Washington, June 11 : A new research has shown that people with tune deafness, an auditory processing disorder in which a person with normal hearing has trouble distinguishing notes in a melody, may actually be able to recognize an incorrect note unconsciously.
The researchers at the National Institute on Deafness and Other Communication Disorders (NIDCD), part of the National Institutes of Health, say that this study may help scientists use the tools of genetic research for a better understanding of the differences between conscious and unconscious thought.
The difference between the states of consciousness from unconsciousness has puzzled neuroscientists for a long time. However, as these disorders are typically caused by brain damage, there is an inconsistency in data from one patient to the next and researchers have difficulty finding a sufficient number of volunteer patients for clinical trials.
"The prevalence of tune deafness is surprisingly high-perhaps as much as 2 percent of the population is tune deaf-and it exists in an otherwise normal, uninjured brain. These factors, combined with the fact that tune deafness is largely genetic in origin, now raises the possibility of using tune deafness as a new way to study consciousness," said James F. Battey, Jr., M.D., Ph.D., director of the NIDCD.
In the study, the researchers randomly screened 1,218 individuals using an online version of the Distorted Tunes Test. The Distorted Tunes Test is a standardized survey that tests a person's ability to identify whether or not a short melody is played correctly.
After further analysis they closed in on seven subjects with severe tune deafness that were otherwise medically normal and were willing to take part in the study. Ten healthy control subjects who performed normally on the Distorted Tunes Test also took part in the study.
Electroencephalography (EEG), a brain imaging technique that places electrodes around a person's head and measures the electrical impulses of millions of neurons in the brain, was used for further studying the subjects.
The researchers measured the volunteers' responses as they listened to an altered version of the Distorted Tunes Test in which the incorrect melodies had a single wrong note at the end. Volunteers listened to 102 familiar melodies, roughly half of which were correct, and half of which contained the errant last note. The researchers then sifted through the EEG data to isolate the brain's response to a specific stimulus-in this case, the right or wrong note.
They focussed on two signals that brains normally generate when they are presented with a stimulus that doesn't match what the brain expects to hear, such as the wrong note in a song. The first, the mismatch negativity (MMN), is a large negative signal that occurs roughly 200 milliseconds after the unexpected stimulus is heard; the second signal, the P300, is a large positive signal occurring roughly 300 milliseconds after the unexpected stimulus.
As tune-deaf people not always recognize when a wrong note is played or sung, the researchers hypothesized that their brains would not generate the MMN or P300 signals, and as expected, this was true for the MMN signal.
However, in the case of the P300 signal, tune-deaf volunteers were processing the wrong note in the same way as the normal participants, even though they weren't consciously aware of the deviation. Other brain signals demonstrated that correct notes were being processed equally well for both tune-deaf and normal volunteers.
As for how a brain can register a wrong note without the person being aware of it, the researchers explain that the MMN and P300 signals are generated in different parts of the brain. The MMN is generated near the primary auditory cortex, in the brain's temporal lobe, while the P300 is generated in the frontoparietal cortex, downstream from the auditory cortex. Normal brains process sounds in a series, with the frontal and parietal cortices receiving signals that have already been processed in the auditory cortex.
However, in someone with tune deafness, the direct route for processing the wrong note may be disrupted, and signals are possibly being routed to the two regions through parallel pathways independent of each other. In this way, information about a wrong note may not be reaching the auditory cortex at all, while information reaching the frontoparietal cortex is not consciously perceived.
The study is published in the latest issue of the online journal PLoS ONE.