The classical area for hearing (primary auditory area, A1) has been described. As in the case of the visual areas it has been shown that fibres of the acoustic radiation end not only in the classical or first auditory area, but extend into some neighbouring areas as well. These include a second acoustic area lying in the superior temporal gyrus anterior to A1. A number of other auditory areas have been described.
Efferent fibres arising in the acoustic areas project to the medial geniculate body, and to the inferior colliculus, and may possibly reach motor nuclei of cranial nerves. Some of these efferents may influence the state of contraction of the stapedius and tensor tympani muscles. The acoustic areas are also connected with other parts of the cerebral cortex.
It may be remembered that the fibres of each lateral lemniscus are bilateral. Hence, the acoustic areas in each cerebral cortex receive fibres from both the right and left cochleae. The close relationship of the acoustic areas to Wernicke’s speech area may be noted. The association is significant in view of the obvious relationship between hearing and speech.
As the auditory areas receive impulses from both sides, a lesion on one side produces only partial loss of hearing. Lesions in the secondary auditory area (area 22) interferes with interpretation of speech (word deafness).