Frequency magnification in human primary auditory cortex nearly predicts behavioural frequency hyperacuity
The auditory system decomposes sounds into different frequencies, but does not represent them equally: in the human cochlea, lower frequencies occupy a larger spatial extent than higher frequencies, creating a quasi-logarithmic frequency map with finer low-frequency resolution. It remains unclear whether the primary auditory cortex preserves this quasi-logarithmic frequency mapping, or further magnifies behaviourally relevant frequencies, analogous to the foveal over-magnification in primary visual cortex that is thought to underpin visual hyperacuity phenomena such as Vernier acuity. Here, we used ultra-high field functional magnetic resonance imaging to quantify frequency magnification in the primary auditory cortex of 20 human listeners. We compared cortical frequency magnification to predictions based on published estimates of either cochlear frequency resolution or behavioural frequency discrimination performance, a form of perceptual frequency hyperacuity. Cortical frequency magnification was better predicted by frequency discrimination performance than by cochlear frequency resolution, with an additional unexpected over-representation of frequencies around 1 kHz. These findings suggest that frequency discrimination is constrained by cortical processes rather than by frequency information available at the cochlea, consistent with similar observations in the visual and tactile systems.