The Tessellated Monkey: Parsing the Functional Fields of the Auditory Cortex

1109 No self-respecting concertgoer of a certain era would consider wearing earplugs at a show, but that was long before Pete Townsend and other rock icons spoke out about the risk of deafness. Today, most people recognize that highintensity noise causes hearing loss—except maybe for those iPod users who routinely blast earsplitting music straight into their brains. Blaring volume causes deafness by destroying soundresponsive hair cells, but it’s unclear how these auditory assaults affect the brain’s auditory system. Much of the auditory cortex is organized by sound frequency, but neuroscientists are still fi guring out the extent of the spatial organization of frequency-selective neurons and how each auditory fi eld contributes to sound perception. While neurophysiological studies have characterized the functional properties of certain auditory cortical fi elds (by recording the electrical activity from individual neurons), anatomical studies have identifi ed other fi elds that had not been functionally characterized. A new study by Christopher Petkov, Nikos Logothetis, and colleagues fi lls in some of these gaps by using high-resolution functional magnetic resonance imaging (fMRI) on macaque monkeys presented with acoustic stimuli. The researchers used the anatomical and neurophysiological data to see how the fMRI data compared with the already described auditory cortical fi elds. With a better sense of how to interpret the functional imaging data, they could use fMRI to probe the functional properties of uncharacterized auditory fi elds. This approach allowed them to show the functional organization of 11 discrete auditory fi elds in the primate auditory cortex— an important step toward understanding how these fi elds operate together to shape what the primate listener perceives of its acoustical environment. Petkov et al. fi rst used a broad spectrum of sound frequencies to globally activate the monkeys’ auditory cortex. (Six anesthetized monkeys and one monkey trained to stay still were placed in fMRI scanners while presented with acoustical stimuli.) Next, they used lowand highfrequency sounds to identify regions with selectively tuned neurons. Based on predictions that auditory fi elds follow an alternating pattern of high to low frequency along a posterior to anterior direction, they expected fMRI activity to follow the same pattern—which it did. This now “grounded” frequency gradient allowed them to match the rest of the activity patterns that they observed with other auditory fi elds. Signifi cantly, they matched an alternating pattern of highand low-frequency selective regions with three fi elds in the primary auditory cortex, or auditory “core” fi elds: A1, R, and RT. These core fi elds are thought to be surrounded by seven or eight so-called “belt” (nonThe Tessellated Monkey: Parsing the Functional Fields of the Auditory Cortex