The responses of ventral cord neurons of Decticus verrucivorus (L) to sound and vibration stimuli

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Behavioural Processes, 5 (1980) 55-74 o Elsevier Scientific Publishing Company, Amsterdam - Printed in Belgium.

THE RESPONSES OF VENTRAL CORD NEURONS OF DECTICUS VERR UCIVOR US (L) TO SOUND AND VIBRATION STIMULI

ROLAND KUHNE, BRIAN LEWIS* and KLAUS KALMRING Fachbereich

Biologie,

Philipps-Universitiit,

D-3550

Marburg

*Permanent address: Department of Biological Sciences, Old Castle Street, London El 7NT (Great Britain)

(Federal

Republic

City of London

of Germany)

Polytechnic,

(Accepted 9 August 1979)

ABSTRACT Kiihne, R., Lewis, B. and Kalmring, K., 1980. The responses of ventral cord neurons of Decticus verrucivorus (L) to sound and vibration stimuli. Behav. Processes, 5: 55-74. The real time analysis of the song of D. verrucivorus recorded in the sun and shade shows that changes occur predominantly in the time parameters and not in the frequency content. Single unit recordings in the ventral nerve cord of D. verrucivorus show that all the acoustic units respond to both sound and vibration. However, on the basis of their response characteristics they may be classified as vibration (V), vibration and sound (VS) and sound (S) neurons. The responses of some of the units depend upon the degree of habituation. Single parameter processing was not observed; the characteristic frequencies of these units range across the whole of the frequency band investigated, and distinct intensity response fields were observed. Some of the V neurons were more sensitive than the receptors, and some units responded well to the species song when both sound and vibration were presented simultaneously. The source of the vibratory input is shown to be predominantly from the ipsilateral foreleg. Many of the units run together in an ‘acoustic bundle’; some run through fibres, passing from the posterior thoracic ganglia to the cervical connectives. In many cases the primary fibres projecting to these central units can be predicted from their response characteristics. An hypothesis of the mechanisms underlying conspecific song recognition at the ventral cord level is presented.

INTRODUCTION

The responses of the central acoustic neurons of Orthoptera to artificial sounds and conspecific songs have been investigated in some detail (e.g. Rheinlaender and Kalmring, 1973; Kalmring, 1975a, b; Rheinlaender, 1975; Kalmring et al., 1978a, b), and much information is now available concerning the central coding of frequency, intensity, direction, duration and repetition

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rate. Similar studies have been made of the central responses to vibration stimuli (Dambach, 1972; Cokl et al., 1977). Complex excitatory and/or inhibitory interactions have been demonstrated at the central level between airborne sound and vibration stimuli and between the vibratory inputs from the three pairs of legs in Locusta. Such information is not yet available for Decticus or other tettigoniids. Furthermore, the earlier detailed studies of central acoustic information processing ma.y have suffered because a definitive analysis of the response characteristics of the primary sensory units had not been made. The work of Rheinlaender (1975) provided some information, but only a few receptor units were recorded and the central units were only tested to airborne sound. The nature of the synaptic recoding processes occurring within the appropriate ganglion is therefore still a matter of speculation. For example, central units which respond to both sound and vibration have been assumed to receive their inputs independently from the ear and subgenual organs. This is a perfectly reasonable assumption in Locusta where the two receptor organs are at different bodily positions, but it may not be entirely true for the ensifera. The primary units have been studied in some detail in D. uerrucivorus (Kalmring et al., 1978c) and their response characteristics are reasonably well established. Apart from the campaniform.units, the primary fibres of the complex receptor of the foreleg have been divided into pure vibration, vibration and sound, and pure sound groups. It is therefore possible that central units responding to vibration and sound receive their inputs exclusively from the second group. If this is the case, their response characteristics should be similar to those of the primary units. This investigation extends the studies performed in Locusta; it outlines the characteristics of some central neurons and shows examples of the complex interactions between the vibration and sound systems. Where possible, the responses of these central neurons are compared to those of the primary units. Finally, we attempt a first hypothesis of the mechanisms underlying conspecific song recognition at the ventral cord level. MATERIALS

AND METHODS

59 adults of Decticus verrucivorus were used for this study. Details of the preparation have been described (Kalmring et al., 1978c). The present preparation differed only in that both the prothoracic and mesothoracic legs were waxed to the T-piece of the minivibrator. Recordings were made using glass micropipettes of approximately 30 MOhm resistance when filled with 3M KCl, in the cervical connectives just anterior to the prothoracic ganglion. In some cases recordings were also obtained from the pro-mesothoracic and meso-metathoracic connectives. To investigate the source of the vibratory inputs to the recorded units, the contralateral and/or the mesothoracic legs were sometimes cut, as were the cervical and promesothoracic connectives. 183 units were well characterised, with up to 11 neurons being tested in one animal. The transections performed during the experiments demonstrate that these units were

57

ventral cord units. These experiments, together with cobalt filling, indicate whether the recorded units passed through the prothoracic ganglion or not. The classification of the units given in the Results section is therefore considered real and not due to interindividual variation. The method of stimulus presentation has also been described (Cokl et al., 1977; Kalmring et al., 1978c). The vibration stimulus shown in the figures is the real acceleration delivered to the preparation and was measured during the experiments by a B u. K Accelerometer (No. 4369). Except where specifically stated, the vibration stimulus duration was always 100 ms and the sound stimulus was 20 ms; the repetition rate of each was always 2 s-‘. Simulated natural song (SNS) was produced by amplitude modulating white noise, and was also used as a stimulus. The method of synthesis is described by Kalmring et al. (1978b). The syllable rate was that of the natural song (ca. 140 s-‘) but the chirp rate was only 2 s-‘. The natural song, recorded in the biotope, gave a chirp repetition rate of about 8 s-’ in the sun, and about 6 s-’ in the shade. A comparison of the responses to a chirp of the natural song and SNS showed no measurable differences. PST histograms were produced as described by Kalmring et al. (1978a). Fifteen successive responses were analysed over a period of 500 ms with 1 ms binwidths. Induced

vibration and sound problem

In studying the responses of central neurons, several problems exist for characterising bifunctionality. Fir&vibrations may be induced in the leg as a result of high intensity airborne sound (c.f. Seymour et al., 1978). Second, vibration may produce some secondary airborne sound. The intensity level of each secondary stimulus was measured and, in every critical case, was compared with the threshold stimulus value for the investigated neuron. Up to 1 kHz, the induced vibration produced at high sound intensities (80-90 dB) was close to the threshold value in response to the vibratory stimulus. But, when the response patterns of the unit are compared for any given value of sound and vibration, the responses are seen to be clearly different. Contact vibration produces a response which is effectively 20 dB above that produced by induced vibration. At frequencies above 1 kHz the induced vibration falls increasingly below threshold intensity very rapidly; at 2 kHz it is lo-20 dB below threshold and is 40 dB at 3 kHz. The sound output of the vibrator is in all instances far below the airborne sound threshold of the neurons. The given examples of bifunctionality are therefore real. RESULTS

1. The natural song The natural song of D. verrucivorus was recorded in the biotope in Yugoslavia on a Nagra tape-recorder, in both sun and shade. Real time com-

58

puter analysis of these songs showed no change in the frequency content. Changes did occur in the time parameters of the song when 50 chirps of the ‘constant singing period’ were compared. In the sun (temp. 30°C) chirps were produced at a rate of 8 s-’ to 9 s-’ (chirp period 111 ms; SD. 4 ms) with a duration of 53 ms (SD. 1.5 ms). In the shade (temp. 25°C) chirp rate fell to 6 s-’ (chirp period 170 ms; S.D. 35 ms) and there was an increase in chirp duration (60 ms; SD. 0.7 ms). Differences in chirp duration were due to differences in both syllable length and syllable rate. The responses of the ventral cord neurons to both these songs were investigated. In all parameters, except for the effects of response habituation (see Discussion), they did not differ significantly from each other or from the responses obtained to simulated natural song. 2. Unit responses The primary units of D. verrucivorus have been classified, on the basis of their response characteristics, as pure vibration, mixed sound and vibration, pure sound, and fibres originating from campaniform sensilla (Kalmring et al., 1978c). In other words, only one group showed bifunctionality. The central auditory neurons, on the other hand, are all influenced in an excitatory and/ or an inhibitory fashion by both sound and vibration. However, the results do fall into three clear classes: units with an equal sensitivity to both vibration and sound (VS neurons); units which respond predominantly to vibration (V neurons); and units which respond predominantly to sound (S neurons). Within these three broad groups, units were classified on the basis of threshold curves, characteristic frequencies, response patterns (i.e. tonic or phasic), latency and the excitatory/inhibitory interactions between the ‘preferred’ and ‘non-preferred’ stimuli.

VS neurons Kalmring et al. (1979) have described a large ventral cord auditory neuron whose axon passes through the prothoracic ganglion. Figs 1 and 2 show the characteristics of another through neuron in the neighbourhood of the first, and of similar morphology except for a lateral branch to the motor neuropile in the prothoracic ganglion. Both neurons have the same response fields to airborne sound (10 kHz to at least 40 kHz), producing phasic responses to tone bursts of 20 ms duration. But this neuron (VS 1) also responds to vibration at frequencies up to 100 Hz, and is phase-locked to the sine wave stimulus at high intensities. No responses occurred to low frequency airborne sound, or vibration in the preferred response range of the subgenual organ receptors. Direct excitatory input from the mixed sound and vibrator primary units is therefore unlikely; this unit probably receives its input from the mid- and/or high frequency sound receptors and from some campaniform sensilla. However, subthreshold excitation by subgenual activity is indicated in Fig.2. When white noise was pre-

59

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Fig.1. The response characteristics of a VSl neuron. Left, the threshold curve for vibration stimuli and an original response (inset) to a vibration stimulus of 40 Hz, 8.5 m s-* acceleration. Stimulus duration 100 ms. Right, the threshold curve for airborne sound stimuli and original responses to WN stimuli of different intensities. Stimulus duration 20 ms, repetition rate 2 s-l.

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Fig.2. The additional influences of vibration stimuli of 100 Hz and 1000 Hz on the VSl neuron stimulated by WN are shown by PST histograms and original responses. Simultaneous stimulation with airborne sound (WN, 68 dB, 20 ms) and vibration (100 Hz, 3.4 m smz or 1000 Hz, 2.3 m se*; 100 ms duration). From left to right, the airborne sound stimulus first leads by 20 ms, and is then successively delayed by 20 ms steps. Top: each box shows the PST histogram of the responses to 15 identical stimuli, the curve of the cumulative frequency distribution and the total number of impulses within the 15 responses. Bin width, 1 ms. Bottom: original responses (films).

60

sented after the onset of vibration, the responses of the unit increased significantly. However, this excitatory effect is slow acting, the response reaching a maximum when the sound stimulus is presented some 20 ms to 40 ms after the onset of vibration. No changes in latency of response occurred during this mtenslty

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61

excitation. The main input to this unit is from the ipsilateral foreleg; some degree of contralateral inhibition was demonstrated by cutting the contralateral tympanal nerve when the ipsilateral response increased. VSl was recorded anterior and posterior to the prothoracic ganglion. Three other VS neurons (VS2, VS3, VS4) were recorded whose threshold curves to airborne sound were very similar (Fig.3). The preferred frequencies occurred in the range 10-20 kHz. These units also respond tovibration (Fig.4) displacement

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62

and show similar threshold curves whose frequency ranges cover the bandwidth of the mixed sound and vibration primary units and, perhaps, the pure vibration units. On the basis of other characteristics, however, these three units are seen to be clearly different. Fig.3 shows that VS2 produces only a phasic response to 100 ms white noise stimulation. VS3, on the other hand, produces a tonic response at all intensities of white noise, but there is some indication that the preferred intensity is around 68 dB. This unit can be compared to the Ctype neuron of Locusta (Kalmring, 1975a). VS4 is a ‘K-type’ neuron; tonic responses are produced for stimuli of 20 ms duration; stimuli of durations greater than 20 ms produce no increase in response duration. The three units also differ in their response patterns to vibration (Figs 3, 4). VS2 responds tonically to high stimulus intensities over the range from below 50 Hz up to 2000 Hz; lower intensities produce only an ‘on’ burst. VS3 responds with strong tonic responses over the whole of the suprathreshold region for frequencies from 500 Hz to 2000 Hz; low frequency vibration stimuli evoke only weak responses. VS4 gives only an ‘on’ burst to stimuli greater than 20 ms duration. There seems little doubt that these units receive inputs from the midrange sound, mixed sound and vibration primary units and perhaps additional inputs from the pure vibrators. Indeed, in response to vibration, the central units have a sensitivity equivalent to that of the receptors. What is also clear from these results is that the weighting given to each of the primary sources of input varies from unit to unit. Considerable reorganisation of sensory information must therefore occur within the prothoracic ganglion. Some of the interactions of sound and vibration stimuli for VS2, VS3, VS4 are shown in Fig.5. VS2 produces only weak responses to SNS (inset); in response to 100 Hz vibration alone, low intensity phasic responses become tonic at high intensities. Simultaneous stimulation with SNS and 100 Hz vibration produces an enhanced response which is synchronised with the syllables of the conspecific song. This effect is the result of an inhibition of the tonic vibratory response by the sound stimulus. This is further demonstrated when white noise and 100 Hz are presented simultaneously (c.f. columns 1,2, VS2, Fig.5). VS3 produces a tonic response to SNS and may be considered a ‘chirp coder’. Summation of response occurs at high intensities of SNS plus 100 Hz vibration. No specific interactions were observed. In VS4, the 100 Hz ‘on’ activity dominates when 100 Hz plus SNS are presented simultaneously. VS2, VS4 are known to be through fibres; VS3 was recorded only anterior to the prothoracic ganglion. Other neurons in the VS group were also recorded. In one case, when white noise was presented alone, intensities of 70 dB were required to produce a single spike response. No activity was recorded to either 100 Hz or 1000 Hz at any intensity. But, when white noise and either 100 Hz or 1000 Hz were presented simultaneously, the single spike response occurred at much lower white noise intensities; i.e. the response threshold was lowered, suggesting subthreshold excitation by vibrator primary units. Another neuron responded

63

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Fig.5. Response characteristics of the VS2, VS3 and VS4 neuron shown in Figs.3, 4, when these are stimulated simultaneously with vibration and airborne sound. VS2 neuron. Left column: responses to vibration stimuli (100 Hz, 100 ms) of different intensities. Middle column: responses to combinations of the same vibration stimuli and WN, 68 dB, 20 ms. Right column: responses to the same vibration stimuli and a simulated chirp of the conspecific song (SNS = simulated natural song), 68 dB. Bottom right corner: responses to SNS alone. VS3 neuron: Responses to stimulation with vibration (100 Hz, 3.4 m-l) only, WN burst (68 dB, 20 ms) only and SNS (68 dB) only, and to combinations of these vibration and airborne sound stimuli. VS4 neuron. Left column: responses to vibration stimuli (100 Hz, 100 ms) of different intensities. Right column: responses to combinations of the same vibration stimuli with SNS (68 dB). Bottom right corner: responses to SNS only.

with phase-locked activity to high intensity and low frequency vibration; white noise stimulation produced one spike from medium to high intensities. When both stimuli were presented simultaneously, the white noise inhibited the phaselocked activity. These units were recorded in the pro-mesothoracic connectives and are through fibres. V neurons Fig.6 shows the threshold curves and the response characteristics of two neurons preferentially influenced by vibration. V2 is a broadband vibrator with high sensitivity; it is also sensitive to a narrow band (l-2 kHz) of airborne sound. There is an unspecific reaction to SNS. All inputs to V2 arise in the ipsilateral foreleg and its response characteristics suggest that these may be the mixed sound and vibration units of the tympanal nerve.

64

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Vl is a vibrator neuron responding only at low frequencies up to 300 Hz. Vibratory receptor cells reacting exclusively in this frequency range are not known, except for the campaniform sensilla. But the latter ones are by far more insensitive. The sensitivity of the Vl neuron is greater than has been described for any receptors. VI 100 Hz

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Fig.6. Top: threshold curves and response magnitudes of a Vl and V2 neuron recorded in the same animal. For each frequency/intensity combination the number of impulses is represented by the height of the bars. Black dots and bars, Vl neuron; white dots and bars, V2 neuron. Bottom: original responses of these neurons to vibration stimuli of different frequencies and intensities (100 ms, 2 se’ ).

66

Therefore, the response characteristics of this neuron must be due to complex interactions with convergence of different vibratory receptors. By comparing the responses of the Vl and of the V2 neuron, a rough frequency discrimination in the CNS of the animal is possible. The preferred intensity of the responses of the Vl neuron occurs near threshold, when it responds tonically with a minimum latency (24 ms). An increase in intensity produces a decrease in response and the latency increases to 42 ms. Fig.7 shows a computer analysis of the responses of this unit to 100 Hz vibration. At low intensities there is a strong after discharge which is inhibited at higher intensities, producing a silent period; at the highest intensity (10.1 m s-l) ‘rebound activity’ occurs. Stimulation with white noise produces an unspecific reaction at low intensities. The long and variable latency of this unit indicates complex multisynaptic processing at the level of the prothoracic ganglion, so that predicting inputs over and above the vibrator primaries is not possible. v3 IOOHz

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67

Both Vl and V2 run in an ‘acoustic bundle’, but recordings were not made posterior to the prothoracic ganglion. Unit V3 (Fig.8) has the same response field and threshold curve as V2 (Fig.G), but the suprathreshold response characteristics are quite different and of the ‘K-type’ neuron. V3 responds unspecifically to airborne sound and with only a few spikes at onset of 100 Hz or 1000 Hz vibration. However, a response enhancement is produced when sound and vibration are presented together. This is shown in the cumulative histograms in Fig.& Furthermore, simultaneous presentation of SNS and 100 Hz produces a response which clearly follows the amplitude modulation pattern within the chirp, i.e. a syllable coder. This phenomenon is less clear with SNS plus 1000 Hz vibration. Thus the response pattern of this neuron depends upon whether it receives a sound and campaniform input or a sound and pure vibratory input; all inputs, however, appear to be excitatory. It is a through neuron which was recorded anterior and posterior to the prothoracic ganglion. displacement

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A number of other V group neurons appear to receive their inputs exclusively from campaniform sensilla. One example (V4) is given in Fig.9. A train of spikes is produced for vibration frequencies from 30 Hz to 100 Hz; at higher frequencies (e.g. 200 Hz) a single spike is produced at stimulus onset. No response was obtained to airborne sound at any intensity. This particular unit is a through fibre occurring in the acoustic bundle. S neurons

A large neuron (Sl) with a preferred response in the range 10 kHz to 40 kHz has already been described (Kalmring et al., 1979). Its response to airborne sound is similar to VSl (Figs 1, 2), but it does not produce phase-locked activity in response to campaniform input. Furthermore, it is inhibited by low frequency sound and by vibration. This inhibition may be derived from the primary mixed sound and vibration units or from both pure vibration and low frequency sound units. In the habituated state Sl produces phasic responses throughout the frequency range, but after a longer period without stimulation tonic responses occur which again habituate with repeated stimulation. The morphology of the unit is also well known and the neuron runs in the abdominal cord; it is the largest unit in the acoustic bundle. Other phasically-responding units occur in the bundle which do receive only weak (unspecific) excitatory or inhibitory inputs from primary vibratory units. Three units responding to airborne sound are shown in Fig.10. All three occur in the acoustic bundle, but recordings were obtained only from the cervical connectives. The unit S3 has a very long latency (30 ms) and its response pattern is peculiar and very variable. It responds to white noise at high intensities and sometimes also at low intensities. Tonic responses are produced to 20 ms pulses at these intensities; 100 ms low intensity white noise also produces a tonic response, but at high intensities only a short burst of spikes comparable to those produced to 20 ms white noise are seen. Few or no spikes are produced at intermediate intensities. The characteristics suggest complex information processing in the prothoracic ganglion. The activity of primary auditory fibres with specific characteristic frequencies and thresholds may be necessary to produce a response in this particular unit. Direct projection is unlikely, however, in view of the 30 ms latency. S4 (Fig.10) is a C-type neuron. It produces a tonic response in the whole of its range and has a characteristic frequency of 20 kHz. Since it also responds weak!y to vibratory stimuli, it may receive its main inputs from the mid-range sound neurons of the ipsilateral ear. S5 is a K-type neuron producing tonic responses to short (20 ms) white noise or pure tone stimuli. To 100 ms stimuli it gives an ‘on’ burst or is weakly tonic (e.g. 58 dB). It has a characteristic frequency of 10 kHz and so may receive its inputs from broadband or midrange primaries.

69

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Fig. 10. A comparison of the response characteristics of three different S neurons. Top: threshold curves of S4 and S5 neurons for airborne sound stimuli. Bottom: original responses of these neurons and a S3 neuron to airborne sound stimuli (WN, different intensities) of 20 ms duration (left column in each box) or 100 ms duration (right column in each box ).

DISCUSSION

The responses of central neurons Less than 50% of the primary neurons recorded from the tympanal nerve of D. uerruciuorus were shown to be bifunctional; that is, responding to both airborne sound and vibration (Kalmring et al., 1978c). In contrast, the present study has shown that all the central acoustic neurons are influenced to some extent by vibration and sound and the effect is either excitatory or inhibitory. Such excitatory effects can be seen in neuron VS3 (Fig.5); summation of response occurs at high intensities of simulated natural song (SNS) and 100 Hz vibration. Inhibition is shown in VSZ; simultaneous stimulation with SNS and 100 Hz results in a response synchronised with the syllables of the song. The inhibition of the tonic vibratory response by the sound is also shown when 100 Hz and white noise are presented simultaneously. Such effects may occur at the subthreshold level and may not be immediately apparent. For example, VSl (Fig.1) does not respond to low frequency sound (< 7 kHz), nor to vibration above 200 Hz. But subthreshold excitation is shown when the white noise stimulus is delayed relative to the onset of vibration. Under these conditions the responses of VS 1 may be increased significantly. Inhibition also occurs in this unit; cutting the contralateral leg nerve produces a slight increase in the response. In the vibrator unit Vl, after inhibition occurs at some intensities (Fig.7). The source of the 100 Hz and 1000 Hz vibration in the biotope is not yei known, and the function of such vibration in the acoustic behaviour of the inject is also problematical. All that can be said at present is that such inputs can often profoundly affect the responses of central acoustic neurons. V3 (Fig.8) responds with ‘on’ activity to 100 Hz vibration and unspecifically with perhaps one spike to SNS. Simultaneous presentation of both stimuli results in precise coding of the syllable pattern of the song. This is less true with SNS plus 1000 Hz vibration. The conclusion must be that this unit acts as a syllable coder only in the presence of 100 Hz environmental vibration. At other vibration frequencies it may act as a chirp coder and supplement the chirp information carried by VS3 (Fig. 5). We may speculate that one possible source of the 100 Hz or so vibration may be the syllable repetition rate of the species song (ca. 140 s-l). Each closure of the tegmina may induce vibration in the body of the singing insect which is then transmitted to the undergrowth. Such vibration may be detected by another insect at close quarters. ‘Drumming’ of the substrate is known in Ephippiger spp. and may be a common close quarters form of communication. We have no suggestion as to the possible source of 1000 Hz. Apart from its bifunctionality, no neuron was recorded which responded to only a single stimulus parameter. Frequency and intensity parameters of vibration and sound are also coded by single units. These may have different characteristics frequencies (e.g. Vl, V2, Fig.6; S4, S5, Fig.lO), but the intensity response curves of any one unit may not be linear and its sensitivity may be

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equal to or greater than that of the receptor units. This is the case for Vl (Fig.6), whose preferred intensity occurs near its threshold; an increase in intensity increases the latency and decreases the response. S3 (Fig.10) has a low and a high preferred intensity region and its response pattern depends also on the stimulus duration. Such ‘preferred response areas have already been described (e.g. Rheinlaender, 1975, for D. uerruciuorus; Kalmring, 1975, for L. rnigratoriu), but their role in acoustic behaviour is not yet known. In contrast, a number of units may have similar characteristic frequencies, but their responses may be quite different in other respects (e.g. VS2, VS3, VS4, Figs 3, 4 and 5). The responses of ventral cord units may depend on the the stimulus duration (S3, S4, S5, Fig.10); tonic responses for 20 ms stimuli may become an ‘on’ burst with longer stimulus durations. Alternatively, a neuron may give tonic responses at all durations. Third, longer durations may produce an inhibition of tonic activity (S5). In such ways, information about onset, duration and repetition rate may be coded by different neurons tuned to similar characteristic frequencies. In this connection the point must be made that most of the responses described here are produced by the neuron when in an habituated state. After a period without stimulation the response to the first stimulus, even of phasic neurons (e.g. Sl, S3), may be tonic. It is therefore possible that a normally tonically-responding unit may become phasic at high stimulus repetition rates. Such factors may underlie the fast, syllable coding features of some neurons. The extent to which the neurons are habituated in the biotope is still an open question and one which can be answered only by replicating the environmental noise level in the laboratory during unit recording.

Central reprocessing Studies of the response characteristics of the primary auditory fibres of D. uerrucivorus (Rheinlaender, 1975; Kalmring et al., 1978c; Zhantiev and Korsunovskaya, 1978) have shown that much complex information is propagated from the receptor to the CNS, and Kalmring et al. (1978c) have grouped the neurons into campaniform units, units responding to vibration, to low-, mid- and high-frequency sound and bifunctional units responding to both vibration and sound. Such a complex peripheral mechanism for coding acoustic and vibratory information is unlikely to have evolved if this information is subsequently lost at successive central stations. Our present results indicate that this information is not lost at the prothoracic ganglion level. A considerable amount of reprocessing occurs here. In some cases, as for S3 with a 30 ms latency and Vl whose latency may vary between 24 ms and 42 ms, it is almost impossible to speculate what primary units project onto these neurons. In other cases, e.g. S4, V4, such speculation seems well founded. Certainly, recording involves a recombination of the parameters of the incident stimulus or indeed a re-emphasis of some of these parameters. But the re-emphasis is always in a positive direction; there is no evidence that any information is de-

graded (c.f. Rheinlaender, 1975). Directionality was not investigated, but the point has already been made (Kalmring et al., 1978c) that directional information is largely inseparable from frequency information. Our results also show that the main vibratory input to the central acoustic fibres always appears to be from the ipsilateral fore leg; cutting the inputs from the other legs has little or no detectable effects on the unit’s responses. This is in contrast to the situation in the locust where the sensory inputs of all vibratory receptors are integrated in the responses of ventral cord neurons (Cokl et al., 1977). The influence of the vibratory receptors of the other legs in D. uerruciuorus remains to be determined. An hypothesis

for conspecific

song recognition

On the basis of the information now available it seems timely to attempt an hypothesis of the neural basis of conspecific song recognition and phonotactic behaviour. Further work will undoubtedly modify any such attempt, but it is necessary at some stage to move beyond the mere cataloguing of unit response patterns. It is probable that, in the absence of specific acoustic stimulation, the VS and S group neurons (at least) are in a non-habituated state. Under such conditions the normally ‘phasic’ neurons (e.g. VSl, VS2, Sl) may respond tonically to the first two or three stimuli. If these stimuli are the chirps of the conspecific song, the units may provide information about the onset of the song, the chirp duration, chirp interval and (as a result) the chirp period. After habituation, such units may continue to fire phasically at the onset of each chirp, providing information on the chirp period and repetition rate. Such units may be instrumental in creating an ‘arousal’ state, in directing attention selectively to the stimulus, and perhaps in the fast recognition of the conspecific song. Non-repetitive stimuli (e.g. undergrowth sounds due to predators) may also be mediated preferentially by these units in which case they may have an ‘alarm’ function. Continuous information about the species song may be mediated by the syllable coders (e.g. V3 and perhaps VS2) and by the chirp coders (e.g. VS3). The characteristics of V3 are of considerable interest in this connection. SNS plus 1000 Hz vibration results in a train of spikes which last for the duration of the sound stimulus, i.e. the unit is a chirp coder. On the other hand, SNS plus 100 Hz vibration results in a response which accurately codes the syllable pattern of the stimulus. At a distance from the singing male this unit may code for the chirps, but its response pattern may become more specific as it approaches the source. If the 100 Hz vibration in the biotope is due to the approx. 140 syllables per second of the species song, and if this vibration pattern is transmitted to the plant stem, syllable recognition will occur when the approaching female stands on the same plant stem as the singing male. Not only will this confirm the species but it may also indicate the close proximity of the male. Species recognition in this sense is the pattern of activity in a number of specific neurons. The distance of the sound source may be correlated with sound intensity;

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sound intensity information may be coded by neurons with preferred intensity ranges and different thresholds (e.g. VS2, VS3, VS4, S3, S4, S5). At the simplest level, confusion as to origin of intensity changes may be avoided if distancedependent intensity changes are carried by non-directional units and directiondependent intensity information by other, directional units. Both unit types should be looked for in the ventral nerve cord, but in fact the situation is likely to be more complex than this. This multichannel method of analysis of conspecific song pattern, intensity and direction can at best be only a first approximation of the real mechanism. Much more work is necessary, but this hypothesis may at least serve as a basis for future work. ACKNOWLEDGEMENT

We are grateful to Professor Dr. H.-U. Schnikler, Marburg, for the use of his real time frequency analyser. Supported by the Deutsche Forschungsgemeinschaft; part of the program of Sonderforschungsbereich 114 (Bionach) Bochum and part of the program ‘Neurale Mechanismen des Verhaltens’ (Ka 498/l).

REFERENCES Cokl, A., Kalmring, K. and Wittig, H., 1977. The responses of auditory ventral-cord neurons of Locusta migratoria to vibration stimuli. J. Comp. Physiol., 120: 161-172. Dambach, M., 1972. Der Vibrationssinn der Grillen. II. Antworten von Neuronen im Bauchmark. J. Comp. Physiol., 79: 305-324. Kalmring, K., 1975a. The afferent auditory pathway in the ventral cord of Locusta migmtoria (Acrididae). I. Synaptic connectivity and information processing among the auditory neurons of the ventral cord. J. Comp. Physiol., 104: 103-141. Kalmring, K., 1975b. The afferent auditory pathway in the ventral cord of Locusta migratoria (Acrididae). II. Responses of the auditory ventral cord neurons to natural sounds. J. Comp. Physiol., 104: 143-159. Kalmring, K., Kiihne, R. and Moysich, F., 1978a. The auditory pathway in the ventral cord of the migratory locust (Locusta migratoriu): Response transmission in the axons. J. Comp. Physiol., 126: 25-33. Kalmring, K., Kiihne, R. and Moysich, F., 1978b. The coding of sound signals in the ventralcord auditory system of the migratory locust, Locustu migratoriu. J. Comp. Physiol., 128: 213-226. Kalmring, K., Lewis, B. and Eichendorf, A., 1978c. The physiological characteristics of the primary sensory neurons of the complex tibia1 organ of Decticus uerruciuorus L. (Orthoptera, Tettigonioidae). J. Comp. Physiol., 127: 109-121. Kalmring, K., Rehbein, H. and Kiihne, R., 1979. An auditory giant neuron in the ventral cord of Decticus verrucivorus (Tettigoniidae). J. Comp. Physiol., 132: 225-234. Rheinlaender, J. and Kalmring, K., 1973. Die afferente Horbahn im Bereich des Zentralnervensystems von Decticus uerruciuorus (Tettigoniidae). J. Comp. Physiol., 85: 361-410. Rheinlaender, J., 1975. Transmission of acoustic information at three neuronal levels in the auditory system of Decticus uerruciuorus (Tettigoniidae, Orthoptera). J. Comp. Physiol., 97: l-53.

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Seymour, C., Lewis, B., Larsen, O.N. and Michelsen, A., 1978. Biophysics of the ensiferan ear. II. The steady-state gain of the hearing trumpet in bushcrickets. J. Comp. Physiol., 123: 205-216. Zhantiev, R.D. and Korsunovskaya, O.S., 1978. Morphofunctional organisation of tympanal organs in Tettgoniu cantans (Orthoptera, Tettigoniidae). Zool. Zh., 57: 1012-1016 (in Russian, with English summary).