This photographic view of the left hemisphere is shown from the lateral perspective (see Figure 14A). The lateral fissure has been opened, and this exposes two gyri, which are oriented transversely. These gyri are the areas of the cortex that receive the incoming auditory sensory information first. They are named the transverse gyri of Heschl (as was also shown in the previous illustration), the auditory gyri, areas 41 and 42 (see Figure 60).
The lateral fissure forms a complete separation between this part of the temporal lobe and the frontal and parietal lobes above. Looked at descriptively, the auditory gyri occupy the superior aspect of the temporal lobe, within the lateral fissure.
Cortical representation of sensory systems reflects the particular sensation (modality). The auditory gyri are organized according to pitch, giving rise to the term tono-topic localization. This is similar to the representation of the somatosensory system on the postcentral gyrus (soma-totopic localization; the sensory "homunculus").
Further opening of the lateral fissure reveals some cortical tissue that is normally completely hidden from view. This area is the insula or insular cortex (see Figure 14B). The insula typically has five short gyri, and these are seen in the depth of the lateral fissure. It is important not to confuse the two areas, auditory gyri and insula. The position of the insula in the depth of the lateral fissure is also shown in a dissection of white matter bundles (see Figure 19B) and in the coronal slice of the brain (see Figure 29).
It should be noted that the lateral fissure has within it a large number of blood vessels, branches of the middle cerebral artery, which have been removed (see Figure 58). These branches emerge and then become distributed to the cortical tissue of the dorsolateral surface, including the frontal, temporal, parietal, and occipital cortex (discussed with Figure 58 and Figure 60). Other small branches to the internal capsule and basal ganglia are given off within the lateral fissure (discussed with Figure 62).
Since the auditory system has a bilateral pathway to the cortex, a lesion of the auditory pathway or cortex on one side will not lead to a total loss of hearing (deafness) of the opposite ear. Nonetheless, the pathway still has a strong crossed aspect; speech is directed to the dominant hemisphere.
Auditry gyri (transverse gyri of Heschl)
Ins = Insula
FIGURE 39: Auditory System 3 — Auditory Gyri (photograph)
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