Representation of auditory distance within a central neuropil of the bushcricketMygalopsis marki

Summary

The functional organization of the afferent auditory system was studied in the bushcricketMygalopsis marki. The acoustic stimuli were computer simulated signals equivalent to the song of a male recorded at various distances in the habitat.

• 1. Individual receptor fibres have different tuning and absolute sensitivities and therefore differ in the specific distance at which they start to respond (‘threshold distance’, Fig. 3). This resulted in a range-fractionation in the distance of a conspecific signaller.

• 2. The target areas of receptor fibre endings in the auditory neuropil of the prothoracic ganglion differed in the rostro-caudal and dorso-ventral axis (Fig. 4). This tonotopic arrangement and the range-fractionation of tympanic receptor fibres result in a specific spatial distribution of afferent nervous activity in the neuropil. The distribution depends on the distance away from a singing conspecific male (Fig. 5).

• 3. Distance response characteristics of interneurons differed in both their shape and ‘threshold distance’ (Fig. 6). Certain neurons showed maximum responses which corresponded to an intermediate distance. These responses are based on a frequency-dependent synaptic interaction of excitation and inhibition (Fig. 7). The different distance response characteristics of interneurons can be related in part to the dendritic branching pattern within the neuropil.