At least six tonotopic maps have been found in the primary auditory cortex and auditory association areas. In each of these maps, high-frequency sounds excite neurons at one end of the map, whereas low-frequency sounds excite neurons at the opposite end. In most, the low-frequency sounds are located anteriorly, as shown in Figure 52-11, and the high-frequency sounds are located posteriorly. This is not true for all
the maps. The question one must ask is, Why does the auditory cortex have so many different tonotopic maps? The answer, presumably, is that each of the separate areas dissects out some specific feature of the sounds. For instance, one of the large maps in the primary auditory cortex almost certainly discriminates the sound frequencies themselves and gives the person the psychic sensation of sound pitches. Another map is probably used to detect the direction from which the sound comes. Other auditory cortex areas detect special qualities, such as the sudden onset of sounds, or perhaps special modulations, such as noise versus pure frequency sounds.
The frequency range to which each individual neuron in the auditory cortex responds is much narrower than that in the cochlear and brain stem relay nuclei. Referring back to Figure 52-6B, note that the basilar membrane near the base of the cochlea is stimulated by sounds of all frequencies, and in the cochlear nuclei, this same breadth of sound representation is found. Yet, by the time the excitation has reached the cerebral cortex, most sound-responsive neurons respond to only a narrow range of frequencies rather than to a broad range. Therefore, somewhere along the pathway, processing mechanisms "sharpen" the frequency response. It is believed that this sharpening effect is caused mainly by the phenomenon of lateral inhibition, which is discussed in Chapter 46 in relation to mechanisms for transmitting information in nerves. That is, stimulation of the cochlea at one frequency inhibits sound frequencies on both sides of this primary frequency; this is caused by collateral fibers angling off the primary signal pathway and exerting inhibitory influences on adjacent pathways. The same effect has been demonstrated to be important in sharpening patterns of somesthetic images, visual images, and other types of sensations.
Many of the neurons in the auditory cortex, especially in the auditory association cortex, do not respond only to specific sound frequencies in the ear. It is believed that these neurons "associate" different sound frequencies with one another or associate sound information with information from other sensory areas of the cortex. Indeed, the parietal portion of the auditory association cortex partly overlaps somatosen-sory area II, which could provide an easy opportunity for the association of auditory information with somatosensory information.
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