Intro | Auditory Cortex | Cochlear Nucleus | Inferior Colliculus | Medial Geniculate | Superior Olive
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
The fifth and final synapse of the primary auditory pathway occurs in the Auditory Cortex. The primary auditory cortex is located along the ventral surface of the temporal lobe inside the lateral fissure. Secondary auditory cortex, which receives projections from the primary auditory cortex, lies along the superior edge of the lateral surface of the temporal lobe thereby surrounding the primary cortex.
Different frequencies of sound are represented in particular regions of the auditory cortex. This arrangement is called tonotopic representation, and is found throughout the auditory pathway starting with the basilar membrane of the inner ear. The most medial portion of the auditory cortex contains the representation of the basal end of the basilar membrane of the inner ear (by the oval window), whereas the apical end of the basilar membrane of the inner ear is represented in the lateral portion of the auditory cortex. This arrangement is called place coding. The tonotopic organization of each auditory cortical subregion remains unclear, but in general it appears that in each subfield that has been mapped low tones are represented posteriorly and high tones more anteriorly. A process called rate coding is also used for the encoding sounds of low frequency. This process occurs, however, only at the inner ear or cochlear level. In rate coding, low frequency sounds are encoded when neurons fire in synchrony with movement of the apical end of the basilar membrane.
Bilateral damage to the auditory cortex is not associated with permanent deficits in the ability to detect sound. Extensive damage to the auditory cortex does, however, often induce a condition called auditory agnosia. Auditory agnosia is characterized by the inability to perceptually identify the meaning of both verbal and nonverbal sounds, or word deafness. Humans with damage to the auditory cortex may also experience difficulty in perceiving the ordering of simple sounds, suggesting a fundamental disorder in processing sound across time, a temporal auditory disorder.
The secondary auditory pathway is essential for sound detection and localization. This pathway consists primarily of projections from the dorsal and ventral nuclei of the medulla to the inferior colliculus of the midbrain. The midbrain in turn transmits relevant information to the secondary auditory cortex. Sound localization occurs via two mechanisms. The medial system, arising from the medial superior olivary complex, responds to slight differences in the timing of sound arrival at each ear (intensity differences). The lateral system, arising from the lateral superior olivary complex, responds to slight differences in sound amplitude arriving at each ear (phase differences).
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The small gyrus in the superior temporal lobe that contains the primary auditory cortex is also known as Heschl's gyrus, AI, and Brodmann's area 41. The secondary auditory cortex includes Wernicke's area, an important region in language function that is discussed with detail in Tutorial 38.
The response of neurons in the primary auditory cortex to specific sound wave frequencies is narrow and binaural (responsive to both ears) (Reser, Fishman, Arezzo & Steinschneider, 2000; Schreiner, Read & Sutter, 2000). Some are excited by both ears (EE), while others are excited by stimulation from one ear and inhibited by stimulation from the other (EI). The primary auditory cortex is organized in vertical columns with the cells in each column sensitive to the same main sound frequency. Adjacent columns are organized according to the tonotopic representation (from lower frequency to higher frequency moving from posterior to anterior). Adjacent columns moving in the lateral to medial direction alternate between EE and EI cells as described above. This arrangement is similar to the arrangement of ocular dominance as found in primary visual cortex.
| References |
Reser, D.H., Fishman, Y.I., Arezzo, J.C., Steinschneider, M. (2000). Binaural interactions in primary auditory cortex of the awake macaque. Cerebral Cortex, 10(6),574-584.
Schreiner, C.E., Read, H.L. & Sutter, M.L. (2000). Modular organization of frequency integration in primary auditory cortex. Annual Review of Neuroscience, 23, 501-529.