Discrimination of calling song models by the cricket,Teleogryllus oceanicus: the influence of sound direction on neural encoding of the stimulus temporal pattern and on phonotactic behavior

Summary

Recordings were made from an identified auditory neuron, the omega neuron, in the cricketTeleogryllus oceanicus. Models of the conspecific calling song and of the song of another species were presented either singly or simultaneously, and the degree to which the temporal pattern of the conspecific model was encoded in the neuron's spike train was determined. When a single stimulus was presented alone, its temporal pattern was faithfully reflected by the cells's spiking activity, no matter what the azimuth of the broadcasting loudspeaker (Fig. 3). When two stimuli were presented simultaneously from opposite sides, encoding of the pattern ipsilateral to the recorded neuron was interfered with only slightly by the contralateral pattern, as long as the two loudspeakers were sufficiently separated (Figs. 2, 3, 4). When the loudspeakers were each 15° from the cricket's midline, however, the encoding of the temporal pattern of the ipsilateral song model was severely disrupted (Figs. 3, 4). Bilateral interactions are important in determining the response level of the neuron, but do not appear to contribute to the direction-selective encoding of the stimulus temporal pattern (Figs. 5, 6).

Phonotactic steering movements of tethered, flying crickets were recorded under stimulus conditions similar to those used in the neurophysiological tests. Under one-stimulus conditions, crickets attempted to turn towards the conspecific model for all tested speaker locations. The heterospecific model elicited reliable steering behavior when it was broadcast from azimuths of 90° and 60°, but often failed to elicit consistent responses when the speaker was positioned closer to the cricket's midline (Figs. 7, 8A and 8B). Responses to the heterospecific pattern were smaller in amplitude than those to the conspecific song model (Figs. 7, 8B). Under two-stimulus conditions, the conspecific model was consistently preferred over the heterospecific song for all tested speaker locations in half the tested crickets. In the remaining animals, preference for the conspecific pattern was only evident for the larger loudspeaker azimuths (Figs. 7, 8C).

These results demonstrate that simultaneouslypresented stimuli can be represented separately in the nervous system as a consequence of auditory directionality. It is postulated that the cricket's ability to choose between these stimuli may result from the interactions between two bilaterallypaired song recognizers, each of which may be driven primarily by sound stimuli from one side.