Temporal and spectral sensitivity in identified auditory units in the cricket
Farris, H.E.; Mason, A.C.; Hoy, R.R.
Society for Neuroscience Abstracts 26(1-2): Abstract 368 16
We characterized the responses of previously undescribed auditory units in the prothoracic ganglion of the cricket Gryllus rubens to stimuli varying in carrier frequency and amplitude modulation (AM). With respect to carrier frequency, we found two classes of interneurons. First, we recorded from an ascending L-shaped unit sensitive to frequencies >10 kHz (BF 20-30 kHz, 65 dB SPL). This cell is similar to AN2 (INT-1) found in other gryllines, with dendritic arborization near the auditory neuropile and an ascending axon contralateral to the soma. A second cell class consisted of omega neurons (ON, intrasegmental) which were most sensitive to 5 kHz (55 dB SPL) but broadly tuned up to 65 kHz. In addition we recorded from afferents entering the ganglion from the tympanal nerve that were sensitive to frequencies <20 kHz. The G. rubens calling song consists of a trill of 10 ms pulses of 4.8 kHz at 50 p/s. As a song propagates through the environment, however, its depth of AM can be reduced. To assess whether modulation depth might influence detection of calling song, we measured the temporal modulation transfer function (TMTF) of units sensitive to 5 kHz. TMTFs were calculated by measuring the vector strength of spike times produced in response to sinusoidal AM stimuli that varied in modulation frequency (25-85 Hz) and depth. For ONs, TMTFs were lowpass with half-power points from 30 to 50 Hz. Some afferents, however, had lowpass responses with half-power points near 60 Hz, whereas others demonstrated all-pass responses over the range of modulation frequencies tested. This suggests that ONs are limited in their ability to process calling song AM, whereas the afferents examined are not.