Functional imaging of temporal auditory processing

Abstract

Functional imaging of the auditory system can be carried out using positron emission tomography (PET) and functional MRI (fMRI). Both techniques depend on the blood flow response to stimuli as a measure of brain activity, a process with a delay of 10 s in the auditory cortex. This limits the ability of the techniques to investigate temporal processing in the auditory system. Experiments to investigate the basis for temporal processing are critically dependent on the model that is used to interpret the brain blood flow data. The blood flow response in such experiments depends on local mean synaptic activity; this can be increased either by the firing rate in a subpopulation of cells or by increased synchronization of neural activity. At the millisecond level, an increase in the fMRI BOLD response with the temporal regularity of a signal at the millisecond level has been demonstrated as early as the cochlear nucleus. This may be due to an increased firing rate of onset chopper cells tuned to a particular autocorrelation delay or to increased synchronized activity in the whole population. The temporal processing of patterns over longer periods (seconds) is likely to involve areas of the cortex distinct from the primary auditory cortex.