Summary
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1.
By means of simultaneous recordings with hook-electrodes the response behavior of a pair of homologous auditory interneurons on either side of the CNS of the bushcricketTettigonia viridissima L. was studied with particular emphasis on bilateral symmetry and directional coding.
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2.
In about 50% of the preparations the threshold curves of both interneurons were symmetrical, tested within a frequency range from 5 to 40 kHz (Fig. 4a). In the remaining cases significant bilateral asymmetries at frequencies above 20 kHz and/or below 12 kHz could be observed (Figs. 4 and 5). Therefore, in this bushcricket species symmetrical thresholds seem to be guaranteed only within a rather limited frequency band around 20 kHz. This frequency coincides with one (the higher) of the two spectral bands within the conspecific call (Fig. 2).
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3.
From recordings of unilaterally activated interneurons it can be inferred that the directional sensitivity of the single tympanic organ is as well tuned to this stimulus frequency of about 20 kHz. Maximal intensity differences of 15 to 20 dB between ipsiversus contralateral stimulation could be found (Fig. 3). Above and below this frequency the directionality of the ear was much lower.
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4.
This frequency dependence of directional hearing could be confirmed on the level of the CNS. Only at a stimulus frequency of about 20 kHz the directional curves of both interneurons were very pronounced (Fig. 10) with precisely encoding the lateralization of the sound source starting at about 10 dB above neuronal thresholds (Figs. 7, 8 and 9).
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5.
Though in single preparations the intensity-response curves of the two interneurons were notably different (Fig. 6), with respect to right versus left discrimination no auditory ‘handedness’ could be observed.
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6.
As some insects already orient to a single or a small number of stimuli, individual neuronal responses of the right versus the left interneuron were compared at different sound directions (Figs. 11 and 12). Surprisingly, with frontal stimulation the probability of ‘correct’ (symmetrical) responses was only 17% which abruptly increased as soon as the sound source became lateral. These results are discussed in terms of the zig-zag walk of the localizing insect.
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This work was supported by Sonderforschungsbereich ‘Bionach-SFB 114’. We especially appreciate the cooperation with Dr. G.J. Dörrscheidt in analyzing the stridulatory sounds and A. Akkermann for her comprehensive technical assistance. We thank Prof. Dr. J. Schwartzkopff, J.W. Green, and Dr. W. Papajewski for critically reading and B. Brücher for typing the manuscript and making the drawings.
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Rheinlaender, J., Römer, H. Bilateral coding of sound direction in the CNS of the bushcricketTettigonia viridissima L. (Orthoptera, Tettigoniidae). J. Comp. Physiol. 140, 101–111 (1980). https://doi.org/10.1007/BF00606302
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DOI: https://doi.org/10.1007/BF00606302