User:Dtran200/sandbox

Tonotopy in the Cortex
Audio frequency, otherwise known as the pitch, is currently the only characteristic of sound that is known with certainty to be topographically mapped in the central nervous system. However, other characteristics may form similar maps in the cortex such as sound intensity, tuning bandwidth , or modulation rate   , but these have not been as well studied.

In the midbrain, there exist two primary auditory pathways to the auditory cortex--the lemniscal classical auditory pathway and the extralemniscal non-classical auditory pathway. The lemniscal classical auditory pathway is tonotopically organized and is comprised of the central nucleus of the inferior colliculus and the ventral medial geniculate body projecting to primary areas in the auditory cortex. The non-primary auditory cortex receives inputs from the extralemniscal non-classical auditory pathway, which shows a diffuse frequency organization.

Tontopic Subregions of the Cortex
The tonotopic organization of the auditory cortex has been extensively examined and is therefore better understood compared to other areas of the auditory pathway. Tonotopy of the auditory cortex has been observed in many animal species including birds, rodents, primates, and other mammals. In mice, four subregions of the auditory cortex have been found to exhibit tonotopic organization. The classically divided A1 subregion has been found to in fact be two distinct tonopic regions--A1 and the dorsomedial field (DM). Auditory cortex region A2 and the anterior audiotry field (AAF) both have tonotopic maps that runs dorsoventrally. The other two regions of the mouse auditory cortex, the dorsoanterior field (DA) and the dorsoposterior field (DP) are non-tonotopic. While neurons in these non-tonotopic regions have a characteristic frequency, they are arranged randomly.

Studies using non-human primates have generated a hierarchical model of auditory cortical organization consisting of an elongated core consisting of three back-to-back tonotopic fields--the primary auditory field A1, the rostral field R, and the rostral temporal field RT. These regions are surrounded by belt fields (secondary) regions and higher-order parabelt fields. A1 exhibits a frequency gradient from high to low in the posterior-to-anterior direction; R exhibits a reversed gradient with characteristic frequencies from low to high in the posterior-to-anterior direction. RT has a less clearly organized gradient from high back to low frequencies. These primary tonotopic patterns continuously extend into the surrounding belt areas.

Tonotopic organization in the human auditory cortex has been studied using a variety of non-invasive imaging techniques including magneto- and electroencephalography (MEG/EEG), positron emission tomography (PET), and functional magnetic resonance imaging (fMRI). The primary tonotopic map in the human auditory cortex is along Heschl's gyrus (HG). However, various researchers have reached conflicting conclusions about the direction of frequency gradient along HG. Some experiments found that tonotopic progression ran parallel along HG while others found that the frequency gradient ran perpendicularly across HG in a diagonal direction, forming a angled V-shaped pair of gradients.