- University of Toronto
Auditory sensitivity has often been measured by identifying neural threshold in real-time (online) which can introduce bias in the audiograms that are produced. We tested this by recording auditory nerve activity of the notodontid moth Nadata gibbosa elicited by bat-like ultrasound and analysing the response offline. We compared this audiogram with a published online audiogram showing that the bias introduced can result in a difference in the audiogram shape. In the second part of our study we compared offline audiograms using spike number as threshold with others that used spike period and stimulus/spike latency, variables that have been suggested as providing behaviourally functional criteria. These comparisons reveal that functional audiograms are more flatly tuned than simple spike audiograms. The shapes of behavioural audiograms are discussed in the context of the selection pressure that maintains their shape, bat predation. Finally, we make predictions on the distance from bats at which notodontid moths use negative phonotaxis or the acoustic startle response.
All moths used in this study were wild caught from mercury vapour light traps at the Queen’s University Biological Station (44° 34′N, 76° 19′W) during the months of July and August, 2007–2009. Moths were identified (Covell 2005; Handfield 1999) and moths were used within 24 h of capture.
A desktop PC (Dell Optiplex GX260, Intel Pentium, 2.53 gHz) was connected to a National Instruments data I/O board (NI-USB-6251), which was used to generate stimulus pulses. Stimuli were amplified (Avisoft 70101) and broadcast through a speaker (ScanSpeak, Avisoft), calibrated using a Brüel & Kjær (B&K) measuring amplifier (Type 2610) with a (B&K) type 4135, 6.35 mm microphone, using continuous pure tones, measured as dB peSPL (rms re 20 μPa) (Stapells et al. 1982). The ultrasonic speaker was placed 30 cm from the ear being measured. Recordings were made in a Faraday cage lined with sound-absorbing foam.
Moths were decapitated and their thorax dissected exposing the auditory nerve (IIIN1b (Nüesch 1957)). Neural activity from this nerve was recorded with a stainless steel hook electrode which was referenced to an electrode placed in the abdomen (Fullard et al. 2003). Neural responses were amplified with a Grass P-15 A.C. pre-amplifier and fed into one channel of a multi-channel data acquisition board (Avisoft UltraSoundGate 416). The un-amplified signal from the DAQ board was fed into another channel to serve as a stimulus marker. Both channels were recorded with Avisoft Recorder USG software as 8 bits/50 kHz .wav files.