A Step Further 6.1: The Auditory Cortical Regions of Many Species Show Tonotopic Organization

Tonotopic organization of the auditory brain regions can be demonstrated by creating maps of activity associated with different frequencies. Here, each animal was injected with the metabolic tracer 2-deoxyglucose (2-DG) and then 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 (see Figure 1).

A horizontal cross-section of the rat brain has the inferior colliculus. Within the circular-shaped inferior colliculus, there are five layers from the top left to the bottom right. The different frequencies of these layers from top to bottom are 2 kilohertz, 6.5 kilohertz, 13.5 kilohertz, 15 kilohertz, 21 kilohertz. — **Figure 1 Mapping Auditory Frequencies in the Cat Inferior Colliculus**
(a)This lateral view of the cat brain shows the plane of the transverse section through the inferior colliculus shown in part (b). (c, d) Locations of the cells labeled with 2-DG via (c) 2,000-Hz stimulation and (d) 21,000-Hz stimulation are indicated here by blue and red, respectively. (e) Complete tonotopic mapping shows the range of frequencies that can stimulate the cat’s auditory system. (After Serviere et al., 1984.)

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 2). 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.

The primary auditory cortex of the Guinea Pig, Cat, and Macaque monkey are given. — **Figure 2 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.)

References

Merzenich, M. M., Schreiner, C., Jenkins, W., and Wang, X. (1993). Neural mechanisms underlying temporal integration, segmentation, and input sequence representation: Some implications for the origin of learning disabilities. Annals of the New York Academy of Sciences 682: 1–22.

Serviere, J., Webster, W. R., and Calford, M. B. (1984). Isofrequency labelling revealed by a combined [14C]-2-deoxyglucose, electrophysiological, and horseradish peroxidase study of the inferior colliculus of the cat. Journal of Comparative Neurology 228: 463–477.