Beats are periodic amplitude modulations resulting from the superposition of two periodic signals of different frequencies. Psychophysical experiments by Georg Simon Ohm, Hermann Helmholtz, and others from the 19th century demonstrate that beats are not only perceived for two close-by frequencies but also at mistuned octaves. However, the physiological mechanisms of this percept are still debated. Motivated by a field study, in which we observed the behavioral relevance of beats at high difference frequencies, we here study the beat encoding over a wide range of difference frequencies in the electric fish Apteronotus leptorhynchus. The activity of P-unit electroreceptor afferents, that share many properties with mammalian auditory fibers, follows a repetitive pattern with slow modulations of their firing rate reoccurring around multiples of the frequency of the carrier signal. By mathematical reasoning and supported by simulations of modified integrate-and-fire models we conclude that neither Hilbert transform, squaring, harmonics of the carrier, nor a threshold operation are sufficient to extract slow beats around the octave. Raising the thresholded signal to a power of three, however, is sufficient to explain the repetitive P-unit responses. Since the threshold-nonlinearity of the afferent's spike generator could be ruled out, it is most likely the transfer function of the electroreceptor synapse that implements such a non-linearity. In the auditory system the hair-cell synapse is known to act as a smooth threshold operation. We thus conclude that this mechanism within each auditory fiber contributes to the perception of beats at mistuned octaves.
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