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Heschl's Gyrus
Heschl's Gyrus is a convolution or fold of the temporal lobe of the brain and is the cortical center for hearing. It runs obliquely outward and forward from the posterior, or back part of the lateral sulcus. This brain area is also known as the Transverse temporal gyrus. This gyrus was first described by an Austrian named Richard Heschl during the 1850's. The main functions of the structure have been found to be related to audition or hearing since it serves as a piece of the primary auditory cortex. Tumors to this brain area have been found to cause problems related to verbal processing and may contribute to the development of certain psychological disorders. Future research advocates for the use of more advanced technology to develop a more complete understanding of the function of the gyrus, as well as the consequences of abnormalities of the structure.

History


Richard L. Heschl (1824-81) is the man credited for first describing the transverse temporal gyrus. Heschl was born in Austria in 1824 and attended the University of Vienna where he later became a professor of pathological anatomy. While at the University of Vienna, he wrote many pieces that impacted the scientific community at the time, including a compendium of pathological anatomy. He also described a new method of biological tissue staining called metachromasia that is still used today in detecting the pathology of amyloidosis.

There are many folds in the cerebral cortex or surface layer of the brain. The depressions are known as sulci and the ridges are known as gyri. It is hypothesized that these folds allow the brain more capacity for storage, retrieval, and other brain activity. These convolutions were first described by various scientists including Adolph Pansch, Pozzi, Ecker, Giacomini, and Heschl. Heschl described the transverse temporal gyrus as “a convolution springing from the first temporal convolution about the middle of the lower border of the Sylvian fissure and ending either singly or by joining the second temporal gyrus”.

Structure and Function


Heschl's gyrus is located bilaterally on the temporal plane. It serves as a general marker for the location of the primary auditory cortex, which still cannot be localized exactly. The primary auditory cortex was identified over 100 years ago, but due to human genetic variation and technology limitations it has not been routinely identifiable in the living human brain.

Studies have been done in attempt to relate variations in the structure of human brain anatomy to variations in the functioning of Heschl's Gyrus. . One such study addressed this by relating physical measurements of Heschl's Gyrus to how the region processes acoustic information. Volume measurements were taken from subjects and as predicted a large amount of structural variation was found. Auditory responses were then recorded using an MRI and the results found structure to be related to function. The larger the volume of Heschl's Gyrus in the right side of the brain, the more cortex response was associated with temporal, or time processing. The larger the volume on the right, the more spatially related cortex was activated. This study helps support one of the overarching theories of science that form dictates function.

Brain areas of the temporal plane respond differently to various sounds in the environment according to spatial arrangement. This is known as tonotopy. Studies have been able to identify tonotopy in the temporal lobes of monkeys. The studies used fMRI readings to map out which brain areas respond to which types of sound frequencies. The relation between the structure and function of a monkey's brain is identifiable through these maps, but technology has not allowed for this research in humans as of yet. This is due to the small size of this brain region in humans in relation to the current instruments available for inspection.

Research
Heschl's gyrus has been the focus of various areas of research. Such areas include: research in mental illness (for example schizophrenia and bipolar disorder), as well as research on pitch perception and acoustic processing.

Mental Illness
Mental Illness is any condition characterized by abnormalities in cognitive, emotional, or behavioral functioning. Mental Illness can be caused by social, psychological, biochemical, genetic, or other factors, such as infection or head trauma. Examples of mental illness include bipolar disorder, anxiety disorder, schizophrenia, and personality disorders among others. In relation to Heschl's Gyrus there has been more research pertaining to schizophrenic symptoms than there has been to other mental illnesses.

The relationship between schizophrenic auditory hallucinations and Heschl's Gyrus has been an expanding area of study. Early studies on this topic found little to no relationship between schizophrenia and Heschl's Gyrus. A 2004 study by David Cotter and company investigated cell density and cortical thickness of Heschl's Gyrus in order to determine any cellular differences between post-mortem patients with diagnoses of either major depressive disorder, schizophrenia and/or bipolar disorder. These were compared to normal experimental controls. No difference was found between the patient group and the control group so it was concluded that there was no support for the presence of cellular pathology within Heschl's Gyrus in schizophrenia. Looking at the size of brain matter, however, is only one way to compare differences between groups. Another method includes comparing the various connections of cells and this technique has been used in more recent studies.



This evolution in thinking beyond size comparisons mirrors the thinking of cognitive science as a whole. In the past it was thought that the larger a species' brain size the more intelligence the species possessed. This was eventually refuted as it was shown that body size plays a role in brain size. Whales for example have larger brains than humans. Later, relative size of cortex was thought to equate with intelligence, but this view was also difficult to uphold. More currently, the amount of brain cell connections are being researched to determine intelligence and proper functioning.

In a more recent 2012 study by Shinn and company some support was found for Heschl’s Gyrus playing a role in auditory hallucinations, which are a symptom of schizophrenia. This study used a resting-state fMRI to investigate the connectivity of the primary auditory cortex, located on Heschl's Gyrus, in patients with schizophrenia induced auditory hallucinations. The results showed that increased left hemisphere Heschl's Gyrus connectivity was positively related to hallucination severity in other brain areas. It was then concluded that abnormal interactions between the left Heschl's gyrus and the brain regions involved in speech/language, memory, and the monitoring of self-generated events may contribute to propensity for hallucinations. Similar studies have been conducted that support the role of primary auditory cortex abnormalities in auditory hallucinations.

Another school of thought advocates that auditory hallucinations cause the abnormalities witnessed in Heschl's Gyrus, and are not an underlying symptom of schizophrenia. A study by Daniela Hubl and company compared the volume of Heschl's Gyrus in patients with schizophrenia induced auditory hallucinations with those without auditory hallucinations. The study found a higher volume of Heschl's Gyrus in the right hemisphere of patients with hallucinations than those without. The scientists conducting this study suggested that the volume increases of Heschl's Gyrus were caused by the hallucinations and not by the underlying disorder of schizophrenia.

There are a variety of techniques and methods involved in the study of the brain, as well as a variety of schools of thought related to the topic. Each process and mode adds more information to the scientific community in its own way and aids in better understanding of the role of Heschl's Gyrus in mental illness. Cognitive Science as a subject increases knowledge in a similar interdisciplinary fashion, combining information learned from a variety of faculties, from cognitive psychology, to neuroscience, to computer science. .

Acoustic Processing
Heschl's Gyrus is one of the primary centers for hearing in the brain. Studies have been performed to better understand how pitch perception and acoustic processing occur in the brain.

A much cited study by Peter Schneider and company demonstrated that pitch perception localization varies based on the type of listener. There are two different types of pitches, f(0) or f(sp) and depending on the subjects dominant perceptual mode, localization will occur in either the right or left hemisphere. It was found that F(0) listeners had more right hemisphere localization and f(sp) listeners had more left hemisphere localization. Localization in this sense refers to the area where brain activity is highest upon exposure to a certain stimulus.

This study was supplemented by a second study from the same colleagues, which supported that pitch preferences in Heschl's Gyrus occur regardless of musical ability. This study also demonstrated that relative size of Heschl's Gyrus may be related to musical ability. The study used MRI scanning to examine and compare activity in Heschl's Gyrus of 334 professional musicians, 75 amateur musicians, and 54 non-musicians. Researchers such as Sebastian Puschmann were still not convinced that localization of pitch was due to perception and not stimulation.

Puschmann and his colleagues conducted a study using dichotic pitch stimuli in order to ensure that it was indeed pitch that was being perceived in Heschl's Gyrus and not just any sound stimulus. The study found that the lateral end of Heschl's gyrus was equally activated for sequences of fixed pitch or random pitch which supports prior research that the brain area is critical to the perception and evaluation of pitch information. The research concluded that Heschl's Gyrus might represent a general pitch processing centre in humans.

A study by John Brugge was conducted to compare the progression that Heschl's Gyrus undergoes for auditory processing in humans compared to monkeys. There is a deeper understanding of the structure and function of the primary auditory cortex in monkeys than there is in humans due to the ability to perform more invasive tests and the flatter structure of the monkey brain area. One technique used in science to understand more about humans is to compare human anatomy to their closest relatives; primates. In the study by Brugge it was found that auditory processing in monkeys was similar to humans. The study used electrodes implanted into the Heschl's Gyrus area of 15 patients to record results of brain activity. Researches exposed subjects to click sounds of varying rates. It was found that two auditory fields seemed to exist. A core field of the Gyrus could detect slower click rates, up until 200Hz, and a separate field responded to the highest click rates of the study. . The study concluded that auditory processing may occur in a similar fashion across humans and primates.

Directions for Future Research
The future understanding of Heschl's Gyrus will depend on new technological innovations. As microscopes and scanning systems are able to view brain areas at the neuronal level, a deeper understanding of the structure and function of Heschl's Gyrus will come about. Developing a tonotopic map of this structure should be researched in the future in order to more accurately identify the primary auditory cortex in humans. Understanding more about the transverse temporal gyrus could help in finding a cure, or prevention method, for schizophrenia and other mental illnesses. Once more, basic research has been conducted on this topic. Scientists and psychologists should apply this knowledge in order to benefit society.