Detection of short pure-tone stimuli in the noctuid ear: what are temporal integration and integration time all about?

Abstract

Temporal integration in the A1 acoustic receptor cell of noctuid moths (Lepidoptera: Noctuidae) was studied. This was done by recording from the auditory nerve of Spodoptera littoralis and Noctua pronuba while stimulating with short-duration tone pulses. Consistent with previous experiments of the same kind, a decrease in threshold was observed with increasing stimulus duration. These threshold improvements, however, were larger than expected from an energy detector model of the receptor based on previous experiments with double clicks. This difference was analysed and could be attributed to a bias introduced by the choice of threshold criterion. If a fixed number of spikes elicited per stimulation is used as criterion, this will tend to lower the measured thresholds for long durations because for long stimuli, more time is available for accumulation of the spikes needed to exceed the criterion. It appears that temporal integration in the noctuid ear can be characterised by different time constants. One, termed the intrinsic time constant, is measured in the double-click experiments and provides information about the temporal integration and resolution within the receptor cell. Behavioural time constants, on the other hand, characterise the temporal integration reflected in the behaviour of the animals.