|Anatomy of the tone deaf brain. Image source:http://musicpsychology.co.uk|
Tone deafness, also known as amusia, is a neurological disorder which mainly affects the processing of pitch. This disorder also involves a decrease in recognition of melodies as well as a decrease of recognition of tunes. Studies suggest that amusic individuals have limitations in the discernment of speech spatial perception and the processing of new stimuli.  There are two main types of tone deafness, congenital and acquired amusia. Congenital amusia is identified by anomalous brain activity, structures, or connections which occur naturally in the person.  Acquired amusia, alternatively, is tone deafness which occurs after an injury to the brain.  Studying both congenital and acquired amusia and the underlying anatomical abnormalities reveals the areas of the brain that are used to process the various aspects of music. Different areas of the brain are used to process pitch and rhythm. The study of amusia can greatly improve the anatomical knowledge of music processing. To diagnose tone deafness, a variety of tests are used by the labs involving the ability of the subject to discriminate pitches. One test developed to standardize the process is called the Montreal Battery of Evaluation of Amusias (MBEA).  A method of rehabilitation for tone-deaf subjects was developed based on computer assisted repetitive melody discrimination tasks. 
|MBEA music recognition schema. Image source: Peretz, Champod & Hyde, 2003.|
To diagnose tone deafness, the individual must be evaluated on his ability to discern dissonant notes in melodies. The only standardized assessment of musical proficiency is the Montreal Battery of Evaluation of Amusias (continuously developed since 1987). This battery is based on the assumption that music memory and sensitivity have to be processed melodically (by differences in pitch) and temporally (by differences in duration of pitches).  The melodic portion is organized into contours, intervals and scales, while the temporal portion focuses on the rhythm and meter. Both pathways lead to the brain which contains the repertoire (music memory).  Each part of the musical pathway is affected by a different type of brain damage.
The Montreal Battery of Evaluation of Amusias is composed of six tests. Each test examines different stages of the musical pathway. These assessments are called: interval, contour, memory, scale, meter and rhythm tests.  These tests controlled for the variables not being tested. Also, the intensity and velocity of all the passages are held constant, and a piano sound is also used throughout.  These six tests are all administered in one session and consist of a base musical stimulus being presented to the patient, followed by the stimulus that has its scale, interval, contour, meter, or rhythm altered.  The patient must discern the modified passage from the original in each test.  In the final test, the patient is played some passages which were used in the test and some new ones, and must discern the novel ones from the ones previously played.  These tests are used to confirm truly tone deaf individuals for testing, and can be used to correlate different areas of the brain to the various parts of the music pathway.
|Oliver Sack's on Amusia|
|This is a video of Oliver Sacks speaking about what what an
tone deaf person experiences when listening to music. The anecdotes
from the video are from his book, "Musicophilia: Tales of Music and the
Congenital amusia, a typically lifelong disorder, is characterized by diminished perception and production of music, with unimpaired hearing and intelligence.  This congenital pitch disorder has a hereditary aspect, as a study showed that 39% of first degree family members experience this impairment.  A study conducted in the UK, which required participants to identify dissonant notes found 4% to perform at an amusic level.  A similar study in the USA identified 5% of people are likely tone deaf. 
Congenital tone deafness can be the result of absent fine-grain pitch discrimination (smaller than a semitone).  Amusic individuals appear to have normal brain responses at the early (unconscious) stages of the music processing pathway when identifying pitch changes from a repetitive tone. The amusics had normal mismatch negativity (MMN) in these stages.  As the signals travel along the pathway, information is lost and the small pitch changes are not consciously registered.  This has been observed by the lack of P3b potential (occurs after an improbable event within a repetitive stream) in amusic people.  When a large pitch change is introduced, the amusic brain overreacts and produces a larger P3b potential compared to the control, showing that the brain is not accustomed to hearing pitch changes. 
|The average voltage map of 235-242
ms waves taken from amusic and control individuals.
Image source: Peretz et al., 2009.
The early stage processing of information in an amusic individual is located in the right auditory cortex and is unable to make schemas about melodies and keys.  This makes the amusic individual unable to detect the improper tones in the inferior frontal region.  The lack of information transfer is consistent with the brain abnormalities shown in imaging studies.
Diffusion tensor imaging (which depicts the diffusion patterns of water in the brain and is very effective at showing the structure of white matter structure in the brain) was used to identify the disconnections present in the congenitally tone deaf brain.  The connectivity in the amusic brain is significantly diminished as is the volume of the arcuate fasciculus (AF).  Specifically, the right AF was not large enough to be identified. Increases in cortical thickness occurred in both hemispheres, but they were more pronounced on the right side since there are normally smaller volumes of fiber there. 
These results from various studies show that in congenital amusia, the problem is not the ability of the brain to sense difference in pitch. The amusic brain is not able to process the sensory information. There is a disconnect between the lower order and higher order pathway. The disconnections in the arcuate fasciculus disrupt the travel of information between the temporal (early pathway) and frontal region (late pathway) in the brain. Thus, the main source of congenital amusia is a lack of information transfer between the musical processing areas of the brain.
Acquired amusia is the impairment of musical capabilities which is not present at birth, but is caused by damage to the brain.  Studies conducted on individuals with acquired amusia lend insight on the regions required for music processing.
A study conducted on 53 patients who suffered a middle cerebral artery stroke found that amusic individuals primarily had a stroke on the right side of the brain, but this could be influenced by a sampling bias against aphasic patients (a disorder where the understanding and production of language is impaired).  The groups of amusic patients had a higher probability of having a lesion in the temporal and frontal areas. 
A musician who experienced a left tempo-parietal stroke developed a type of amusia which made it impossible for him to discriminate or recreate rhythms.  The arrhythmia is limited, though, to listening to music as the musician could play in rhythm from a musical score as well as sing from memory without impairment.  When the patient was asked to write the notes of two pieces from ear, the pitch was correct, but the rhythms were not.  The findings indicate that rhythm and melody require different parts of the brain to be processed; the short term analysis of rhythm is present in a different area of the brain from the long term rhythm memory. 
|An MRI taken 1 month after the patient's stroke. The infarct is located in the left temporo-parietal
area within the superior temporal gyrus. A shows a coronal slice, B is a axial slice. Image source:
Di Pietro et al., 2004.
A different musician, who experienced a right temporoparietal infarct (lack of oxygen) experienced trouble producing music.  He was not able to play the organ with the same proficiency, but the recognition of melodies was not particularly affected.  This damage for this individual caused expressive instrumental amusia (tone deafness which mainly affects the production of music), showing that the right hemisphere is necessary to play music. 
|The images show an MRI of the patient's
right (top) and left (bottom) hemispheres showing
significant atrophy in the right temporoinsular region.
Image source:Confavreux et al., (1992)
The first case where amusia and aprosody (a disorder which affects the speech of the patient; the patient's voice has a lack of normal variation in pitch, speed and accent) were found in a patient with focal cortical degeneration was reported in 1992.  Brain imaging revealed that the degeneration was limited to the right frontal and temporal regions.  The presence of aprosody and amusia suggests that musical capabilities and variations in speech are closely related in the brain. 
In a separate case study, a woman experienced electrographic seizures in the right temporooccipital region, which caused amusia and aprosody.  She was not able to speak or express emotion in her voice.  This case study demonstrates that the right temporooccipital region must be functioning normally for music processing to occur. When phenytoin (a seizure controlling drug) was administered, the patient's amusia and aprosody were resolved. 
Other neurowiki pages which further explore different aspects of seizures are the: Hallucinations in Psychopathologies, Traumatic Brain Injury, Sezual Serial Killers, and Neuroethology of Parasites That Alter Host Behaviour pages.
In a study which covered patients who possessed unilateral cerebrovascular cortical lesions, musical ability was tested.  The patients suffered from right and left hemispheric lesions and the musical instrument digital interface (MIDI) was used to test the different aspects of musical processing (contours, intervals, scales, rhythm and meter).  The patients with left-hemispheric lesions and musical impairments were generally unable to process interval and rhythm, while the patients with right-hemispheric damage expressed deficits in the recognition of contour and meter.  This proves that musical abilities rely on both hemispheres and the localization of processing in the brain can depend on the individual's music experience. 
For more information on the other effects of stroke, read the Alzheimer's Disease and Stroke Neurowiki page.
To learn about stroke rehabilitation techniques which use music, read the The Role of Music Therapy in Stroke Rehabilitation Neurowiki page.
Amusia is a disorder which can be triggered by damage to various parts of the brain; these regions have alternate functions which are also impaired. One study on the electrical responses of the brain in response to music finds that dyslexia is closely related to it.  Also, aprosody typically accompanies amusia: the patient's speech becomes extremely monotonous and the person is not able to express emotion due to damage to the temporal lobe.  Another common disorder caused by injury to the temporal lobe is aphasia.  Finally, another study suggests that musical abilities closely correlate to higher brain functions such as memory and mental flexibility, as after acquired amusia, these functions are impaired. 
There has been relatively little progress to rehabilitate amusia. A lab in Paris, France has made progress on creating a rehabilitation regimen similar to work done on rehabilitating word deafness (a disorder where the patient is not able to understand speech).  The therapy presumes the two pathway (melodic and temporal) musical model and only aims at treating defects with the melodic portion.  The treatment consists of computer assisted pitch discrimination tasks.  The conditions of the therapy fostered errorless learning (the conditions prevented the creation of false memories) and fading visual cues were used.  This type of therapy is only effective at treating amusia related to the melody pathway and not the temporal pathway. The MBEA score of the patient after this therapy improved significantly, and as the patient suffered from amusia for 4 years, spontaneous recovery could not have been a factor.  According to a follow up test, the improvement lasted for at least 7 months after the therapy.  This shows that it may be possible to rehabilitate amusic patients through persistent exercise. Further research will need to be done with an increased sample size to determine whether it is truly an effective treatment.
For more information on memory pathways which could be damaged in tone deafness, see the False Memories Neurowiki page.