Preferred intervals in birds and mammals: a filter response to noise?

Quasi-periodic spontaneous activity (preferred intervals, PIs) has been reported from avian primary auditory afferents. In mammals, PIs have not been reported, as yet. As the length of PIs is close to 1/characteristic frequency, it has been suggested that this type of spontaneous activity indicates particular mechanisms in avian inner ear transduction. However, the present paper shows that pigeon auditory fibres possessing preferred intervals in their spontaneous activity always belong to the most sensitive and the most sharply-tuned fibres recorded. This leads to the assumption that preferred intervals are the response of narrow-band filters to noise. This view is supported by three additional findings: (i) Near-threshold noise provokes PIs in avian fibres that show no spontaneous PIs. (ii) Similarly, PIs can also be evoked in mammalian (gerbil) auditory afferents by low level noise. (iii) Phase-locking of auditory afferents can be achieved by sound stimuli 10-20 dB below rate threshold. It is argued that no conclusions may be drawn from the presence of PIs about the nature of the underlying filter.