ALBERT

Notes: Summary 1. A method of recording nervous activity by microelectrodes in the brain (supraoesophageal ganglion) of L. migratoria is described. An auditory centre is located at the border of proto- and deutocerebrum, comprising parts of both. In it neurons can be activated by acoustic signals. Those cells located in the protocerebrum (Fig. 1a, field A), were at first stimulated by simple acoustic stimuli of changing intensity and frequency. Subsequently, more and more complicated stimuli and finally natural sound were used. 2. The threshold curves show sensitivity-minima at 4–8 kcps and 15–30 kcps at 35 db (rel. 0,0002 dyn/cm2. Most neurons have a wide range of sensitivity, i.e. between 3 and 30 kcps (Fig. 4). Less common are cells sensitive in the lower or upper half of the acoustic range only. 3. If the correlation between excitation and stimulus intensity is plotted („Kennlinie“), different types of reaction become apparent. The „Kennlinie“ of a neuron may differ strongly from that of another. Furthermore, the “Kennlinien” of one and the same neuron have usually different shapes depending on the pitch (Fig. 5). „Gipf eiförmige Kennlinien“ (showing one maximum or more), whose maxima have different positions, are considered to represent different intensities (Fig. 6). 4. Inside the response areas — with frequency and intensity as parameters of the stimulus — the neurons show ranges of excitation and inhibition. Therefore more suitable for description are “Frequenzwirkfelder” (fields of effectivities for different frequency- and intensity-conditions). These and similar „Wirkfelder“ always show the quantity of excitation — sum of nerve impulses — for different stimuli combinations as hights of columns. At frequencies below 10 kcps, they show “Hemmzungen” (tongue-shaped inhibition areas), which may be more (Fig. 12) or less (Fig. 13a)- extensive. Simpler zones of inhibition, which comprise the entire upper range of intensity, partly permit excitation only directly above the threshold (Fig. 11). 5. In the “Frequenzwirkfelder” of contralateral sound (Figs. 11–13, outlined columns) inhibitory influences are stronger than in those of ipsilateral sound (solid columns). The dynamics of the types of response of single elements can be explained by a relative simple system of excitatory and inhibitory influences (Fig. 17). Experiments in which the tympanal organ was destroyed indicate that the contralateral organ causes predominantly inhibition, while the ipsilateral organ causes mainly excitation. In addition, contralateral excitatory and ipsilateral inhibitory effects were observed. 6. Further nervous information is contained in the patterns of excitation which are plotted as impulse-time-diagrams (method in Fig. 2). In these, transitory and tonic kinds of behaviour, sometimes in the same element, as well as postexcitatory inhibition and postinhibitory excitation become visible (Figs. 14–16). 7. The finding of a well developed sensitivity to ultrasound is complemented by the discovery of mandibel snapping in L. migratoria, which is composed of frequencies of approximately 10–30 kcps (Figs. 18a, b) with a duration of sometimes less than 0,5 ms. Thus, the ability to perceive ultrasound has found a biologically meaningful explanation. 8. Under stimulus conditions which correspond to complicated natural sound (variation of intensity, pitch and its superpositions, duration, sequences and direction of stimulus, Figs. 19–27) the acoustic neurons have an enhanced sensitivity to some conditions i.e. they filter out: lower or higher pitch (Figs. 8, 9), optimal stimulus sequences of 40/s or 15/s (Figs. 23, 25), duration of 0,5–5 ms or 20 ms (Fig. 22 neurons 309/2, 312/5). Different stimulus parameters can be filtered out by the same neuron. The above filtration seems to be an active process, caused by neuron-interaction. 9. The results obtained by using artificial sound were confirmed by using natural sound signals (Fig. 28): for instance, a neuron prefers brief pips to long sound pulses and correspondingly mandible snapping to stridulation. Superposition of lower and higher pitch — as in species-specific stridulation — causes strong interaction-effects: for example facilitation or, mostly, inhibition. The stimulus conditions, under which the nervous filters react, correspond often and for different parameters (see paragraph 8) to those of species-specific sound. 10. The high efficiency of the auditory path of insects, which is composed of comparably few elements, can be explained by the following: a, one element has the ability to transmit “compatibly” heterogeneous information; b, information overlaps, i.e. it is transmitted independently by different elements; c, it can be separated again by “recombination” (Figs. 29, 30). This principle explains the complicated types of the “Wirkfelder”.