Primary Auditory Cortex - Function

Function

As with other primary sensory cortical areas, auditory sensations reach perception only if received and processed by a cortical area. Evidence for this comes from lesion studies in human patients who have sustained damage to cortical areas through tumors or strokes, or from animal experiments in which cortical areas were deactivated by surgical lesions or other methods. Damage to the Primary Auditory Cortex in humans leads to a loss of any awareness of sound, but an ability to react reflexively to sounds remains as there is a great deal of subcortical processing in the auditory brainstem and midbrain.

Neurons in the auditory cortex are organized according to the frequency of sound to which they respond best. Neurons at one end of the auditory cortex respond best to low frequencies; neurons at the other respond best to high frequencies. There are multiple auditory areas (much like the multiple areas in the visual cortex), which can be distinguished anatomically and on the basis that they contain a complete "frequency map." The purpose of this frequency map (known as a tonotopic map) is unknown, and is likely to reflect the fact that the cochlea is arranged according to sound frequency. The auditory cortex is involved in tasks such as identifying and segregating auditory "objects" and identifying the location of a sound in space.

Human brain scans have indicated that a peripheral bit of this brain region is active when trying to identify musical pitch. Individual cells consistently get excited by sounds at specific frequencies, or multiples of that frequency.

The auditory cortex is an important yet ambiguous part of the hearing process. When the sound pulses pass into the cortex, the specifics of what exactly takes place are unclear. Distinguished scientist and musician James Beament puts it into perspective when he writes, “The cortex is so complex that the most we may ever hope for is to understand it in principle, since the evidence we already have suggests that no two cortices work in precisely the same way."

In the hearing process, multiple sounds are absorbed simultaneously. The role of the auditory system is to decide which components form the sound link. Many have surmised that this linkage is based on the location of sounds. However, there are numerous distortions of sound when reflected off of different mediums, which makes this thinking unlikely. Instead, the auditory cortex forms groupings based on more reliable fundamentals; in music, for example, this would include harmony, timing, and pitch.

The primary auditory cortex lies in the posterior half of the superior temporal gyrus and also dives into the lateral sulcus as the transverse temporal gyri (also called Heschl's gyri).

The primary auditory cortex is located in the temporal lobe. There are additional areas of the human cerebral cortex that are involved in processing sound, in the frontal and parietal lobes. Animal studies indicate that auditory fields of the cerebral cortex receive ascending input from the auditory thalamus, and that they are interconnected on the same and on the opposite cerebral hemispheres.The auditory cortex is composed of fields, which differ from each other in both structure and function.

The number of fields varies in different species, from as few as 2 in rodents to as many as 15 in the rhesus monkey. The number, location, and organization of fields in the human auditory cortex are not known at this time. What is known about the human auditory cortex comes from a base of knowledge gained from studies in mammals, including primates, used to interpret electrophysiological tests and functional imaging studies of the brain in humans.

When each instrument of a symphony orchestra or the jazz band plays the same note, the quality of each sound is different — but the musician perceives each note as having the same pitch. The neurons of the auditory cortex of the brain are able to respond to pitch. Studies in the marmoset monkey have shown that pitch-selective neurons are located in a cortical region near the anterolateral border of the primary auditory cortex. This location of a pitch-selective area has also been identified in recent functional imaging studies in humans. The primary auditory cortex is subject to modulation by numerous neurotransmitters, including norepinephrine, which has been shown to decrease cellular excitability in all layers of the temporal cortex. Norepinephrine decreases glutamatergic excitatory postsynaptic potentials at AMPA receptors by the activation of alpha-1 adrenergic receptors.