The Auditory Cortical Regions of Many Species Show Tonotopic Organization

As described in the text and illustrated in Figure 9.8, tonotopic organization can be demonstrated by the mapping of auditory brain regions using 2-deoxyglucose (2-DG). Following 2-DG injection, an animal is exposed to a tone of a particular frequency. Because 2-DG is taken up like glucose by neurons, but not metabolized, we can use its presence within neurons as an indicator of neuronal activity. Postmortem processing of 2-DG distribution reveals which cells were most active when the stimulus frequency was presented.

Most species of animals have several auditory cortical fields. Different fields of the auditory cortex may be specialized for location of sounds in space, movement of sound sources, perception of species-specific sounds, and so on (see Figure 1). By and large, cortical auditory areas are dedicated to the processing of “biologically relevant” sounds; that is, these regions have been shaped through evolution to be especially responsive to sound patterns that signal threats or opportunities in the environment.

Figure 1  Tonotopic Organization of Auditory Cortical Regions in Three Species of Mammals
The arrows show the direction of tonotopic representation, from low to high frequencies. (After Merzenich et al., 1993.)