‘Switching-off’ of an auditory interneuron during stridulation in the acridid grasshopperChorthippus biguttulus L.

Summary In freely moving grasshoppers of the speciesChorthippus biguttulus compound potentials were recorded from the neck connectives with chronically implanted hook electrodes. The spikes of one large auditory interneuron, known as the G-neuron (Kalmring 1975a, b) were clearly distinguishable in the recordings and the neuron was identified by its physiology and morphology. In quiescent grasshoppers the G-neuron responds to auditory and vibratory stimuli, but responses to both stimuli are suppressed during stridulation in males (Fig. 1, top, Fig. 7). When a male's wings were removed so that the stridulatory movements of its hindlegs produced no sound, the suppression of the G-neuron response still occurred (Fig. 1, bottom). When proprioceptive feedback from the hindlegs was reduced, by forced autotomy of the legs, the switching-off remained incomplete (Fig. 3) (production of stridulatory patterns was inferred on the basis of electromyograms from the relevant thoracic musculature). Imposed movement of the hindlegs, on the other hand, suppressed the G-neuron response in a graded fashion, depending on the frequency of the movement (Figs. 4 and 5). These experiments suggest that the switching-off is brought about by a combination of proprioceptive feedback and central efferences. The switching-off phenomenon could either protect the grasshopper's auditory pathway from undesired effects of overloading by its own intense song (e.g. self-induced habituation as described by Krasne and Wine 1977) and should therefore apply for most auditory neurons. Alternatively it could prevent escape reflexes from being triggered by stridulatory self-stimulation and consequently might apply only for neurons involved in such networks (as the G-neuron might be).     

Filtering of behaviourally relevant temporal parameters of a grasshopper's song by an auditory interneuron

Summary In females of the acridid grasshopperChorthippus biguttulus, thoracic auditory interneurons were investigated with respect to their selectivity for temporal parameters of the conspecific song. Special attention was given to the detection of small gaps in the syllables of the song, since behavioural experiments have shown that the presence or absence of gaps is critical for the female's Innate Releasing Mechanism (cf. Fig. 1).The spiking response of one ascending interneuron, the AN4, shows filtering properties which closely resemble the behavioural reactions (cf. Figs. 1, 3 and 5b). The difference in the AN4's reaction to stimuli with gaps and uninterrupted stimuli is maintained over the behaviourally relevant intensity range (Fig. 4). This reaction is reliable enough that the stimulus type could be inferred by higher centres even from single stimulus presentations. Hence, this neuron is likely to participate in the task of gap detection and probably is a part of the neuronal filter network which determines the characteristics of the Innate Releasing Mechanism of this species. However, this interneuron is not species-specific: A homologue exists in other acridids as well and, inLocusta migratoria, has similar response characteristics (Fig. 6). The inferences of this observation for the evolution of an Innate Releasing Mechanism are discussed.Abbreviations CNS central nervous system - PST-histogram post-stimulus-time-histogram - SPL sound pressure level - IRM Innate Releasing Mechanism     

The function of auditory neurons in cricket phonotaxis

Summary The activity of auditory receptor cells and prothoracic auditory neurons of the cricket,Gryllus bimaculatus, was recorded intracellularly while the animal walked on a sphere or while passive movement was imposed on a foreleg.During walking the responses to simulated calling song is altered since (i) the auditory sensory cells and interneurons discharged impulses in the absence of sound stimuli (Figs. 1, 3) and (ii) the number of action potentials in response to sound is reduced in interneurons (Figs. 2, 3).These two effects occurred in different phases of the leg movement during walking and therefore masked, suppressed or did not affect the responses to auditory stimuli (Figs. 3, 4). Hence there is a time window within which the calling song can be detected during walking (Fig. 5).The extra excitation of receptors and interneurons is probably produced by vibration of the tympanum because (i) the excitation occurred at the same time as the leg placement (Fig. 4), (ii) during walking on only middle and hindlegs, no extra action potentials were observed (Fig. 6), (iii) in certain phases of passive movements receptor cells and interneurons were excited as long as the ipsilateral ear was not blocked (Figs. 8, 9).Suppression of auditory responses seems to be peripheral as well as central in origin because (i) it occurred at particular phases during active and passive leg movements in receptor cells and interneurons (Figs. 1, 4, 9), (ii) it disappeared if the ear was blocked during passive leg movements (Fig. 9) and (iii) it persisted if the animal walked only on the middle and hind legs (Fig. 6).     

Changes in the auditory system of locusts (Locusta migratoria and Schistocerca gregaria) after deafferentation

Deafferentation experiments during postembryonic development show morphological and/or physiological changes of receptor fibers and of identified auditory interneurons in the CNS of the locusts Locusta migratoria and Schistocerca gregaria after unilateral ablation of one tympanic organ either in the larva or the adult animal.

Auditory interneurons in locusts produce directional head and abdomen movements

Locusts (Locusta migratoria) were stimulated with pulses of pure tones of frequencies between 5 kHz and 25 kHz. Interneurons responding to these stimuli (auditory interneurons) were recorded intracellularly and identified by dye injection. Their output functions were investigated by injection of depolarizing current during simultaneous registration of components of flight steering behavior of the animals, i.e. movements of the head and the abdomen and flight activity. Three different types of effects were found, corresponding to 3 functional classes of interneurons:

Physiology and tonotopic organization of auditory receptors in the cricketGryllus bimaculatus DeGeer

Summary Physiological recordings were obtained from identified receptors in the tympanal organ ofGryllus bimaculatus. By immersing the prothoracic leg in Ringer solution and removing the anterior tympanic membrane the auditory receptors were exposed without significantly altering the frequency response of the auditory organ (Fig. 1). Each receptor was tuned to a specific sound frequency. For sound frequencies below this characteristic frequency the roll-off in sensitivity decreased from 20–30 dB/octave to 10–15 dB/octave as the characteristic frequency of receptors increased from 3–11 kHz (Fig. 4A). For each individual receptor the slope, dynamic range and maximum spike response were similar for different sound frequencies (Fig. 9A). The receptors were tonotopically organized with the characteristic frequency of the receptors increasing from the proximal to the distal end of the array (Figs. 5, 6). Several receptors had characteristic frequencies of 5 kHz. These receptors were divided into two groups on the basis of their maximum spike response produced in response to pure tones of increasing intensity (Fig. 7). Independent of the tuning of the receptor no two-tone inhibition was observed in the periphery, thus confirming that such interactions are a property of central integration.     

Noise filtering in the auditory system of Locusta migratoria L

Many acoustically communicating grasshoppers live in crowded populations where sound of many individuals may cause permanent noise. Tympanic receptors and first-order auditory interneurons of Locusta migratoria code such noise tonically, whereas many higher order interneurons react only weakly. In response to simultaneously presented sound they exhibit a better signal-to-noise ratio than their presynaptic elements. Two possible filter mechanisms are suggested for noise reduction in higher-order interneurons: (i) high-pass filtering of receptor spike frequencies and (ii) filtering due to synchronization of receptor spikes. Different receptor spike frequencies were elicited by series of short noise pulses with variable repetition rates. Receptor activities differing in their degree of synchronization were elicited by sound stimuli with variable rising times. In contrast to the first order interneurons some higher order interneurons responded best to receptor spike frequencies above 150–200 Hz, thus showing the postulated filtering. Only one higher order interneuron (AN4) distinguished between synchronous and asynchronous receptor activities. It is suggested that high-pass filtering of receptor spike frequencies is responsible for the noise filtering observed in these interneurons. The synchronization selectivity of AN4 is proposed to be responsible for temporal pattern detection of conspecific sounds.     

Physiological characteristics of the tympanic organ in noctuoid moths

Summary We show the variations in the spike activity of both auditory receptors inSpodoptera frugiperda, Mocis latipes, Ascalapha odorata (Noctuidae),Maenas jussiae andEmpyreuma pugione (Arctiidae) immediately after 45 ms and 5 s acoustic stimuli at different intensities. The frequency of the applied stimuli was 34 kHz forE. pugione and 20 kHz for the other species. The electrical activity of the auditory receptors was recorded at the tympanic nerve with a stainless steel hook electrode. When the 45 ms pulses cease there is an afterdischarge from both auditory receptors in all the species. The number of spikes in the afterdischarge activity of both receptor cells (A₁ and A₂) shows a linear relation with stimulus intensity (Table 1). This number increases monotonically with increments in stimulus intensity, except for the A₁ cell activity inE. pugione, which decreases at intensities higher than 55 dB (Fig. 1). There are significant species-specific differences in the slope values of the number of spikes in the afterdischarge of both auditory receptors. After a 5 s stimulusM. latipes andM. jussiae show a rapid recovery of the standard spontaneous A₁-cell discharge level. Poststimulus A₁-cell spike activity inS. frugiperda shows a silent period, the duration of which increases with stimulus intensity (Fig. 3).E. pugione andA. odorata show such a silent period after low and moderately intense stimuli, but at high intensities the post-stimulus activity exceeds the pre-stimulus spontaneous discharge (Fig. 3). We demonstrate statistically that these variations cannot be explained by the random fluctuations of the standard spontaneous discharge. They are thus considered a silent and a rebound period respectively (Fig. 5). The presence and duration of either type of period seem to depend on the magnitude of the response to the acoustic stimulus. They thus seem related to the adaptation rate and the previously suggested existence of peripheral inhibitory interaction between the auditory receptors.     

Physiological characteristics of the tympanic organ in noctuoid moths

Summary The tympanic organ ofSpodoptera frugiperda, Mocis latipes, Erebus odorata (Noctuidae) andMaenas jussiae (Arctiidae) was stimulated with acoustic stimuli of 20 kHz, 45 ms and 5 s duration, and intensities ranging from 30 to 100 dB. The electric activity of the auditory receptors was recorded at the tympanic nerve with a stainless steel hook electrode. In all of these moth species there is an intensity range (ca. 20 dB) in which the response of each auditory receptor (A₁ and A₂ cells) to 45 ms pulses varies in a linear relation to the logarithm of stimulus intensity. For intensities higher than this value, depending on the species and the cell analysed, the spike discharge may continue to increase, may saturate or may diminish (Fig. 2). InE. odorata andM. latipes the A₁-cell response shows a decrease for stimulus intensities higher than 30 dB above the threshold. In the former species there is a statistically significant linear relation between the A₂-cell response and the decrease of the A₁-cell response, but this is not the case inM. latipes (Fig. 3). The similarity of the responses ofE. odorata to those described inEmpyreuma pugione (Coro and Pérez 1984) suggest that also in this noctuid species one may assume that the A₂ cell inhibits the A₁ receptor. In all of these moth species there is a maximum firing rate of the auditory cells at the beginning of the response to pure tones of 5 s and an exponential decrease of their discharge frequency with the course of time (Fig. 5). The analysed species differ in the adaptation rates of their auditory receptors. In all of these species the A₂ cell adapts more rapidly than the A₁ cell. In most of these species the stimulus intensity influences the adaptation rate of the auditory receptors (Fig. 7). These results are compared with data obtained by other authors, and it is concluded that there are more interspecific differences in the physiological characteristics of the auditory receptors in noctuoid species than those reported so far.Abbreviation AP action potential     

Long rise times of sound pulses in grasshopper songs improve the directionality cues received by the CNS from the auditory receptors

Auditory receptors of the locust (Locusta migratoria) were investigated with respect to the directionality cues which are present in their spiking responses, with special emphasis on how directional cues are influenced by the rise time of sound signals. Intensity differences between the ears influence two possible cues in the receptor responses, spike count and response latency. Variation in rise time of sound pulses had little effect on the overall spike count; however, it had a substantial effect on the temporal distribution of the receptor's spiking response, especially on the latencies of first spikes. In particular, with ramplike stimuli the slope of the latency vs. intensity curves was steeper as compared to stimuli with steep onsets (Fig. 3). Stimuli with flat ramplike onsets lead to an increase of the latency differences of discharges between left and right tympanic receptors. This type of ramplike stimulus could thus facilitate directional hearing. This hypothesis was corroborated by a Monte Carlo simulation in which the probability of incorrect directional decisions was determined on the basis of the receptor latencies and spike counts. Slowly rising ramps significantly improved the decisions based on response latency, as compared to stimuli with sudden onsets (Fig. 4). These results are compared to behavioural results obtained with the grasshopper Ch. biguttulus. The stridulation signals of the females of this species consist of ramplike pulses, which could be an adaptation to facilitate directional hearing of phonotactically approaching males.Abbreviations HFR high frequency receptor - ILD interaural level difference - LFR low frequency receptor - SPL sound pressure level - WN white noise     

Ascending auditory interneurons in the cricketTeleogryllus commodus (Walker): comparative physiology and direct connections with afferents

Summary Ascending auditory interneurons of the cricket,Teleogryllus commodus (Walker), were investigated using simultaneous intracellular and extracellular recording in order to identify units which had previously been characterized only by extracellular recording. The morphology and physiology of the large adapting unit (LAU: Fig. 1) and of the small tonic unit (STU: Fig. 2) ofTeleogryllus correspond well to those of the ascending neuron 2 (AN2) and the ascending neuron 1 (AN1) ofGryllus (Figs. 1, 2), respectively.A summary of the ascending auditory interneurons described by various authors in 5 species of crickets is presented in order to establish common identities.Physiological evidence for direct connections between auditory afferents and the ascending auditory interneurons AN1 (STU) and AN2 (LAU) is presented. Simultaneous intracellular recordings from receptors and interneurons in response to sound as well as the activity of auditory interneurons upon electrical stimulation of the tympanal nerve reveal short and constant latencies of receptor-evoked synaptic activity in AN1 (STU) and AN2 (LAU).Abbreviations STU small tonic unit - LAU large adapting unit - AN ascending neuron - EPSP excitatory postsynaptic potential     

Song recognition in the grasshopper Chorthippus biguttulus is not impaired by shortening song signals: implications for neuronal encoding

The communication signals in many grasshopper species are composed of multiple repetitions of highly stereotyped subunits, and thus provide redundancy. In a behavioural paradigm, we tested the ability of males of the grasshopper Chorthippus biguttulus to recognize shortened versions of the communication signals of conspecific females. Males reliably responded to a three-subunits signal (250 ms), i.e. to a signal of less than a quarter of the natural duration. This performance is remarkable in view of the substantial adaptation and the variability present in the spiking responses of auditory interneurones. These behavioural results will impose constraints for investigating possible encoding mechanisms used by the grasshoppers' auditory system. Accepted: 1 September 1998     

The responses of auditory ventral-cord neurons ofLocusta migratoria to vibration stimuli

Summary Most of the auditory neurons in the ventral nerve cord ofLocusta migratoria carry information not only from the tympanal organs but also from the subgenual organs (vibration sensors). Six of the eight neuron types studied electrophysiologically respond to at least these two modalities. Artificial sounds (white noise and pure tones varying in frequency and intensity) and sinusoidal vibration (200 Hz with an acceleration of 15.8 cm/s₂ or 2000 Hz and 87 cm/s₂) were used as stimuli.Complex excitatory and/or inhibitory interactions of the signals from both tympanal organs form the discharge patterns of auditory ventral-cord neurons in response to stimulation with air-borne sound. Normally the input of the ipsilateral sense organ dominates. The response patterns of these same neurons elicited by vibration stimuli are formed differently, as follows: (1) the sensory inputs of all subgenual organs are integrated in the responses of the ventral-cord neurons; in a single neuron they have either excitatory or inhibitory effects, but not both. (2) The more legs vibrated, the larger is the response. (3) The subgenual organs in the middle legs are most effective, those in the hind legs least so. (4) Ipsilateral vibration has more effect than contralateral.The six auditory neurons react to vibration combined with air-borne sound in different ways. The B neuron is the only one inhibited by vibration stimuli. The G neuron has been studied more intensively; because its anatomical arrangement and the location of the endings of the subgenual receptor fibers are known, it could be inferred from effects of transection of the connectives that interneurons are interposed between receptor cells and the G neuron.Part of the program Sonderforschungsbereich 114 (Bionach) Bochum, under the auspices of the Deutsche Forschungsgemeinschaft, with the support of the Slovenic Research Society (RSS)     

Phonotactic specificity of the cricketTeleogryllus oceanicus: intensity-dependent selectivity for temporal parameters of the stimulus

Summary We have investigated the effects of alterations of several temporal parameters of auditory stimuli, as well as of stimulus intensity changes, on the attractiveness of these stimuli to femaleTeleogryllus oceanicus, as measured by monitoring sound-elicited flight steering responses. AlthoughT. oceanicus has a rhythmically complex calling song, females are attracted by a simpler model consisting of regularly repeating sound pulses. We have found that the two major temporal features of this model, sound pulse duration and pulse repetition rate, are both important for eliciting phonotactic steering responses.Stimuli with altered temporal features had intensity thresholds indistinguishable from the control stimulus (Fig. 3). The majority of crickets, however, ceased to respond to the altered stimuli when the stimulus intensity was sufficiently increased (Figs. 4–7). In some cases, intensity increases resulted in a reversal of the steering response from positive to negative (Fig. 10). Effects of altered temporal parameters were also apparent at lower stimulus intensities, where the amplitudes of steering responses to stimuli with altered parameters were smaller than those in response to the control stimulus (Figs. 8, 9).We considered the possibility that the cessation of responsiveness to stimuli with altered temporal features was due to a temporal pattern-specific diminution of binaural cues for sound localization at high intensities. Experiments performed with unilaterally deafened crickets (Fig. 11) led us to conclude that this was not the case, and that our findings instead reflect the properties of the song recognition mechanism.Abbreviations UIL upper intensity limit - RAF ratio of abdominal flexion     

Acoustic pattern recognition in a grasshopper: processing in the time or frequency domain?

Males of the grasshopper Chorthippus biguttulus produce songs which consist of the stereotyped and rhythmic iteration of a sound unit (termed syllable) separated by distinct syllable pauses. Virgin females respond to this signal, and to similar artificial signals, with song phrases of their own. In behavioural experiments the response probability of virgin females can be measured with artificial acoustic stimuli. The stimuli consisted of an amplitude modulated noise the envelope of which was altered. We investigated several hypotheses on the mechanisms of conspecific song recognition with special emphasis on the question whether recognition occurs in the frequency domain or in the time domain. (1) Females of Ch. biguttulus required only the first five Fourier components of the envelope function (corresponding to 50 Hz for a fundamental frequency of 10 Hz) to detect the syllable/pause structure. In addition, they detected small gaps within syllables if the signal contained at least ca. 15 Fourier components (corresponding to a frequency of 150 Hz). Further experiments showed that the correct phase information of the Fourier components is necessary for recognition, indicating that pattern recognition is not achieved merely on the basis of band pass filtering. (2) A cross correlation between the signal and an assumed internal template yields only inconsistent predictions of the response probabilities. (3) The recognizer system probably works in the time domain, possibly by direct comparison of adjacent syllable and pause durations. It is not yet clear whether the duration of a syllable is evaluated with respect to the preceding or succeeding pause. We emphasize that the neural recognizer of the grasshopper does not only examine a signal for its similarity to an internal template, but that it also takes into account features that indicate an incorrect signal. This may be a general feature of neuronal pattern recognition systems which have been shaped by natural selection. Received: 4 October 1997 / Accepted in revised form: 26 August 1998     

Noise affects the shape of female preference functions for acoustic signals

The shape of female mate preference functions influences the speed and direction of sexual signal evolution. However, the expression of female preferences is modulated by interactions between environmental conditions and the female's sensory processing system. Noise is an especially relevant environmental condition because it interferes directly with the neural processing of signals. Although noise is therefore likely a significant force in the evolution of communication systems, little is known about its effects on preference function shape. In the grasshopper Chorthippus biguttulus, female preferences for male calling song characteristics are likely to be affected by noise because its auditory system is sensitive to fine temporal details of songs. We measured female preference functions for variation in male song characteristics in several levels of masking noise and found strong effects of noise on preference function shape. The overall responsiveness to signals in noise generally decreased. Preference strength increased for some signal characteristics and decreased for others, largely corresponding to expectations based on neurophysiological studies of acoustic signal processing. These results suggest that different signal characteristics will be favored under different noise conditions, and thus that signal evolution may proceed differently depending on the extent and temporal patterning of environmental noise.     

Comparison of motor patterns in the intact and deafferented flight system of the locust

Summary The activity of flight interneurons was recorded intracellularly in intact, tethered flying locusts (Locusta migratoria) and after removal of sensory input from the wing receptors. Depolarization patterns and spike discharges were characterized and compared for the two situations.In general, depressor interneurons (n=6) showed only minor changes in their activity as a result of deafferentation (Fig. 1). Exceptions were interneurons 308 and 506 (Fig. 2). By contrast, all but one of the elevator interneurons (n=9) produced distinctly different depolarization patterns in intact locusts and following deafferentation. Three different groups of elevator interneurons were found (excluding the one exceptional neuron, Fig. 6). (i) One group of interneurons (n=4) produced different, superthreshold depolarizations in intact and deafferented animals (Fig. 3). Characteristic, biphasic depolarizations were recorded from these fibres at lower wingbeat frequencies in the intact situation but only single, delayed potentials were recorded after deafferentation. (ii) The second group of interneurons (n=3) exhibited distinct rhythmic activity only in intact animals. After deafferentation their depolarizations were small and often below the threshold for spike initiation (Fig. 4). (iii) One interneuron produced rhythmic flight motor oscillations only after deafferentation. In intact locusts the membrane potential of this neuron showed very small oscillations and remained subthreshold (Fig. 5).Four main conclusions emerge from these data. (i) The activity of elevator interneurons is under greater sensory control than that of the depressors. This confirms the results of our previous electromyographic and motoneuronal analyses, (ii) A considerable portion of elevator activity is generated as a result of phasic sensory feedback. An essential input is from the hindwing tegulae (Table 1; Pearson and Wolf 1988). (iii) The activity of depressor interneurons appears to be determined by central mechanisms to a major extent. (iv) Different sets of central neurons appear to be involved in flight pattern generation in intact and deafferented locusts —although the two sets share many common elements.Abbreviations EMG electromyogram - PSP postsynaptic potential (EPSP excitatory andIPSP inhibitory)     

Species Recognition Influences Female Mate Preferences in the Common European Grasshopper (Chorthippus biguttulus Linnaeus, 1758)

Species recognition and intraspecific mating preferences constitute two basic aspects of animal communication. Both can be considered as variations in response to signals and it has been suggested that they represent a continuum. Selection on species recognition could therefore influence intraspecific mating preferences. We show that females of the common European grasshopper Chorthippus biguttulus prefer conspecific male signals that can be distinguished more reliably from sympatrically occurring heterospecific signals. This suggests that in C. biguttulus, sexual selection might be influenced by pleiotropic effects from species recognition. The results show how the heterospecific signal environment could have determined why and in which direction specific traits become sexually selected.     

Ocellar interneurons in the honeybee

Summary Morphologically identified spiking ocellar interneurons (L_B and L_D-neurons) of the honeybee (Apis melliferd) were investigated by combined intracellular recording and staining techniques using multimodal stimulus programs.Response patterns containing both graded and action potentials (mixed response), and pure spiking responses were analysed. Mixed responses allow a comparison of information coded simultaneously by graded and action potentials in one neuron. In most cases the intensity dependence coded by spikes was found to be similar to the intensity dependence coded by one of two different parameters evaluated from the graded signal. Lneurons with mixed responses were unimodal, i.e. they reacted exclusively to stationary illumination of the ocelli, as do nonspiking L-neurons.In contrast, spiking L-neurons that lacked a graded response component could also respond to stimuli of other sensory modalities: moving patterns, compound eye illumination, airstreams, mechanical and gustatory stimulation. One L_D-neuron was also excited by the wing beat.Recordings from the same type of neuron in different individuals demonstrate that the input modalities and response patterns of L-neurons vary remarkably. Consequently many recordings are required to properly characterise the physiological properties of these neurons even though anatomically they are identified.The existence of graded and action potentials in the same cell and the fact that these two signals carry different information is discussed in the context of a possible role for information transmission from L-neurons to postsynaptic cells.Abbreviation R/I response/intensity     

A putative hybrid in nature between Chorthippus brunneus and C. biguttulus(Orthoptera: Acrididae)

The characteristics of the song and morphology of a probable natural hybrid between the two grasshoppers Chorthippus brunneus (Thunberg) and C. biguttulus (L.) are described and compared with those of the parent species. The status of C. eisentrauti (Ramme) is briefly discussed.