Auditory neurons learning to hear

The Biotechnology and Biological Sciences Research Council’s recent business report (January 2008) had an interesting research report on auditory neurons and the perception of complex sounds. (Science Daily has a short report on the longer piece available here). (The BBSRC is the UK’s principal funder of basic biological research.)

As the BBSRC piece discusses, sound perception is extremely difficult because similar objects often make quite different sounds, and the medium (typically air) through which we hear does not allow for the spatialization or easy decomposition that, say, light allows in vision. The Oxford-based research team is using neural imaging to try to figure out how the brain makes sense of sound, and one thing that they’re finding is that background noise appears to be extremely important to sound processing. The auditory cortex does not simply respond to isolated qualities of specific sounds but to variations in the statistical properties of the entire sound scape. As the article reports: ‘Cortical neurons appear to anticipate this particular level of statistical regularity, and respond best to sounds that vary in pitch and intensity according to this natural rate of ebb and flow, which is found in many natural scenes and most musical compositions.’

The piece also discusses plasticity and variability in the auditory cortex, a characteristic that should not be a surprise to frequent readers of this blog. It turns out that our cortex constantly responds to the auditory environment by shifting its sensitivity. According to Dr. Jan Schnupp (U of Oxford), ‘Hearing is different from person to person. Nerve cells in the auditory cortex do not respond to sounds in the same way if we simulate listening to these sounds through the ears of another individual, and when we listen to the world through another person’s ear our sound location judgments are impaired, at least until our auditory cortex has learned the new mapping from acoustic cues to spatial location.’ Okay, so the ‘transplanted ears’ science fiction scenario is not terribly helpful, but it is important to note that the Oxford team’s research supports the idea that the auditory cortex fundamentally needs perceptual input in order to develop and take a particular configuration (something that Bruce Wexler also discusses in his book, Brain and Culture: Neurobiology, Ideology, and Social Change, which I’ve just finished reading).

Perhaps the most interesting part of the short article, however, is the fact that the auditory cortex appears to be different from the visual cortex, which has groups of neurons specifically tuned to different perceptual traits. That is, in the visual cortex, some parts deal with color, others with shape, position in the visual field, and the like. In the auditory cortex, however, the neurons appear to respond to a range of different sound qualities, including pitch, location and timbre.The material on neural plasticity, even very quick, responsive reconfiguration in the auditory cortex, is something that wouldn’t have caught my eye before Paul Mason really pushed me on it (and got me to reread Gerard Edelman more closely), but it’s significant on many levels. The constant adaptation of the brain, even in such basic areas as primary sense perception, really undermines assertions of neural architecture being fixed, something that’s crucial to theorists who want to argue for ‘massive modularity’ and even that the brain has pre-programmed abilities shaped by evolution. But the other material on the relative non-differentiation between sensory dimensions of sound in auditory neurons is also intriguing. It suggests that are native categories of perception — folk concepts, phenomenological traits, or even physical categories from audiology such as timbre, pitch, and location — may not be relevant categories for the neural division of perceptual labor.

I’ve been working on an introduction to a volume I want to do on the anthropology of the senses, and I think that this point — that phenomenology is absolutely necessary, but not in any way sufficient — to understand perceptual mechanisms, is crucial. Knowing our senses from the inside is not really the same as understanding the sensory, neural and behavioral mechanisms that produce perceptions. Our brains are so damn good at providing us with ‘more than we perceive,’ convincing us that we have an unmediated experience of the sense environment.

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