Perception of complex sounds: N1 latency codes pitch and topography codes spectra

Objectives: This work aimed to find out whether the human cortical ‘tonotopy’ represents the true fundamental frequency (Fo) of complex sounds, or the center frequency CF at which harmonics peak in the audio spectrum. Indeed, complex periodic sounds (such as those of the human voice, musical instruments, etc.) comprise a ‘fundamental component’ (Fo) and its ‘harmonics’ (2Fo, 3Fo, …nFo). These often peak around a certain frequency CF. As Fo and CF are confounded in pure (sinusoidal) tones, the question of whether Fo or CF is represented through tonotopy had been hitherto unresolved. Methods: Whole-head recordings of brain electrical activity were obtained for 16 subjects submitted to an array of 9 different series of sounds (3 Fo×3 CF). Electrophysiological data were analyzed separately for each sound and each subject with brain functional imaging and dipole reconstruction. Results: Equivalent dipole sources of N1 components were, significantly for all subjects, more and more frontally oriented as CF increased, independently of Fo. Conclusions: Sounds are mapped in both the right and the left primary auditory cortices according to the spectral profiles of their harmonics (CF), rather than their fundamental frequencies (Fo).