Synaptic mechanisms of monaural and binaural processing in the locust

(1) Responses of auditory interneurones were recorded intracellularly within the metathoracic ganglion of the locust when stimulating each tympanic membrane with a piezoelectric transducer. Thus, in contrast to conventional sound stimulation, each of the two ears could be activated independently from the other at variable intensities, duration and stimulus onsets. By means of this ‘earphone-like’ stimulation technique the binaural integration properties of auditory interneurons could be analysed. (2) A minority of units (3 out of 43) was affected by input from one side only. Their synaptic input was purely excitatory and the intensity characteristics reflected those of auditory receptor fibres. (3) Most interneurones received input from both ears, each being excitatory or one excitatory or one excitatory and one inhibitory. In some units the unilateral synaptic response already included both an EPSP and an IPSP. As a result of varying temporal interactions between the EPSP and the IPSP within the unilaterally evoked complex response the intensity characteristics differed widely from unit to unit. (4) With binaural simultaneous stimulation the complexity of the postsynaptic responses of most interneurones increased as the synaptic input from both ears coincided at the level of the recorded interneurone. Although both ears were stimulated symmetrically (at the same time and intensity), units were recorded where the latencies of ipsilateral and contralateral synaptic input were different. Contralateral inhibition could either follow or precede ipsilateral excitation and in some cases both EPSP and IPSP had the same latency. On the basis of these findings the binaural synaptic mechanisms of directional coding are discussed and compared with corresponding results under free field stimulus conditions.     

Harmonic analysis of two-tone discharge patterns in cochlear nerve fibers

Discharges in cochlear nerve fibers evoked by low frequency phase-locked sinusoidal acoustic stimuli are synchronized to the stimulus waveform. Excitation and suppression regions of single units were explored using a stimulus composed of either a fixed intensity test tone at the characteristic frequency, a variable intensity interfering tone with a simple integer frequency relation to the characteristic frequency, or both. Compound period histograms were constructed from period histograms in response to normal and reversed polarity stimuli. Discharge patterns were characterized by Fourier components of the histogram envelopes. The two stimulus frequencies constituted the principal harmonics in the histogram envelopes and their combination accounted for observed rate changes. Suppression of the test tone harmonic as a function of interfering tone intensity was always seen; rate suppression was not. The harmonic was typically suppressed by 20–30 dB compared to the value for the test tone alone and often reached the 40–60 dB resolution limit of the experiment. Suppression plots were nearly linear on a power scale with an average slope of-0.8. The onset of suppression occurred for an interfering tone 9 dB greater on average than the test tone intensity. Information transfer through the peripheral system was described by the ratio of the principal harmonic amplitudes versus the ratio of the intensities of the two stimulus tones. These plots were nearly linear on a power scale with an average slope of 0.9. Neither the onset of suppression nor the slopes of the harmonic plots displayed strong dependence on characteristic frequency or interfering tone frequency. These features of harmonic behavior, however, are closely related to system nonlinearity. Comparison of measured harmonics to the predictions of two phenomenological models suggest the presence of complex nonlinear transformations in the peripheral auditory system.     

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     

Accession of sweet stimuli to receptors. I. Absolute dominance of one molecular species in binary mixtures

Intensity/time studies of sweetness response in pure solutions of each of nine different sweet stimuli have been carried out. Both variables exhibit simple power functions of the form Intensity (S) = kscns and Persistence (P) = kpcnp. In binary mixtures of these nine stimuli a depression (or negative synergism) of both sweetness intensity and persistence is observed which is predictable from the low exponents of the power functions. Combination of both power functions allows the "effective concentration" of each stimulus in a binary mixture to be calculated from its observed intensity/time characteristics. All "effective concentrations" calculable in this way show absolute dominance of one stimulus in mixtures of two irrespective of the relative proportions of the two stimuli. It is suggested that the "effective concentrations" may reflect real concentrations of a single molecular species in the microenvironment of the receptor. Thus the accession of sweet molecules to ordered, localized concentrations at the receptor is ultimately dependent on chemical structure.     

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.     

An electrophysiological study of sound sensitive neurons in the ‘Primitive ear’ of Acheta domesticus

Crickets have two types of mechanisms for the reception of environmental sounds: (1) the tympanal organs in the two forelegs and (2) the freely articulated setal receptors on the abdominal ceri. The cercal setal receptors have hitherto received much less experimental attention as decoders of biologically significant sounds than have the tympano-receptors. In the present study the cercal auditory system of Acheta domesticus was examined electrophysiologically to determine its auditory frequency sensitivity, the tuning characteristics of individual units, and the synchronization between nerve impulses and stimulus frequency. Both pre- and postsynaptic units were examined in the fifth abdominal ganglion; several of the observed response patterns were compared with those of homologous cercal sensory neurons in Periplaneta americana. The results show that (1) A. domesticus possesses an elaborate array of cercal receptors which are highly sensitive to sounds, (2) the cercal setal receptors are more sensitive and numerous in the cricket than in the cockroach, and (3) the cercal auditory system can decode stimulus information by narrow tuning in individual cells and by synchronous discharge patterns; firing frequencies range up to 300 Hz in presynaptic sensory units and 60 Hz in the postsynaptic giants. The response patterns were related to the structure of the receptor and the behavioural adaptations of the insect.     

Short-latency auditory evoked potentials during change in the physical parameter of a sound stimulus

The amplitude-temporal and spectral characteristics of the short-latency auditory evoked potentials (SLAEP) recorded under conditions of monoaural stimulation with sound clicks with initial phase of rarefaction followed by compression and alteration, with the intensity of 60 dB and frequency of 11.1 Hz, were studied in ipsi- and contralateral derivations. Substantial changes in SLAEP morphology in response to polarity inversion of the acoustic stimulus were found. Waves II, IV, VI, and VII changed to the greatest extent. The spectral analysis detected three main SLAEP components: low- (LF), medium- (MF), and high-frequency (HF) components as well as the respective frequency bands. Change in the click phase from rarefaction to compression resulted in bilateral redistribution of power between the MF and HF components. This was expressed as a decrease in the HF peak power and simultaneous rise of MF power. Selective effects of the polarity inversion of the sound stimulus on the MF and HF components support the finding that the activity of SLAEP-generating structures are mainly reflected in the mentioned components. It is suggested that two populations of phase-sensitive units are represented in the auditory analyzer. These populations determine the characteristic changes in SLAEP morphology and spectral characteristics.     

Chemoreception by a cell with age-dependent receptor binding sites

The authors determine the time-dependent ligand current into a spherical cell that is covered with a large number of age-dependent receptors. These receptors can be in either of two states: active (i.e., available for ligand binding) or inactive. An active receptor turns inactive upon binding a ligand, and it can reappear as active at some later time. The transition inactive----active is treated as a probabilistic process. The ligand distribution around the cell is determined analytically in terms of this distribution at the cell surface. A set of nonlinear integral equations is derived for the distribution at the cell surface, which is solved numerically. In this way the time-dependent ligand current into the cell as well as the average active receptor population at the cell surface are determined.     

On the information received by sensory receptors

It is hypothesized that a sensory neuron, a neuron issuing from a sensory receptor, encodes the rate at which entropy or uncertainty is removed at the receptor level. This hypothesis is tested for the case of the entropy associated with the magnitude of a signal (stimulus) applied at the sensory receptor. A simple mathematical model of the process is presented and a number of well-known stimulus-response relationships are seen to emerge. For example, the adaptation of a receptor may be seen to occur as a consequence of reduced uncertainty regarding stimulus intensity. A general equation relating stimulus and response is developed, and this equation will simplify, depending upon the ratio of signal power to noise power, to either a logarithmic or a power law.     

Intensity Characteristics of the Noctuid Acoustic Receptor

Spiking activity of the more sensitive acoustic receptor is described as a function of stimulus intensity. The form of the intensity characteristic depends strongly on stimulus duration. For very brief stimuli, the integral of stimulus power over stimulus duration determines the effectiveness. No response saturation is observed. With longer stimuli (50 msec), a steady firing rate is elicited. The response extends from the spontaneous rate of 20–40 spikes/sec to a saturated firing rate of nearly 700 spikes/sec. The characteristic is monotonic over more than 50 db in stimulus intensity. With very long stimuli (10 sec), the characteristics are nonmonotonic. Firing rates late in the stimulus decrease in response to an increase in stimulus intensity. The non-monotonic characteristics are attributed to intensity-related changes in response adaptation.     

Comparison of binaural release from forward masking in animals and humans. Electrophysiologic study

The EPs of the inferior colliculus and auditory cortex in anaesthetized guinea pigs and the long latency auditory EPs in alert humans were studied. The stimuli consisted of binaurally presented pairs of clicks used as a masker, and the probe, with a variable time delay between them. The greatest relative differences between out-of-phase and in-phase probe responses were observed at the beginning of the recovery course. They averaged as 1.6, 1.5 and 1.4 for the responses of the inferior colliculus, auditory cortex and long latency potentials, resp., at the stimuli intensities of 50-65 dB SPL, and then decreased to zero during the time course of the probe response recovery. Correlation of this parameter with the stimulus intensity was positive.     

Stimulus-dependent oscillations and evoked potentials in chinchilla auditory cortex

Besides the intensity and frequency of an auditory stimulus, the length of time that precedes the stimulation is an important factor that determines the magnitude of early evoked neural responses in the auditory cortex. Here we used chinchillas to demonstrate that the length of the silent period before the presentation of an auditory stimulus is a critical factor that modifies late oscillatory responses in the auditory cortex. We used tetrodes to record local-field potential (LFP) signals from the left auditory cortex of ten animals while they were stimulated with clicks, tones or noise bursts delivered at different rates and intensity levels. We found that the incidence of oscillatory activity in the auditory cortex of anesthetized chinchillas is dependent on the period of silence before stimulation and on the intensity of the auditory stimulus. In 62.5% of the recordings sites we found stimulus-related oscillations at around 8-20 Hz. Stimulus-induced oscillations were largest and consistent when stimuli were preceded by 5 s of silence and they were absent when preceded by less than 500 ms of silence. These results demonstrate that the period of silence preceding the stimulus presentation and the stimulus intensity are critical factors for the presence of these oscillations.     

Processing of vibratory and acoustic signals by ventral cord neurones in the cricket Gryllus campestris

The responses of single vibratory receptors and ascending ventral cord interneurones were studied extracellularly in Gryllus campestris L. The physiology of the vibration receptors resembled those found in tettigoniids and locusts. The frequency responses of the subgenual receptors provide two possible cues for central frequency discrimination: differences in mean tuning between groups of receptors in the different leg pairs and a range of receptors tuned to different frequencies within one subgenual organ.Most of the ascending vibratory interneurones were highly sensitive in either the low or high frequency range. Broadbanded neurones were less sensitive. The characteristic sensitivity peaks of these units are due mainly to receptor inputs from a particular leg pair, although most central neurones receive inputs from all 6 legs. Only one neurone type, TN1 received excitatory inputs from both auditory and vibratory receptors; its responses were greatly enhanced by the simultaneous presentation of both stimulus modes. The responses to sound stimuli of AN2, on the other hand, were inhibited by vibration. No other auditory interneurones investigated were influenced by inputs from vibration receptors. Central processing of vibratory information in the cricket is compared with that of tettigoniids and locusts.     

Peripheral frequency mis-match in the primitive ensiferan Cyphoderris monstrosa (Orthoptera: Haglidae)

Peripheral auditory frequency tuning in the ensiferan insect Cyphoderris monstrosa (Orthoptera: Haglidae) was examined by comparing tympanal vibrations and primary auditory receptor responses. In this species there is a mis-match between the frequency of maximal auditory sensitivity and the frequency content of the species' acoustic signals. The mis-match is not a function of the mechanical properties of the tympanum, but is evident at the level of primary receptors. There are two classes of primary receptors: low-tuned and broadly tuned. Differences in the absolute sensitivity of the two receptor types at the male song frequency would allow the auditory system to discriminate intraspecific signals from sounds containing lower frequencies. Comparisons of tympanal and receptor tuning indicated that the sensitivity of the broadly tuned receptors did not differ from that of the tympanum, while low-tuned receptors had significantly narrower frequency tuning. The results suggest that the limited specialization for the encoding of intraspecific signals in the auditory system of C. monstrosa is a primitive rather than a degenerate condition. The limited specialization of C. monstrosa may reflect the evolutionary origin of communication-related hearing from a generalized precursor through the addition of peripheral adaptations (tympana, additional receptors) to enhance frequency sensitivity and discrimination. Accepted: 13 March 1999     

Multidimensional auditory stimulus control in a chimpanzee (Pan troglodytes)

An adolescent female chimpanzee (Pan troglodytes) was trained to discriminate auditory compound stimuli differing in tonal frequency and/or tone on-off rate. Following acquisition training and overtraining, she was shifted to multidimensional stimulus control testing using redundant relevant auditory stimulus sets with discriminability of elements in each dimension varied systematically. Although the control by both dimensions changed significantly as a function of discriminability, the degree of dimensional control was stronger in the tone on-off rate than in the tonal frequency. These results clearly demonstrated “attentional” control of the chimpanzee's auditory discrimination behavior and the interaction between two dimensions of auditory stimuli. The author is now at the Department of Psychology, Primate Research Institute, Kyoto University as a transfer student of the Doctor course.     

用脑干电位的互相关函数客观估计听力阈值

将脑干诱发电位记录分为大小相等的两个子集均,求其互相关函数。互相关函数在原点附近的平均幅度作为估计听力阈值的参数,并通过实验得出表达该参数与阈上刺激强度关系的直线回归方程,从而求出听力阈值的估计公式。把三种阈上刺激强度下的参数值分别代入估计公式,平均结果作为听力阈值的估计。对8个有不同听力损失的病耳进行了估计,其结果与主观阈值平均相差4.6dB。     

The valve action of intensity receptor

The valve action of intensity receptor has the following properties which are essential to an encoder. (1) The response amplitude corresponds to the stimulus intensity in a one-to-one manner. (2) The dynamic ranges of the stimulus intensity and the response amplitude are respectively set and able to move in parallel with their axes. (3) The probability density distribution of the stimulus intensity may be transformed into that of the response amplitude by a stimulus-response relation. Concerning stimulus-response relations, three empirical formulae, i.e. logarithmic, power and tanh log functions, are well known. These formulae can be arranged for the forms to express the above three properties. Then, power and tanh log functions are converted to a logarithmic function as the exponents of them approach zero. From a viewpoint of information theory, the stimulus-response transformation may play a role to improve the probabilistic nature of stimuli in order to make an efficient code for a given channel of sensory fibre.     

Frequency response characteristics of electroreceptors in weakly electric fish (Gymnotoidei) with a pulse discharge

Summary Three species of Gymnotid fish, two species ofHypopomus andRhamphichthys rostratus, each having pulse type electric organ discharges (EOD) of different durations were studied to learn if any correlation exists between the spectral composition of the species specific EOD pulse and the frequency response characteristics of that species' electroreceptors. The receptor population consisted of two major categories (examples in Fig. 3). One category, termed pulse marker receptors, responded to suprathreshold stimulus pulses with a single spike at a short (<2 ms) latency. These receptors were tuned to the higher frequency components of a species' EOD (Fig. 4A) and were always 5 to 10 dB less sensitive than any other electroreceptors within a given species. The second major receptor category, burst duration coders, responded to an electrical stimulus with a burst of spikes at a longer latency, burst length was a function of stimulus amplitude. This second category could be further divided into three sub-categories according to the receptors' frequency response characteristics. The most commonly seen subcategory, wide band receptors (Fig. 4B), responded best to stimuli having frequencies equal to the dominant frequency component of the species' EOD in the two species ofHypopomus studied. A second subcategory, narrow band receptors (Fig. 4 A), had frequency response characteristics similar to those of the pulse marker receptors; however, these had thresholds 10 dB lower than those of the pulse marker. The third subcategory of burst duration coders, low frequency receptors (Fig. 4 C, D), responded best to stimulus frequencies ranging from about 50 to 150 Hz. Mechanisms of coding stimulus amplitude and responses to prolonged sinusoidal electrical stimuli were also studied in the various receptor types.It is suggested that the differences in the major receptor types and the different frequency response characteristics of the electroreceptors within a given species allows the animals to identify and evaluate signals resulting from their own EOD, the EODs of conspecifics and electrical stimuli generated by other species of electric fish.Supported by NIH Grant #1 RO1 NS 12337-01     

Functioning of catfish electroreceptors: relation between skin potential and receptor activity

Properties of catfish electroceptors were investigated by simultaneous recording of the skin potential and the activity of an afferent nerve. 1. The normal threshold stimulus intensity induces a potential amplitude of about 10 to 30 muV across the skin (Table I). 2. The average spike frequency in the nerve increases approximately with the logarithm of the stimulus intensity (Fig. 1). 3. The direct current restoring the receptor activity in calcium deficient media makes the skin potential more negative. 4. Presumably, not the skin potential itself but a difference between the skin potential and the e.m.f. generated by the receptor epithelium influences receptor functioning. 5. Amplitude and phase characteristics can be described by a filter circuit (Fig. 4 and 5).