Dual representations of temporal modulations in human auditory cortex

Our ability to understand speech and other sounds relies crucially on the capacity to detect and perceive temporal amplitude fluctuations in the range of about 1-100 Hz. However, most individual neurons in auditory cortex are capable of precisely aligning their activities only to modulation rates at the lower end of this range. This raises the question of how higher modulation rates might be encoded, and of how the auditory cortex might be organised to accommodate the full range of perceptually relevant amplitude envelope modulations. Here we show, with noninvasive magnetoencephalography and electroencephalography measurements, that population oscillatory responses of human auditory cortex transition between a mode of strong phase locking to modulation rates below about 40-50 Hz, to a non phase-locked mode of responding at rates higher than about 50 Hz. Such dual response modes are predictable from the behaviours of single neurons in auditory cortex of non-human primates, but only the low rate phase locking mode has been previously observed in the neuronal population responses indexed in human MEG/EEG recordings. Taken together, the single neuron and MEG and EEG results from current thesis work suggest that two distinct types of neuronal encoding are required to represent the full range of temporal modulation rates that are relevant to everyday perception.