In the primary auditory cortex neighboring sound frequencies are processed by neighboring points in the brain – an orderly arrangement called tonotopy. Our lab uses high-resolution functional MRI at 7 Tesla to image the tonotopic organization of human primary auditory cortex, non-invasively and we related those maps, for the first time, to the underlying cortical anatomy. Our fine-scale mappings of auditory cortex revealed several surprises about the location of primary auditory cortex in the human brain.
First, we saw that sound frequency gradients (i.e. tonotopy) consistently run across Heschl’s gyrus, not along it as previously thought. Second, we saw that A1 and R locations are closely linked to the shape of the underlying anatomy. The diagram below summarizes the location of primary auditory cortex across the range of common morphological variants of Heschl’s gyrus.
It was previously thought that the location of primary auditory cortex was highly variable and unpredictable, but we found a remarkably orderly pattern. Primary auditory cortex covers both divisions of Heschl’s gyrus (when present), not only the anterior division as previously thought. Given this new perspective, the variants of Heschl’s gyrus appear to be part of a continuum rather than distinct subtypes.
Tuning into sound: frequency-selective filtering by atention
Cocktail parties, busy streets, and other noisy environments challenge the auditory system: how to focus attention onto selected sounds while ignoring others? Neurons of primary auditory cortex, which are tuned to sound frequency, could help solve this problem by filtering selected sound information based on frequency-content. To investigate whether this occurs, we mapped the fine-scale frequency-tuning of primary auditory areas A1 and R in humans. Then in a selective attention experiment, participants heard concurrent low (250 Hz) and high (4000 Hz) frequency sound streams and focused attention onto only one stream at a time, switching back and forth every 30s. Attention to low frequency tones enhanced neural responses within low frequency-tuned voxels relative to high, and when attention switched the pattern quickly reversed. Thus, like a radio, primary auditory cortex can tune into attended frequency channels and can switch channels on demand.
This spectral filtering could be an important function of the primary auditory cortex, contributing to our ability to focus attention onto complex sounds like speech.