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

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 high-intensity noise causes hearing loss—except maybe for those iPod users who routinely blast earsplitting music straight into their brains.

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 cortexan 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 low-and 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 high-and 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" (non-

Non-invasive functional imaging reveals many discrete functional areas in the auditory cortex of a non-human primate. (Photo: Marc D. Hauser)
primary) fi elds. However, neurophysiological data on RT and many of the belt fi elds are scant, making it unclear how many functional fi elds actually exist.
It's thought that auditory fi elds in the core are tuned to simple sounds, like single-frequency tones, while fi elds in the belt respond to complex sounds. To better locate activity in the belt regions, Petkov et al. also studied brain responses to more-complex sounds. The results included frequency selectivity patterns consistent with known patterns for four belt fi elds that had previously been studied neurophysiologically and provided a base outline for the other fi elds-basically functionally tessellating the monkey auditory cortex. Petkov et al. then took advantage of evidence that tones produce a stronger response in the core than they do in the belt fi elds to outline a border between the core and belt, which helped them to further resolve the position of the core relative to the belt fi elds.
The extensive patterns of frequency gradients indicated that the three core regions were surrounded by eight belt fi elds, four on each side, supporting anatomical evidence for about a dozen auditory fi elds. The researchers then went on to show that neurons in the belt fi elds responded preferentially to sounds with a broad frequency spectrumin other words, more complex sounds that would have some of the properties of natural sounds. These results fall in line with a model of hierarchical auditory processing in which the core operates during the initial stages of auditory cortex processing, contributing to a frequency analysis of the sounds in the environment. The belt fi elds function at a higher level to deal with more-complex sounds by integrating sound frequencies. The challenge now is to understand how each of the many fi elds contributes toward and interacts with others to shape the perception of primates in their typically opulent acoustical-and multisensory-environments.
This study underscores the value of pooling data from different experimental approaches to study something as intricate as the brain. With this high-resolution functional MRI map of the monkey auditory cortex, researchers can now use both fMRI and neurophysiological techniques to refi ne each fi eld's particular role within the primate auditory cortex. The map will also guide efforts to better understand the functional organization of the human auditory system-information that could certainly identify the impact of peripheral hearing loss on this part of the auditory system. And with functional maps of both monkey and human auditory cortex, researchers can better understand how the specialized auditory fi elds evolved, ultimately offering insights into the evolution of human language.