Topographic Representation of Periodicity Information: The 2nd Neural Axis of the Auditory System

For acoustic communication the auditory system has to deal with two typical signal properties: first, many signals are embedded in noise and second, many signals are broadband harmonic sounds. Harmonic sounds, particularly voiced speech sounds and many animal communication signals, are characterized by a periodic envelope or amplitude modulation (AM). Signals with the same periodicity have the same pitch, while their fi-equency content defines their timbre. Note, that the terms 'period, periodic, or periodicity' are used here for the temporal envelope of a signal which may or may not correspond to the period of a pure tone, as in AM where each of its three firequency components may have a period differing fi-om the period of the envelope. For the auditory system periodicity is a very useful property, because in periodic sounds the same feature, for example the peak of the envelope, appears repeatedly thereby facilitating signal detection in noise. Acoustic signals may be described in the temporal as well as in the fi-equency domain. In order to imderstand the processing of complex signals by the auditory system one has to take into account that the first processing stage, the cochlea, performs a frequency analysis. From the beginning of modem hearing research this analysis was compared to a Fourier analysis (von Helmholtz, 1863). However, it turned out that the bandwidths of single