Brain projections and information processing of biologically significant sounds by two large ventral-cord neurons ofGryllus bimaculatus DeGeer (Orthoptera, Gryllidae)

Abstract

Two ventral-cord neurons in the auditory system ofGryllus bimaculatus were studied electrophysiologically by stimulation with pulses of sound at a single frequency (sine-wave pulses), stridulatory songs, and artificial sounds constructed to imitate the conspecific songs. The sine-wave pulses were varied in frequency, sound intensity, duration, and repetition rate. The stridulatory songs were the conspecific calling, aggressive, and courtship songs and the calling songs of 8 sympatric gryllids (played back at different sound intensities). The artificial songs were varied in carrier frequency, pulse rate, chirp rate, and sound intensity. The LF1 neuron precisely duplicates the temporal structure of the conspecific calling (and aggressive) song over the whole intensity range (Figs. 7, 8, 10). It is sharply tuned to the carrier frequency of the song (5 kHz) and shows little or no response above 10 kHz and below 3 kHz (Figs. 1, 2). By variation of the calling song's temporal structure it can be demonstrated that the LF1 neuron is particularly suited to respond to the pulse duration and the pulse and chirp repetition rates of this song pattern (Figs. 6, 9). On the other hand, the HF1 neuron is a broad-band neuron with a maximal sensitivity at 16 kHz (Figs. 1, 4); it is tuned to the conspecific courtship song with respect to carrier frequency, the short pulse duration, and the very low pulse repetition rate (Figs. 6, 7, 8). The results demonstrate that the two ventral-cord neurons represent highly evolved channels of the auditory pathway in gryllids, each of which transmits important features of the corresponding conspecific songs to several areas of the brain (Fig. 11). But they are not ideal filters for these conspecific songs, since they also respond to many other sound signals (Fig. 10).