Responses to features of the calling song by ascending auditory interneurones in the cricket Gryllus campestris

Abstract

ABSTRACT. In female Gryllus campestris L., three functional types of ascending auditory intemeurones have been studied by recording from them extracellularly in the split cervical connectives using suction electrodes. Type 1 neurones are characterized by an optimal sensitivity to the carrier frequency of the species calling song (4–5 kHz). They copy the syllable and pause structure of the call at all intensities. The patterned spike discharge is observable at least 8 dB above absolute threshold. With suprathreshold stimulation, the neurones exhibit maximal responses (number of spikes/chirp) around the carrier frequency. The intensity response curves are approximately linear in the range of 40–90 dB SPL. The envelope of each syllable is reflected by a corresponding change in the firing rate, and syllable periods of 24ms and longer are resolved. This type can be considered as a neural correlate for phonotactic behaviour of the female where the syllable period has been found to be the most important temporal parameter. Type 2 neurones are most sensitive in the range of 4–6 and 11–13 kHz. They copy the syllable and pause structure of the species calling song at low and moderate intensities. However, the spikes invade the intersyllable pauses, when stimulated with the calling song at higher intensities (above 85 dB). This is particularly apparent at the onset of a chirp series. The slope of the intensity—response curve mimics that of type 1 units. The neurones cannot follow syllable periods shorter than 32 ms. Type 3 neurones differ from types 1 and 2 by a rather broad‐band sensitivity in the range of 3–16 kHz, and in copying the chirp as a whole. Even at low stimulus intensities, the intersyllable pauses are filled with spikes, and information about the syllable—pause structure is lost. Stimulation with suprathreshold intensities gives rise to a rather uniform, broad‐band response without distinctive peaks. The intensity—response curve is characterized by a higher absolute threshold, and by the reduction in the response magnitude starting above 70–80 dB. These units are not suitable for copying the calling song temporal structure in detail, but would indicate the chirping rhythm. Their strong response in the range of the species courtship song carrier frequency make them suitable to copy the courtship song.