% of the population. It arises in early childhood and continues throughout adulthood.
Neuroimaging studies show that amusia is not associated with abnormal neural activity in the auditory cortex (the region of the temporal lobe involved in processing auditory information). It is also now known that the condition is specific to musical tones; tone deaf individuals (hereafter referred to as amusics) are able to perceive the subtle differences of inflection used in spoken language, and can differentiate a question from a statement because (in the English language, at least) one raises one's pitch slightly at the end of the sentence when asking a question. But if the words are replaced with tones, the amusic cannot tell the difference between the two types of sentence.
Thus, amusia is not simply an impairment in the sensory processes of pitch detection, but intead is likely to be caused by deficits at a higher level of auditory processing, at which musical pitch is represented more abstractly. Now, a new study published in the July issue of Nature Neuroscience provides evidence that this is indeed the case. The findings of the study suggest that the brain uses a spatial code to represent muscial pitch.
Katie Douglas and David Bilkey, of the Department of Psychology at the University of Otaga in New Zealand, recruited 34 right-handed students to take part in their study, and divided them into 3 groups: an experimental group consisting of 6 female and 2 male amusics, whose condition had been determined using a standardized test called the Montreal Battery of Evauation of Amusia (MBEA), and two control groups, one consisting of musicians and the other of non-musicians.
The performance of all the participants on a number of cognitive tasks was then tested. The first task involved mental rotation; the participants were shown an image of an object and then asked to determine which of a series images presented to them corresponded to that object viewed from a different angle. In the second task, they were shown an image of an animal and then asked to identify the same image on one of a series cards.
It was found that the amusics performed significantly more poorly than both control groups on the mental rotation task. There was no difference in the reaction time between the groups, but the amusic participants were markedly more prone to making errors. The researchers also found a correlation between task performance and MBEA score - the more severe the amusic's condition, the greater the number of errors that individual made. On the other hand, there was no significant difference between the performance of all 3 groups on performance of the animal matching task.
The participants were then asked to perform a third task. They were played two successive musical tones through a set of headphones; if they perceived the second tone as having a higher pitch than the first, they pressed the number '6' located near the top of the keyboard, but if they thought that the second tone was lower than the first, they pressed the letter 'b' which was lower down. Thus, the stimulus and the response are said to be compatible. Under another condition, called the incompatibility condition, the configuration of the keys was reversed, so that responding "higher" involved pressing a key at the bottom and vice versa. All 3 groups made more errors in the second trial, in which the stimulus and response were incompatible. But importantly, the amusics performed the task far more quickly than the non-amusics.
Finally, the participants were made to perform the pitch discrimination task simultaneously with either the mental rotation or the animal matching task. When the pitch discrimination and animal matching tasks were performed together, there was little difference between the amusics and the controls. But when the pitch discrimation was performed together with the mental rotation, the amusics had markedly quicker reaction times than the controls. So, it seems that the mental rotation task caused less interference with the pitch discrimination in the amusics than in the controls.
All of the experiments carried out by Douglas and Bilkey demonstrate a strong correlation between amusia and poor performance on tasks involving the manipulation of objects in space. The study therefore provides evidence that in amusics the link between pitch and the cognitive processes of spatial representation are weaker than in non-amusics. Interestingly, 2 left-handed amusics, whose data were excluded from the study, performed better on the mental rotation task than the right-handed amusics in the exerimental group, suggesting that handedness and the lateralization of cerebral functions that goes with it may have some influence on th erelationship between pitch and spatial representation. But how pitch and spatial representation might be related is as yet unclear; if the relationship is bidirectional, improvements in spatial tasks may enhance pitch discrimination and vice versa. If so, spatial training may enable amusics to improve their pitch discrimination.
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