The neuromagnetic representation of the scale of instrumental tones in human auditory cortex
Martin Andermann; Andre Rupp

The size of a sound source determines the scale of its resonances (“acoustic scale”) in harmonically complex periodic sounds like vowels or instrumental tones. Here, the representation of acoustic scale in human auditory cortex was investigated in two experiments using Magnetoencephalography (MEG). Using the STRAIGHT software (Kawahara et al., 2004), French horn tones were scaled to sound larger or smaller by shifting their spectral envelope as a unit along a logarithmic frequency axis independent of the fundamental frequency. In experiment 1, auditory evoked fields (AEFs) were recorded in fifteen subjects as they listened to scaled French horn tones which had additionally been “whitened” to eliminate pitch cues. In experiment 2, the same subjects were presented with French horn tone triplets varying in pitch and scale. The resulting cortical activity was evaluated by spatio-temporal source analysis with two equivalent dipoles in each hemisphere. In the first experiment, the change of acoustic scale elicited a well-defined N1m/P2m AEF-complex in all subjects with the cortical generators of the N1m component being located in the Planum temporale and the P2m being generated in Heschl’s gyrus. The N1m model was then applied to the data from the second experiment, where the neuromagnetic responses to a scale change could be clearly separated from responses to changes in pitch. The results demonstrate that acoustic scale as a physical property of sound can be distinguished in its cortical representation from pitch-associated cortical activity. Further, the existence of a “Scale-N1m” within the complex structure of the N1m highlights the importance of this component for the processing of spectral features in auditory cortex.