Brain Auditory Pathways

Tutorial 26: Brain Auditory Pathways

The auditory system differs somewhat from the visual system, having a more diffuse network of pathways. Nonetheless, the auditory system can in general be broken down into primary and secondary subsystems similar to those of the visual system. In the primary auditory system, impulses from the sensory receptors are transmitted to one of two nuclei (dorsal or ventral cochlear nuclei) in the medulla. The axons of cells in these nuclei form the lateral lemniscus, which terminates in the inferior colliulus. Two distinct pathways emerge from the inferior colliculus, heading for the ventral and dorsal medial geniculate nucleus of the thalamus. The ventral region of the medial geniculate nucleus projects to the primary auditory cortex, whereas the dorsal region projects to the secondary auditory cortex. Both primary and secondary auditory cortex, are located deep within the lateral fissure of the temporal lobe, with some extension into the parietal lobe. Secondary auditory cortex surrounds primary auditory cortex. The primary auditory system is essential for sound identification or perceptual processing, but is not necessary for the detection or localization of sounds.

The major structures along the auditory pathway are illustrated and described in this tutorial.

Advanced

Research concerning the auditory system ranges across the life span from infancy to late adulthood (Abdala & Sininger, 1996; Boothroyd, 1997; Pasman, Rotteveel, Maassen, Visco, 1999). The growth of auditory function within the first year of life is extensive; neurons in the brain stem mature and billions of important neuronal connections are formed during this period (Nara, Goto, Nakae & Okada, 1993; Peck, 1995; Rubsamen, 1992). Connections between brain stem nuclei, thalamic nuclei, and auditory cortex are just beginning to form during this very important year, and we know that sensory stimulation is essential for this development to proceed normally. When hearing is impaired during early stages of development, the morphology and function of auditory neurons may be adversely affected. Sometimes when hearing is restored with hearing aids, the deleterious effects of no stimulation are reduced. There is, however, indication of a critical period for intervention (Sininger, Doyle & Moore, 1999). It is difficult to detect hearing loss within the first two years of life, but it may be essential to intervene early in such cases for the proper development of the auditory pathway, and normal development of speech and language.

At the other end of the life span, concerns focus on recovery of function following damage in the fully developed, auditory system. Current research programs are studying the neuronal changes that accompany recovery of function after damage and following cochlear implants (Manrique et al., 1999; Nishimura, 2000; Ponton, Moore & Eggermont, 1999; Robinson, 1998; Shepherd et al., 1997). They are also addressing the effects of both training and deprivation on neuronal plasticity and functional recovery, any regional specificity in these effects, neurochemical mechanisms responsible for plasticity, and the relative effectiveness of different therapies. These lines of research may also add to our knowledge of mechanisms underlying normal auditory learning.

References

Abdala, C. & Sininger, Y.S. (1996). The development of cochlear frequency resolution in the human auditory system. Ear and Hearing, 17(5), 374-385.

Boothroyd, A. (1997). Auditory development of the hearing child. Scandinavian Audiology, 46, 9-16.

Manrique, M., Cervera-Paz, F.J., Huarte, A., Perez, N., Molina, M, & Garcia-Tapia. (1999). Cerebral auditory plasticity and cochlear implants. International Journal of Pediatric Otorhinolaryngology, 49, S193-197.

Nara, T., Goto, N., Nakae, Y. & Okada, A. (1993). Morphometric development of the human auditory system: ventral cochlear nucleus. Early Human Development, 32(2-3), 93-102.

Nishimura, H., Doi, K., Iwaki, T., Hashikawa, K., Oku, N., Teratani, T., Hasegawa, T. & Watanabe, A., Nishimura, T. & Kubo, T. (2000). Neural plasticity detected in short- and long-term cochlear implant users using PET. Neuroreport, 11(4), 811-815.

Pasman, J.W., Rotteveel, J.J., Maassen, B. & Visco, Y.M. (1999). The maturation of auditory cortical evoked responses between (preterm) birth and 14 years of age. Europeon Journal of Paediatric Neurology, 3(2), 79-82.

Peck, J.E. (1995). Development of hearing. Part III. Postnatal development. Journal of the American Academy of Audiology, 6(2), 113-123.

Ponton, C.W., Moore, J.K. & Eggermont, J,J. (1999). Prolonged deafness limits auditory system developmental plasticity: evidence from an evoked potentials study in children with cochlear implants. Scandanavian Audiology Supplements, 51, 13-22.

Robinson, K. (1998). Implications of developmental plasticity for the language acquisition of deaf children with cochlear implants. International Journal of Pediatric Otorhinolaryngology, 46(1-2), 71-80.

Rubsamen, R. (1992). Postnatal development of central auditory frequency maps. Journal of Comparative Physiology, 170(2), 129-143.

Shepherd, R.K., Hartmann, R., Heid, S., Hardie, N. & Klinke, R. (1997). Acta Otolaryngologica Supplement, 532, 28-33.The central auditory system and auditory deprivation: experience with cochlear implants in the congenitally deaf.

Sininger, Y.S., Doyle, K.J. & Moore, J.K. (1999). The case for early identification of hearing loss in children. Auditory system development, experimental auditory deprivation, and development of speech perception and hearing. Pediatric Clinics of North America, 46(1), 1-14.

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Tutorial 26: Brain Auditory Pathways

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