fMRI of the central auditory system

Over the years, blood oxygen level-dependent (BOLD) fMRI has made important contributions to the understanding of central auditory processing in humans. Although there are significant technical challenges to overcome in the case of auditory fMRI, the unique methodological advantage of fMRI as an indicator of population neural activity lies in its spatial precision. It can be used to examine the neural basis of auditory representation at a number of spatial scales, from the micro-anatomical scale of population assemblies to the macro-anatomical scale of cortico-cortical circuits. The spatial resolution of fMRI is maximized in the case of mapping individual brain activity, and here it has been possible to demonstrate known organizational features of the auditory system that have hitherto been possible only using invasive electrophysiological recording methods. Frequency coding in the primary auditory cortex is one such example that we shall discuss in this chapter. Of course, noninvasive procedures for neuroscience are the ultimate aim and as the field moves towards this goal by recording in awake, behaving animals so human neuroimaging techniques will be increasingly relied upon to provide an interpretive link between animal neurophysiology at the multi-unit level and the operation of larger neuronal assemblies, as well as the mechanisms of auditory perception itself. For example, the neural effects of intentional behavior on stimulus- driven coding have been explored both in animals, using electrophysiological techniques, and in humans, using fMRI. While the feature-specific effects of selective attention are well established in the visual cortex, the effect of auditory attention in the auditory cortex has generally been examined at a very coarse spatial scale. Ongoing research in our laboratory has started to address this question and here we present preliminary evidence for frequency-specific effects of attentional enhancement in the human auditory cortex. We end with a brief discussion of several future directions for auditory fMRI research.