|AP vs. RP on notes recognition|
Absolute pitch (AP) or more commonly known as perfect pitch is a rare ability to recognize any musical note without an external reference.  Research into this ability began in the 19th century, focusing on 2 main areas, the neuroanatomy and functionality of Absolute pitch. For many years, planum temporale was believed to be the key area responsible for possession of absolute. However, recent study discovered that absolute pitch can also be identified by expansion of right Heschl’s gyrus.  This discovery not only provides one more viable method for identifying individuals that possesses the absolute pitch,but it also highlights the role of the right hemisphere regarding this skill.  With the presence of these indicators, researchers suggested that this skill is primarily responsible for infants to acquire their mother tongue during the early stage of development, but is retained due to the lack of development in planum temporale 
Table of Contents
1 Absolute pitch vs. Relative pitch
|Young Absolute Pitch Musician|
|Gavin George, a gifted nine-year-old pianist demonstrating absolute pitch|
Relative pitch (RP) refers to the ability to figure out the note by judging the relationships between the notes with the use of external reference, which is also called a cue.  This is a common skill among the professional musicians, which can be obtained through proper trainings and experience. Musician with high relative pitch proficiency are able to memorize an external reference and use it as a temporary absolute pitch cue.  Since possessors of both skills are capable for identifying musical notes, RP possessors with high proficiency usually claim themselves as AP processors mistakenly.  Therefore, in order to distinguish between AP and RP possessors during experiments, AP tests are carried out. The main purpose of this test is to eliminate the use of temporary absolute pitch cue by interfering the memory of tones with distorted sounds that do not have any harmonic relation with the standard pitch.  Once the cue is interfered, RP possessors are no longer able to make any judgement on the relationship between notes.
In order to discover the anatomical differences between AP and non AP possessors, multiple types of neuroimaging techniques are used by researchers in different studies. The most common ones are magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), positron emission tomography (PET), and Electroencephalography (EEG).
PET and MRI are responsible for providing structural images of the brains. Thanks to their excellent spatial resolution, researchers are able to use them in order to detect any anatomical differences between AP possessors’ and non AP possessors’ brains. On the other hand, MEG and EEG are responsible for detecting the late auditory evoked response, which is the neural activity onset time after hearing the sound stimulus.  Due to the above imaging techniques in temporal resolution, scientists are allowed to compare the primary neural activity evoked by the pure tone between professional AP possessors and data from human intracranial recordings.  With the consistence between the anatomical and functional results from multiple neuroimaging procedures, the important roles of planum temporale and heschl’s gyrus to the acquisition of AP were discovered.
3 Neuroanatomy of Absolute Pitch
3.1 Right Heschl's gyrus Expansion
3.1a Heschl’s Gyrus
|Morphological Changes in HG|
|(a) and (b) are taken with MEG comparing brain structure non-AP musicians and AP musician. There is an average increase in the right auditory cortex in AP musician, which is due to the morphological expansion of right HG|
Heschl’s gyrus (HG), which is also called transverse temporal gyrus, is named after Austrian anatomist Richard Heschl. It is located at the primary auditory cortex (PAC) and is the first area in the brain to process any incoming auditory information. There are two HG located in the brain, one on the left hemisphere and one on the right hemisphere. With the help of MEG, functional differences between the two HG are found. For left HG, it is responsible for holistic pitch perception, the ability to perceive musical information as a whole. On the other hand, the right HG is responsible for spectral sound perception, the ability to perceive the musical information by breaking the information into small components and carrying out individual analysis to each of the pieces. 
3.1b Heschl’s Gyrus comparison between professional musicians and non-musicians
In Schneider’s experiment (2002), he and his colleagues discovered that the professional musicians generally have larger gray matter volume (about 130% greater) in both sides of their HG than normal people, indicating that HG play an important role in tonal perception and early evoked response in auditory cortex even in people without AP.  They also found out that the cause of the morphological expansion is due to the thickening of gray matter only, with normal size white matter.  These results conclude that the main cause of this phenomenon is increase in axon pruning
3.1c Heschl’s Gyrus comparsion between AP musicians and non AP musicians
A recent study carried out by Wengenroth, further explaines the correlation between HG and musical perception in terms of absolute pitch. This is done by comparing brain structures between AP and non AP musicians. The results of that study shows that AP musicians generally have a larger right HG than non AP musicians but similar size of left HG, which when combined with the earlier studies carried out by Schneider in 2002 showing the relationship between right HG and spectral sound perception, researchers can conclude the spectral sound perception properties of AP processors. However, whether the expansion in right HG is consequence or the cause of AP still remain unclear. It can potentially serve as a new structural marker for AP identification and also highlight the predominant role of right hemisphere in AP possessions. 
3.2 Leftward asymmetry of planum temporale
3.2a Planum Temporale
|Morphological Changes in PT|
|Showing a drastic morphological expansion and reduction left and right PT respectively in AP musicians|
Planum temporale (PT) is located behind the Heschl’s gyrus within the Sylvian fissure. Since it is located at the center of the Wernicke's area, which is one of the most important areas for language, researchers suggests that PT is also associated with language development and music perception. In most of the general population, PT is the most asymmetric area in the brain, where left PT is much bigger than right PT. 
3.2b Morphological expansion in left Planum Temporale
For many years, before morphological expansion of right HG is discovered, PT was believed to be the only physiological indicator for AP possession. Multiple studies showed that there is an increased leftward asymmetry in PT among AP musicians’ brains due to pruning in left PT instead of right PT reduction.  This conclusion is based on the fact that there is general decrease in size of the right PT in AP possessors and therefore, the cause of the leftward asymmetry must be due to the large degree of expansion in left PT.  Due to the discovery of this result, researchers began to hypothesize that AP possession is caused by an dis-inhibition of spectral sound perception ability in some individuals due to their under-developed right PT. 
3.2c Genetic determined possession of Absolute Pitch
Studies on association between PT and AP also point out the importance of genetic in the PT asymmetry. Generally, AP can be trained during childhood by exposing children to music during the critical period. However, the increased leftward asymmetry in PT only occur in innate AP possessors, and leftward asymmetry of PT in AP musicians due to early music exposure is normal, which is proof that PT asymmetry is determined and influenced by genetics during the embryotic development.  This suggests that the brain structure for those who possess AP are genetically determined, and cannot be changed or acquired by any training, including early exposure to music.
Functionality of Absolute pitch
One of the reasons why scientists are interested in the functionality of absolute pitch is because of the fact that this ability is incapable of providing survival advantages to the human species;  this led the scientists to question the vital functions that AP provides for the general population.
Researchers hypothesized that AP should be associated with language due to the fact that one of the structural markers of AP possession, PT, is located at the heart of the language center.
This hypothesis is proven by some studies showing that AP is actually available in all infants when they are born, but the ability is inhibited within a year when the infants start to acquire their mother tongue.  This result shows that infants process language with raw details due to their lack of knowledge, unlike adults, who process language holistically with meaning. The AP ability in infants can then be retained in part by introducing infants to music in the early stages of childhood. This will allow the child to grow up and have the ability of non-innate AP musician mentioned earlier. 
Another piece of evidence that support the relationship between AP and language acquisition is that AP aids in the development of an accent-free second language in adults.  Experimenters found that people with AP are able to acquire a new language without any accent. They concluded that this is because of AP possessors’ ability to break down the sound information into raw components, and listen more accurately to the foreign phonemes separately  Based on these facts, researchers concluded that AP is primarily responsible for infants to acquire their mother tongue during the early stage of development. However the ability remains with some individuals, since to inhibition failed during maturation.