Frequency-intensity characteristics of cricket cercal interneurons: units with high-pass functions

Interneurons in the cercal sensory system of crickets respond in a cell-specific manner if the cercal hair sensilla are stimulated by air-particle oscillations at frequencies below about 2000 Hz. We investigated the filter properties of several of these interneurons, and tested the effect of stimulus intensity (typically 0.3–50 mm s⁻¹ peak-to-peak air-particle velocity) on the frequency response in the range 5–600 Hz. We focus on three interneurons (the lateral and medial giant interneurons and interneuron 9-3a) of Acheta domesticus which are characterized by a relatively high sensitivity above ca. 50–200 Hz. The responses of the medial giant interneuron usually increase monotonically with frequency and intensity. Interneuron 9-3a and the lateral giant interneuron exhibit saturation or response decrement at high frequencies and intensities. The lateral giant interneuron has an additional peak of sensitivity below about 40 Hz. Small individual variations in the relative locations of the two response areas of this interneuron within the frequency-intensity field are responsible for a large variability obtained if frequency-response curves are determined for particular intensities. Stimulus frequency does not affect the principal directional preferences of the three interneurons. Nevertheless, if tested individually, the lateral giant interneuron and interneuron 9-3a exhibit small changes of directional tuning. Accepted: 12 November 1997     

Frequency-intensity characteristics of cricket cercal interneurons: low-frequency-sensitive units

Three identified interneurons of the cercal system were investigated electrophysiologically; these interneurons are sensitive only to stimulation of cercal filiform-hair sensilla by low-frequency sound. Measurement of the frequency ranges revealed cut-off frequencies between ca. 20 and 70 Hz. Analysis of the responses near threshold and at higher intensities in the frequency range 5–500 Hz shows that one of them (Interneuron 9-1b) exhibits a sensitivity maximum at the frequency-intensity combination necessary for the perception of an intraspecific signal at 30 Hz. This band-pass behavior disappears at higher stimulus intensities. In order to investigate the mechanism of the low-frequency selectivity of the interneurons, two-tone stimulation experiments were performed. When stimuli in the best-frequency range were superimposed by a 100-Hz tone, the spiking activity was suppressed in an intensity-dependent manner. Accepted: 22 July 1998     

Directionality of auditory nerve fiber responses to pure tone stimuli in the grassfrog, Rana temporaria. II. Spike timing

We studied the directionality of spike timing in the responses of single auditory nerve fibers of the grass frog, Rana temporaria, to tone burst stimulation. Both the latency of the first spike after stimulus onset and the preferred firing phase during the stimulus were studied. In addition, the directionality of the phase of eardrum vibrations was measured. The response latency showed systematic and statistically significant changes with sound direction at both low and high frequencies. The latency changes were correlated with response strength (spike rate) changes and were probably the result of directional changes in effective stimulus intensity. Systematic changes in the preferred firing phase were seen in all fibers that showed phaselocking (i.e., at frequencies below 500–700 Hz). The mean phase lead for stimulation from the contralateral side was approximately 140° at 200 Hz and decreased to approximately 100° at 700 Hz. These phaseshifts correspond to differences in spike timing of approximately 2 ms and 0.4 ms respectively. The phaseshifts were nearly independent of stimulus intensity. The phase directionality of eardrum vibrations was smaller than that of the nerve fibers. Hence, the strong directional phaseshifts shown by the nerve fibers probably reflect the directional characteristics of extratympanic pathways. Accepted: 23 November 1996     

Directionality of auditory nerve fiber responses to pure tone stimuli in the grassfrog, Rana temporaria. I. Spike rate responses

We studied the directionality of spike rate responses of auditory nerve fibers of the grassfrog, Rana temporaria, to pure tone stimuli. All auditory fibers showed spike rate directionality. The strongest directionality was seen at low frequencies (200 – 400 Hz), where the spike rate could change by up to nearly 200␣spikes s⁻¹. with sound direction. At higher frequencies the directional spike rate changes were mostly below 100 spikes s⁻¹. In equivalent dB SPL terms (calculated using the fibers' rate-intensity curves) the maximum directionalities were up to 15 dB at low frequencies and below 10 dB at higher frequencies. Two types of directional patterns were observed. At frequencies below 500 Hz relatively strong responses were evoked by stimuli from the ipsilateral (+90^o) and contralateral (−90^o) directions while the weakest responses were evoked by stimuli from frontal (0^o or +30^o) or posterior (−135^o) directions. At frequencies above 800 Hz the strongest responses were evoked by stimuli from the ipsilateral direction while gradually weaker responses were seen as the sound direction shifted towards the contralateral side. At frequencies between 500 and 800 Hz both directional patterns were seen. The directionality was highly intensity dependent. No special adaptations for localization of conspecific calls were found. Accepted: 23 November 1996     

Responses of the infrared sensilla of Melanophila acuminata (Coleoptera: Buprestidae) to monochromatic infrared stimulation

The pit organs of the beetle Melanophilaacuminata were stimulated with monochromatic infrared radiation using a continuous wave CO overtone infrared laser. Best sensitivity was in the wavelength range 2.8–3.5 μm. In this range a stimulus intensity of 14.7 mW cm⁻² was sufficient to generate single action potentials. At a wavelength of 5 μm receptor performance significantly decreased. An increase in stimulus intensity caused a decrease in response latency and an increase in the number of action potentials elicited. At a given wavelength (3.4 μm) the dynamic amplitude range of action potential responses covered 12 dB. At high stimulus intensities (94.2 mW cm⁻²) a stimulus duration of 4 ms was sufficient to generate one to two action potentials and a stimulus duration of 60 ms already caused response saturation (with up to nine action potentials). In a repetitive stimulus regime distinct receptor potentials were visible up to a frequency of 600 Hz. Accepted: 18 March 2000     

Temporal resolution in olfaction III: flicker fusion and concentration-dependent synchronization with stimulus pulse trains of antennular chemoreceptor cells in the American lobster

To understand how chemoreceptor organs may extract temporal information from odor plumes, we investigated the frequency filter properties of lobster chemoreceptor cells. We used rapid stimulation and high-resolution stimulus measurement for accurate stimulus control and recorded extracellular responses from chemoreceptors in the lobster lateral antennule in situ. We tested 16 hydroxyproline-sensitive cells with a series of ten 100-ms pulses at 10, 100 and 1000 μmol l⁻¹ at stimulation frequencies from 0.5 Hz to 4 Hz. Receptor cell responses could accurately encode 10 μmol l⁻¹, but not 100 or 1000 μmol l⁻¹ pulses, delivered at rates of 4 Hz. Flicker-fusion frequency and synchronization with the stimulus pulse train were concentration dependent: performance rates above 1 Hz became poorer both with increasing pulse amplitude and frequency. Flicker fusion frequency was 3 Hz for 100 μmol l⁻¹ pulses and 2 Hz for 1000 μmol l⁻¹ pulses. Individual cells showed differences in their stimulus pulse following capabilities, as measured by the synchronization coefficient. These individual differences may form a basis for coding temporal features of an odor plume in an across-fiber pattern. Accepted: 7 July 1999     

Responses of midbrain lateral line units of the goldfish, Carassius auratus, to constant-amplitude and amplitude-modulated water wave stimuli

 Responses of mechanosensory lateral line units to constant-amplitude hydrodynamic stimuli and to sinusoidally amplitude-modulated water movements were recorded from the goldfish (Carassius auratus) torus semicircularis. Responses were classified by the number of spikes evoked in the unit's dynamic range and by the degree of phase locking to the carrier- and amplitude-modulation frequency of the stimulus. Most midbrain units showed phasic responses to constant-amplitude hydrodynamic stimuli. For different units peri-stimulus time histograms varied widely. Based on iso-displacement curves, midbrain units prefered either low frequencies (≤33 Hz), mid frequencies (50–100 Hz), or high frequencies (≥200 Hz). The distribution of the coefficient of synchronization to constant-amplitude stimuli showed that most units were only weakly phase locked. Midbrain units of the goldfish responded to amplitude-modulated water motions in a phasic/tonic or tonic fashion. Units highly phase locked to the amplitude modulation frequency, provided that modulation depth was at least 36%. Units tuned to one particular amplitude modulation frequency were not found. Accepted: 10 July 1999     

Sensitivity and response dynamics of elasmobranch electrosensory primary afferent neurons to near threshold fields

Elasmobranch fishes localize weak electric sources at field intensities of <5 ηV cm⁻¹, but the response dynamics of electrosensory primary afferent neurons to near threshold stimuli in situ are not well characterized. Electrosensory primary afferents in the round stingray, Urolophus halleri, have a relatively high discharge rate, a regular discharge pattern and entrain to 1-Hz sinusoidal peak electric field gradients of ≤20 ηV cm⁻¹. Peak neural discharge for units increases as a non-linear function of stimulus intensity, and unit sensitivity (gain) decreases as stimulus intensity increases. Average peak rate-intensity encoding is commonly lost when peak spike rate approximately doubles that of resting, and for many units occurs at intensities <1 μV cm⁻¹. Best neural sensitivity for nearly all units is at 1–2 Hz with a low-frequency slope of 8 dB/decade and a high-frequency slope of −23 dB/decade. The response characteristics of stingray electrosensory primary afferents indicate sensory adaptations for detection of extremely weak phasic fields near 1–2 Hz. We argue that these properties reflect evolutionary adaptations in elasmobranch fishes to enhance detection of prey, communication and social interactions, and possibly electric-mediated geomagnetic orientation. Accepted: 20 June 1997     

Physiology of vibration-sensitive afferents in the femoral chordotonal organ of the stick insect

The femoral chordotonal organ in orthopterans signals proprioceptive sensory information concerning the femur-tibia joint to the central nervous system. In the stick insect, 80 out of 500 afferents sense tibial position, velocity, or acceleration. It has been assumed that the other sensory cells in the chordotonal organ would serve as vibration detectors. Extracellular recordings from the femoral chordotonal organ nerve in fact revealed a sensitivity of the sense organ for vibrations with frequencies ranging from 10 Hz to 4 kHz, with a maximum sensitivity between 200 and 800 Hz. Single vibration-sensitive afferents responded to the same range of frequencies. Their spike activity depended on acceleration amplitude and displacement amplitude of the vibration stimulus. Additionally, 80% of the vibration-sensitive afferents received indirect presynaptic inputs from themselves or from other afferents of the femoral chordotonal organ, the amplitude of which depended on stimulus frequency and displacement amplitude. They were associated with a decrease of input resistance in the afferent terminal. From the present investigation we conclude that the femoral chordotonal organ of the stick insect is a bifunctional sensory organ that, on the one hand, measures position and movement of the tibia and, on the other hand, detects vibration of the tibia. Accepted: 6 November 1998     

Temperature and the temporal resolving power of fly photoreceptors

A hot head gives an insect a clearer view of a moving world because warming reduces motion blur by accelerating photoreceptor responses. Over a natural temperature range, 19–34 °C, the speed of response of blowfly (Calliphora vicina) photoreceptors more than doubles, to produce the fastest functional responses recorded from an ocular photoreceptor. This acceleration increases temporal resolving power, as indicated by the corner frequency of the response power spectrum. When light adapted, the corner frequency increases from 53 Hz to 119 Hz with a Q ₁₀ of 1.9, and when dark adapted from 8 Hz to 32 Hz with a Q ₁₀ of 3.0. Temperature sensitivity originates in the phototransduction cascade, and is associated with signal amplification. The temperature sensitivity of photoreceptors must be taken into account when studying the mechanisms, function and ecology of vision, and gives a distinct advantage to insects that thermoregulate. Accepted: 2 February 2000     

A gain-control mechanism for processing of chorus sounds in the afferent auditory pathway of the bushcricket Tettigonia viridissima (Orthoptera; Tettigoniidae)

The representation of alternative conspecific acoustic signals in the responses of a pair of local interneurons of the bushcricket Tettigonia viridissima was studied with variation in intensity and the direction of sound signals. The results suggest that the auditory world of the bushcricket is rather sharply divided into two azimuthal hemispheres, with signals arriving from any direction within one hemisphere being predominantly represented in the discharge of neurons of this side of the auditory pathway. In addition, each pathway also selects for the most intense of several alternative sounds. A low-intensity signal at 45 dB sound pressure level is quite effective when presented alone, but completely suppressed when given simultaneously with another signal at 60 dB sound pressure level. In a series of intracellular experiments the synaptic nature of the intensity-dependent suppression of competitive signals was investigated in a number of interneurons. The underlying synaptic mechanism is based on a membrane hyperpolarisation with a time-constant in the order of 5–10 s. The significance of this mechanism for hearing in choruses, and for the evolution of acoustic signals and signalling behaviour is discussed. Accepted: 20 November 1999     

Spike timing dependent plasticity promotes synchrony of inhibitory networks in the presence of heterogeneity

Recently Haas et al. (J Neurophysiol 96: 3305–3313, 2006), observed a novel form of spike timing dependent plasticity (iSTDP) in GABAergic synaptic couplings in layer II of the entorhinal cortex. Depending on the relative timings of the presynaptic input at time t _pre and the postsynaptic excitation at time t _post, the synapse is strengthened (Δt = t _post − t _pre > 0) or weakened (Δt < 0). The temporal dynamic range of the observed STDP rule was found to lie in the higher gamma frequency band (≥40 Hz), a frequency range important for several vital neuronal tasks. In this paper we study the function of this novel form of iSTDP in the synchronization of the inhibitory neuronal network. In particular we consider a network of two unidirectionally coupled interneurons (UCI) and two mutually coupled interneurons (MCI), in the presence of heterogeneity in the intrinsic firing rates of each coupled neuron. Using the method of spike time response curve (STRC), we show how iSTDP influences the dynamics of the coupled neurons, such that the pair synchronizes under moderately large heterogeneity in the firing rates. Using the general properties of the STRC for a Type-1 neuron model (Ermentrout, Neural Comput 8:979–1001, 1996) and the observed iSTDP we determine conditions on the initial configuration of the UCI network that would result in 1:1 in-phase synchrony between the two coupled neurons. We then demonstrate a similar enhancement of synchrony in the MCI with dynamic synaptic modulation. For the MCI we also consider heterogeneity introduced in the network through the synaptic parameters: the synaptic decay time of mutual inhibition and the self inhibition synaptic strength. We show that the MCI exhibits enhanced synchrony in the presence of all the above mentioned sources of heterogeneity and the mechanism for this enhanced synchrony is similar to the case of the UCI.     

Biomechanical properties and a kinetic simulation model of the smooth muscle I2 in the buccal mass of Aplysia

The muscle I2 is a smooth muscle from the buccal mass of the marine mollusc Aplysia californica whose neural control, in vivo kinematics, and behavioral role have been extensively analyzed. In this study, we measured the activation and contractile dynamics of the muscle in order to construct a Hill-type kinetic model of the muscle. This is the first study to our knowledge, of Aplysia muscle contractile dynamics. The isometric force-frequency relationship of I2 had a frequency threshold of about 6–8 Hz, and its force output saturated at 20–25 Hz, properties that match the high frequency (20 Hz) bursts generated by the B31/B32 neurons that innervate it. Peak isometric force was generated at about 118% of the in situ relaxed length. These results and I2's estimated in vivo kinematics suggest that it generates maximum force at the onset of protraction. The muscle tension during iso-velocity lengthening and shortening was an asymmetric function of velocity. Short range stiffness and yielding responses were observed in lengthening, whereas muscle tension decreased smoothly in shortening. These visco-elastic properties suggest that the I2 muscle can serve to brake forceful retraction movements. A Hill-type model, parameterized from the measurements, captured many of the mechanical properties of I2. Our results provide a quantitative understanding of the biomechanical significance of the muscle's neural control and provide a basis for simulation studies of the control of feeding behavior. Received: 5 February 1999 / Accepted in revised form: 18 May 1999     

Pupillary dilation response as an indicator of auditory discrimination in the barn owl

The pupil of an awake, untrained, head-restrained barn owl was found to dilate in response to sounds with a latency of about 25 ms. The magnitude of the dilation scaled with signal-to-noise ratio. The dilation response habituated when a sound was repeated, but recovered when stimulus frequency or location was changed. The magnitude of the recovered response was related to the degree to which habituating and novel stimuli differed and was therefore exploited to measure frequency and spatial discrimination. Frequency discrimination was examined by habituating the response to a reference tone at 3 kHz or 6 kHz and determining the minimum change in frequency required to induce recovery. We observed frequency discrimination of 125 Hz at 3 kHz and 250 Hz at 6 kHz – values comparable to those reported by others using an operant task. Spatial discrimination was assessed by habituating the response to a stimulus from one location and determining the minimum horizontal speaker separation required for recovery. This yielded the first measure of the minimum audible angle in the barn owl: 3° for broadband noise and 4.5° for narrowband noise. The acoustically evoked pupillary dilation is thus a promising indicator of auditory discrimination requiring neither training nor aversive stimuli. Accepted: 28 February 2000     

Peripheral encoding of behaviorally relevant acoustic signals in a vocal fish: single tones

The midshipman fish, Porichthys notatus, generates acoustic signals for intraspecific communication. Nesting males produce long-duration “hums” which attract gravid females and can be effectively mimicked by pure tones. In this study we examine the encoding of tonal signals by the midshipman peripheral auditory system. Single-unit recordings were made from afferents innervating the sacculus while presenting sounds via an underwater loudspeaker. Units were characterized by iso-intensity spike rate and vector strength of synchronization curves, as well as by peri-stimulus time histograms. Additionally, response-intensity curves and responses to long-duration (up to 10 s) stimuli were obtained. As has been seen in other teleosts, afferents had highly variable activity profiles. Excitatory frequencies ranged from 60 to over 300 Hz with most units responding best around 70 or 140 Hz. Thresholds at 90 Hz ranged from 95 to 145 dB re 1 μPa. Strong synchronization provided a robust temporal code of frequency, comparable to that described for goldfish. Spike rate showed varying degrees of adaptation but high rates were generally maintained even for 10-s stimuli. The midshipman peripheral auditory system is well suited to encoding conspecific communication signals, but nonetheless shares many response patterns with the auditory system of other teleosts. Accepted: 10 February 1999     

Thalamocortical transformations of periodic stimuli: the effect of stimulus velocity and synaptic short-term depression in the vibrissa–barrel system

Recent works on the response of barrel neurons to periodic deflections of the rat vibrissae have shown that the stimulus velocity is encoded in the corti cal spike rate (Pinto et al., Journal of Neurophysiology, 83(3), 1158–1166, 2000; Arabzadeh et al., Journal of Neuroscience, 23(27), 9146–9154, 2003). Other studies have reported that repetitive pulse stimulation produces band-pass filtering of the barrel response rate centered around 7–10 Hz (Garabedian et al., Journal of Neurophysiology, 90, 1379–1391, 2003) whereas sinusoidal stimulation gives an increasing rate up to 350 Hz (Arabzadeh et al., Journal of Neuroscience, 23(27), 9146–9154, 2003). To explore the mechanisms underlying these results we propose a simple computational model consisting in an ensemble of cells in the ventro-posterior medial thalamic nucleus (VPm) encoding the stimulus velocity in the temporal profile of their response, connected to a single barrel cell through synapses showing short-term depression. With sinusoidal stimulation, encoding the velocity in VPm facilitates the response as the stimulus frequency increases and it causes the velocity to be encoded in the cortical rate in the frequency range 20–100 Hz. Synaptic depression does not suppress the response with sinusoidal stimulation but it produces a band-pass behavior using repetitive pulses. We also found that the passive properties of the cell membrane eventually suppress the response to sinusoidal stimulation at high frequencies, something not observed experimentally. We argue that network effects not included here must be important in sustaining the response at those frequencies.

Noise improves transfer of near-threshold, phase-locked activity of the cochlear nerve: evidence for stochastic resonance?

 Stochastic resonance can be described as improved detection of weak periodic stimuli by a dynamic nonlinear system, resulting from the simultaneous presentation of a restricted dynamic range of low-intensity noise. This property has been reported in simple physical and biological activities. The present study describes data consistent with the interpretation that stochastic resonance can be observed in the response of cochlear neurons. These experiments utilized low levels (−5 to 25 dB SPL) of stimuli and noise (5 to 30 dB SPL). Stimuli consisted of simultaneously presented 8 kHz (F ₁) and 8.8 kHz (F ₂) tone bursts, which generated an 800 Hz F ₂–F ₁ cochlear nerve envelope ensemble response in the gerbil. The mean response threshold was approximately −3 dB SPL. Simultaneous presentation of a low-intensity wideband noise increased the amplitude of this response. This was observed with tonal stimuli having intensities of 0–5 dB SPL; responses to stimulus levels >10 dB were attenuated by noise. Response amplitude was increased by noise levels of 10–15 dB; the amplitude was unaffected by lower levels of noise, and decreased in the presence of higher noise levels. These properties are compatible with those of stochastic resonance. Accepted: 11 March 1999     

Directionality of the tympanal vibrations in a cicada: a biophysical analysis

1. Laser vibrometry and acoustic measurements were used to study the biophysics of directional hearing in males and females of a cicada, in which most of the male tympanum is covered by thick, water filled tissue “pads”. 2. In females, the tympanal vibrations are very dependent on the direction of sound incidence in the entire frequency range 1–20 kHz, and especially at the main frequencies of the calling song (3–7 kHz). At frequencies up to 10 kHz, the directionality disappears if the contralateral tympanum, metathoracic spiracle, and folded membrane are blocked with Vaseline. This suggests some pressure-difference receiver properties in the ear. 3. In males, the tympanal vibrations depend on the direction of sound incidence only within narrow frequency bands (around 1.8 kHz and at 6–7 kHz). At frequencies above 10–12 kHz, the directionality appears to be determined by diffraction, and the ear seems to work as a pressure receiver. The peak in directionality at 6–7 kHz disappears when the contralateral timbal, but not the tympanum, is covered. Covering the thin ventral abdominal wall causes the peak around 1.8 kHz to disappear. 4. Most observed tympanal directionalities, except around 1.8 kHz in males, are well predicted from measured transmissions of sound through the body and measured values of sound amplitude and phase at the ears at various directions of sound incidence. Accepted: 18 October 1996     

Topographic analysis of dimension estimates of EEG and filtered rhythms in epileptic patients with complex partial seizures

Nonlinear dynamic properties were analyzed on the EEG and filtered rhythms recorded from healthy subjects and epileptic patients with complex partial seizures. Estimates of correlation dimensions of control EEG, interictal EEG and ictal EEG were calculated. The values were demonstrated on topograms. The delta (0.5–4 Hz), theta (4–8 Hz), alpha (8–13 Hz), beta (13–30 Hz) and gamma (30–40 Hz) components were obtained and considered as signals from the cortex. Corresponding surrogate data was produced. Firstly, the influence of sampling parameters on the calculation was tested. The dimension estimates of the signals from the frontal, temporal, parietal and occipital regions were computed and compared with the results of surrogate data. In the control subjects, the estimates between the EEG and surrogate data did not differ (P > 0.05). The interictal EEG from the frontal region and occipital region, as well as its theta component from the frontal region, and temporal region, showed obviously low dimensions (P < 0.01). The ictal EEG exhibited significantly low-dimension estimates across the scalp. All filtered rhythms from the temporal region yielded lower results than those of the surrogate data (P < 0.01). The dimension estimates of the EEG and filtered components markedly changed when the neurological state varied. For each neurological state, the dimension estimates were not uniform among the EEG and frequency components. The signal with a different frequency range and in a different neurological state showed a different dimension estimate. Furthermore, the theta and alpha components demonstrated the same estimates not only within each neurological state, but also among the different states. These results indicate that the theta and alpha components may be caused by similar dynamic processes. We conclude that the brain function underlying the ictal EEG has a simple mechanism. Several heterogeneous dynamic systems play important roles in the generation of EEG. Received: 10 December 1999 / Accepted in revised form: 8 May 2000     

Water wave discrimination in the surface-feeding fish Aplocheilus lineatus

The surface-feeding fish Aplocheilus lineatus uses its cephalic lateral line to detect water surface waves caused by prey insects. The ability of Aplocheilus to discriminate between surface waves with aid of the lateral line system was tested by go/no-go conditioning. Our results show that Aplocheilus can distinguish between single-frequency surface wave stimuli with equal velocity or equal acceleration amplitudes which differ only in frequency. Frequency difference limens were about 15%, i.e. fish distinguished a 20-Hz wave stimulus from a 23-Hz stimulus in 100% of the trials. Aplocheilus can also discriminate between pure sine-wave stimuli and sine waves which show abrupt frequency changes. In contrast, fish were unable to distinguish amplitude-modulated wave stimuli (carrier frequency 20, 40 and 60 Hz, modulation frequency 10 and 20 Hz) from pure sine waves of the same frequency, even if amplitude modulation depth was 80%. Accepted: 27 December 1996