Temporal coding in the auditory receptor of the moth ear

Temporal coding in the moth ear was inferred from the response of the auditory receptor to acoustic stimuli with different temporal characteristics.

1.Determinations of the threshold with different stimulus pulse durations showed that the moth ear behaves as an energy detector with a maximum time constant (the integration time) of 25 ms. Pulse durations beyond this value did not result in decreased thresholds (Fig. 1).

2.The synchronization to amplitude modulations was determined by stimulating the moth ear with amplitude modulated (AM) tones (carrier frequency: 40 kHz) and AM white noise presented as 450 ms pulses separated by pauses of similar length. The modulation depth was constant (100%) whereas the modulation frequency,fm, was varied. The maximumfm which the auditory receptors could follow was 200 Hz (P<0.05) (Figs. 2, 3, 4).

3.The relatively broad tuning of the only receptor which was functional at the relevant stimulus intensities suggested that AM detection could only be based on temporal cues. This was confirmed by the results showing the same degree of synchronization independent of carrier.

4.A minimum time constant for the receptor was also determined by interrupting a 400 ms noise pulse by a gap (Figs. 5, 6). The threshold for gap detection of the moth ear was ca. 2 ms on a 2.5% significance level (one sided test).

5.The temporal acuity reported here seems to be fine enough to explain the temporal resolution suggested by behavioral results from other insect species. The results are discussed in relation to acoustic communication in insects as well as in relation to temporal resolution in vertebrates.