The peripheral auditory characteristics of noctuid moths: information encoding and endogenous noise

The ability of the noctuid A1 cell acoustic receptor to encode biologically relevant information from bat echolocation calls is examined. Short-duration stimuli (less than approximately 6 ms) reduce the dynamic resolution of the receptor, making intensity, and hence range, estimates of foraging bats unreliable. This low dynamic range is further reduced by inaccurate encoding of stimulus intensity, reducing the real dynamic range of the A1 cell to 1 bit at stimulus durations below 3.1 ms. Interspike interval is also an unreliable measure of stimulus intensity at low stimulus levels and/or for short-duration stimuli. The quantity of information encoded per stimulus is reduced as the presentation rate of stimuli is increased. The spontaneous generation of A1 cell action potentials may reduce the ability of the moth to discriminate bat from non-bat signals. Even with a recognition criterion of three A1 cell spikes per call, the moth would regularly make wrong decisions about a bat being present in the immediate environment. Removing this noise would necessitate a considerable loss of information through filtering at the interneurone level. It is proposed that, for bats using short-duration calls, the moth would only be able to recognise an approaching bat from the repetitious nature of the incoming signal.