Short and middle latency vestibular evoked responses to acceleration in man

We have succeeded in recording short and middle latency vestibular evoked responses in human subjects. The head was held rigidly in a special, patented head holder, constructed individually for each subject, which gripped the teeth of the upper jaw. The stimulus consisted of 2/sec steps of angular acceleration impulses produced by a special motor with intensities of about 10,000°/sec² and with a rise time of 1–2 msec. The electrical activity was recorded as the potential difference between special forehead and mastoid electrodes having a large, secure contact area with the skin. The activity was digitally filtered and averaged in 2 separate channels by means of a Microshev 2000 evoked response system. The short latency responses, with peaks at about 3.5 msec (forehead positive), 6.0 msec (forehead negative) and 8.4 msec (forehead positive; bandpass: 200–2000 Hz; average of 1024 trials), had amplitudes of about 0.5 μV. The middle latency responses had peaks at about 8.8 msec (forehead positive), 18.8 msec (forehead negative) and 26.8 msec (forehead positive; 30–300 Hz; N = 128 trials), with larger amplitudes (about 15 μV). These responses were consistently recorded in the same subject at different times and were similar in different normal subjects. Strenuous control experiments were conducted in order to ensure that these responses are not artefacts due to the movement of conducting media (head, electrodes and leads) in the electromagnetic field of the motor and are elicited by activation of normal labyrinths. Among other controls, they were not present in a cadaver, in patients with bilateral absence of nystagmus to caloric stimuli and in conducting volumes the size of the human head. They were also not masked by white noise.     

Vestibular-dependent spinal reflexes evoked by brief lateral accelerations of the heads of standing subjects

An impulsive acceleration stimulus, previously shown to activate vestibular afferents, was applied to the mastoid. Evoked EMG responses from the soleus muscles in healthy subjects (n = 10) and patients with bilateral vestibular dysfunction (n = 3) were recorded and compared with the effects of galvanic stimulation (GVS). Subjects were stimulated while having their eyes closed, head rotated, and while tonically activating their soleus muscles. Rectified EMG responses were recorded from the leg contralateral to the direction of head rotation. Responses were characterized by triphasic potentials that consisted of short-latency (SL), medium-latency (ML), and long-latency (LL) components beginning at (mean ± SD) 54.2 ± 4.8, 88.4 ± 4.7, and 121 ± 7.1 ms, respectively. Mean amplitudes for the optimum stimulus rise times were 9.05 ± 3.44% for the SL interval, 16.70 ± 4.41% for the ML interval, and 9.75 ± 4.89% for the LL interval compared with prestimulus values. Stimulus rise times of 14 and 20 ms evoked the largest ML amplitudes. GVS evoked biphasic responses (SL and ML) with similar latencies. Like GVS, the polarity of the initial interval was determined by the polarity of the stimulus and the evoked EMG response was attenuated when subjects were seated. There was no significant EMG response evoked when subjects were stimulated using 500-Hz vibration or in patients with bilateral vestibular dysfunction. Our study demonstrates that a brief lateral acceleration, likely to activate the utricle, can evoke spinal responses with properties similar to those previously shown for vestibular activation by GVS. The triphasic nature of the responses may allow the nervous system to respond differently to short compared with long-duration linear accelerations, consistent with their differing significance.     

The Role of Cervical and Ocular Vestibular Evoked Myogenic Potentials in the Assessment of Patients with Vestibular Schwannomas

Objectives

To investigate the clinical utility of VEMPs in patients suffering from unilateral vestibular schwannoma (VS) and to determine the optimal stimulation parameter (air conducted sound, bone conducted vibration) for evaluating the function of the vestibular nerve.

Methods

Data were obtained in 63 patients with non-operated VS, and 20 patients operated on VS. Vestibular function was assessed by caloric, cervical and ocular VEMP testing. 37/63 patients with conclusive ACS ocular VEMPs responses were studied separately.

Results

In the 63 non-operated VS patients, cVEMPs were abnormal in 65.1% of patients in response to AC STB and in 49.2% of patients to AC clicks. In the 37/63 patients with positive responses from the unaffected side, oVEMPs were abnormal in 75.7% of patients with ACS, in 67.6% with AFz and in 56.8% with mastoid BCV stimulation. In 16% of the patients, VEMPs were the only abnormal test (normal caloric and normal hearing). Among the 26 patients who did not show oVEMP responses on either side with ACS, oVEMPs responses could be obtained with AFz (50%) and with mastoid stimulation (89%).

Conclusions

The VEMP test demonstrated significant clinical value as it yielded the only abnormal test results in some patients suffering from a unilateral vestibular schwannoma. For oVEMPs, we suggest that ACS stimulation should be the initial test. In patients who responded to ACS and who had normal responses, BCV was not required. In patients with abnormal responses on the affected side using ACS, BCV at AFz should be used to confirm abnormal function of the superior vestibular nerve. In patients who exhibited no responses on either side to ACS, BCV was the only approach allowing assessment of the function of the superior vestibular nerve. We favor using AFz stimulation first because it is easier to perform in clinical practice than mastoid stimulation.     

The effect of stimulus type and background noise on hearing abilities of the round goby Neogobius melanostomus

The auditory abilities of the round goby Neogobius melanostomus were quantified using auditory evoked potential recordings, using tone bursts and conspecific call stimuli. Fish were tested over a range of sizes to assess effects of growth on hearing ability. Tests were also run with and without background noise to assess the potential effects of masking in a natural setting. Neogobius melanostomus detected tone bursts from 100 to 600 Hz with no clear best frequency in the pressure domain but were most sensitive to 100 Hz tone stimuli when examined in terms of particle acceleration. Responses to a portion of the N. melanostomus call occurred at a significantly lower threshold than responses to pure tone stimulation. There was no effect of size on N. melanostomus hearing ability, perhaps due to growth of the otolith keeping pace with growth of the auditory epithelium. Neogobius melanostomus were masked by both ambient noise and white noise, but not until sound pressure levels were relatively high, having a 5-10 dB threshold shift at noise levels of 150 dB re 1 μPa and higher but not at lower noise levels.     

Effects of Near-Threshold Stimulation of the Vestibular Apparatus on Postural Responses Evoked by Displacements of the Visualized Surrounding

In healthy subjects in the relaxed upward stance and perceiving a virtual visual environment (VVE), we recorded postural reactions to isolated visual and vestibular stimulations or their combinations. Lateral displacements of the visualized virtual scene were used as visual stimuli. The vestibular apparatus was stimulated by application of near-threshold galvanic current pulses to the proc. mastoidei of the temporal bones. Isolated VVE shifts evoked mild, nonetheless clear, body tilts readily distinguished in separate trials; at the same time, postural effects of isolated vestibular stimulation could be detected only after averaging of several trials synchronized with respect to the beginning of stimulation. Under conditions of simultaneous combined presentation of visual and vestibular stimuli, the direction of the resulting postural responses always corresponded to the direction of responses induced by VVE shifts. The contribution of an afferent volley from the vestibular organ depended on the coincidence/mismatch of the direction of motor response evoked by such a volley with the direction of response to visual stimulation. When both types of stimulations evoked unidirectional body tilts, postural responses were facilitated, and the resulting effect was greater than that of simple summation of the reactions to isolated actions of the above stimuli. In the case where isolated galvanic stimulation evoked a response opposite with respect to that induced by visual stimulation, the combined action of these stimuli of different modalities evoked postural responses identical in their magnitude, direction, and shape to those evoked by isolated visual stimulation. The above findings allow us to conclude that the effects of visual afferent input on the vertical posture under conditions of our experiments clearly dominate. In general, these results confirm the statement that neuronal structures involved in integrative processing of different afferent volleys preferably select certain type of afferentation carrying more significant or more detailed information on displacements (including oscillations) of the body in space.     

The cyclooxygenase inhibitors indomethacin and Rofecoxib reduce regional cerebral blood flow evoked by somatosensory stimulation in rats

The present study was designed to investigate whether administration of indomethacin (IMC), a non-selective cyclooxygenase (COX-1 and COX-2) inhibitor, and Rofecoxib, a highly selective COX-2 inhibitor, affect the regulation of regional cerebral blood flow response evoked by somatosensory activation (evoked rCBF). IMC and Rofecoxib were applied intravenously (6.25 and 3 mg/kg/hr, respectively). Somatosensory activation was induced by electrical hind paw stimuli of 0.2, 1, and 5 Hz (5-sec duration, 1.5 mA). The evoked rCBF was measured in alpha-chloralose anesthetized rats using laser-Doppler flowmetry. Before and after drug application, the evoked rCBF showed a frequency-dependent increase in the range of 0.2-5 Hz stimulation. IMC reduced significantly (about 50%-60%) evoked rCBF in response to all frequencies of hind paw stimulation (P< 0.05). Rofecoxib reduced significantly (about 50%) evoked rCBF in response to 1 and 5 Hz stimulation (P< 0.05), but did not affect evoked rCBF at 0.2 Hz. After IMC or Rofecoxib application, the normalized evoked rCBF curves peaked earlier as compared with that before their application (P< 0.05), although the rise time of 0.5 sec was nearly constant regardless of the stimulus frequency. The termination time of evoked rCBF curves was changed significantly after IMC application at 0.2 Hz stimulation (P< 0.05), but was not affected after Rofecoxib application. Neither COX inhibitor significantly affected the baseline level of CBF. The results suggest a participation of COX products in the regulation of evoked rCBF in response to somatosensory stimulation in the brain.     

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.     

Intracellular responses of antennal chordotonal sensilla of the American cockroach

The responses of mechanoreceptor neurons in the antennal chordotonal organ have been examined in cockroaches by intracellular recording methods. The chordotonal organ was mechanically stimulated by sinusoidal movement of the flagellum. Stimulus frequencies were varied between 0.5 and 150 Hz. Receptor neurons responded with spike discharges to mechanical stimulation, and were classed into two groups from plots of their average spike frequencies against stimulus frequency. Neurons in one group responded to stimulation over a wide frequency range (from 0.5 to 150 Hz), whereas those in a second group were tuned to higher frequency stimuli. The peak stimulus frequency at which receptor neurons showed maximum responses differed from cell to cell. Some had a peak response at a stimulus frequency given in the present study (from 0.5 to 150 Hz), whereas others were assumed to have peak responses beyond the highest stimulus frequency examined. The timing for the initiation of spikes or of a burst of spikes plotted against each stimulus cycle revealed that spike generation was phase-locked in most cells. Some cells showed phase-independent discharges to stimulation at lower frequency, but increasing stimulus frequencies spike initiation began to assemble at a given phase of the stimulus cycle. The response patterns observed are discussed in relation to the primary process of mechanoreception of the chordotonal organ.     

Differential sensitivity to rotation measured on potentials evoked by electrical stimulation of the guinea-pig ear

Responses to electrical stimulation of the ear applied between round-window and vertex electrodes were recorded in awake guinea-pigs from the same electrodes or from separate vertex/mastoid subdermal needle electrodes. They were averaged during opposite phases of sinusoidal rotation or before and after constant velocity rotation. In both cases the responses were subtracted from each other and yielded differential per- or post-rotatory “electrovestibular” responses. For comparison, responses were also recorded in the same animals and conditions of electrical stimulation during silence and during presentation of a broad-band noise. The difference yielded “electroacoustic” responses. In round-window records, electrovestibular and electroacoustic responses presented typical compound nerve action potential patterns. Electrovestibular responses could be recorded for head angular velocities as low as 3° sec⁻¹ at 0.1 Hz. Response amplitude showed a logarithmic relation to head velocity. Changes in amplitude, as a function of time after rotation, were comparable to those reported for vestibular nerve fibre responses. In vertex/mastoid records, electroacoustic responses presented a sequence of peaks similar to the click-evoked auditory brain-stem responses, and electrovestibular responses presented two peaks, presumably representing contributions of central vestibular structures. Such “electrovestibulography” permits the study of an individual ear and makes available to the investigator a large range of vestibular stimulation conditions.     

Enhancing human balance control with galvanic vestibular stimulation

 With galvanic vestibular stimulation (GVS), electrical current is delivered transcutaneously to the vestibular afferents through electrodes placed over the mastoid bones. This serves to modulate the continuous firing levels of the vestibular afferents, and causes a standing subject to lean in different directions depending on the polarity of the current. Our objective in this study was to test the hypothesis that the sway response elicited by GVS can be used to reduce the postural sway resulting from a mechanical perturbation. Nine subjects were tested for their postural responses to both galvanic stimuli and support-surface translations. Transfer-function models were fit to these responses and used to calculate a galvanic stimulus that would act to counteract sway induced by a support-surface translation. The subjects' responses to support-surface translations, without and with the stabilizing galvanic stimulus, were then measured. With the stabilizing galvanic stimulus, all subjects showed significant reductions in both sway amplitude and sway latency. Thus, with GVS, subjects maintained a more erect stance and followed the support-surface displacement more closely. These findings suggest that GVS could possibly form the basis for a vestibular prosthesis by providing a means through which an individual's posture can be systematically controlled. Received: 11 May 2000 / Accepted in revised form: 20 November 2000     

Correlation between auditory thalamic area evoked responses and species-specific call characteristics

Evoked potentials were recorded from the posterior dorsal thalamus of green treefrogs (Hyla cinerea) in response to single tones and combinations of two and three tones. 1. The responses to two tones were largest when one of the component tones was 500 Hz and when the second component was between 2000 and 4000 Hz (Fig.3). 2. The response to 500 + 3000 Hz showed nonlinear facilitation; i.e., the amplitude of the response was greater than the sum of the responses to the component tones alone (Figs. 4, 5). This result provides evidence that cells functioning as 'AND' gates will be found in this center. 3. When a third tone around 1200 Hz was added to a stimulus of 500 + 3000 Hz a 65% decrease in the evoked response amplitude occurred (Fig. 6). 4. The largest evoked response amplitude to a two-tone stimulus (500 + 3000 Hz) occurred when the rise-time was less than 50 ms (Fig. 7). 5. The two-tone tuning was found to be temperature dependent. The optimal lower frequency tone shifted downward with decreasing temperatures (Fig. 8). 6. When the temperatures of the neurophysiological and the behavioral experiments are matched, the optimal stimuli for evoking a large response are closely correlated to the parameters of the acoustic stimuli preferred by gravid H. cinerea females in discrimination tests. This center therefore appears to be very important for the processing of complex species-specific sounds.     

Stabilometry and definition of the threshold of galvanic vestibular stimulation in normal subjects: an experimental study

The effect of the galvanic stimulation on the vestibular apparatus has been evaluated by registration on the postural deviations, using a stabilometry platform. We have studied the galvanic body-sway responses in a group of normal subjects, using a binauricolar bipolar stimulation, with the electrodes attached by means of surgical tape to the mastoid area. The records of body-sway responses have demonstrated in 80% of the considered cases a significant variation of all positional parameters after a current intensity of 2 mA, according the body sways toward the positive stimulus. At the same current intensity only five of the studied subjects have shown multidirectional swinging, in three cases joined with a subjective slight sway toward the ear stimulated with positive polarity. Therefore the galvanic test, joined with the posturography, proves to be a useful auxiliary method in vestibular investigation, allowing us to lower the threshold of galvanic stimulation and to make the electric stimulus better supported for the patient.     

Behavioral and neurophysiological assessment of lateral line sensitivity in the mottled sculpin,Cottus bairdi

1. The unconditioned feeding response of the mottled sculpin, Cottus bairdi, was used to measure threshold sensitivity of the lateral line system to a vibrating sphere as a function of stimulus position (i.e., sphere near head, trunk or tail) and vibration frequency. In addition, extracellular recording techniques were used to measure threshold sensitivity curves for posterior lateral line nerve fibers for the same stimulus positions used for measuring trunk sensitivity in behavioral measurements. 2. For all stimulus positions, behaviorally-measured threshold sensitivity was relatively independent of vibration frequency from 10 to 100 Hz when defined in terms of water acceleration, rather than velocity or displacement. Best thresholds for stimuli placed 15 mm away from the head were around -75 dB re: 1m/s(2), approximately 20 dB less than that for stimuli placed at the same distance near the tail. Trunk sensitivity was intermediate. 3. Physiologically-measured threshold sensitivity, in terms of acceleration, was also relatively independent of of frequency from 10 to 100 Hz in most fibers. A smaller number of fibers showed a decline in acceleration sensitivity after 10-30 Hz, with the rate of decline being equivalent to equal velocity sensitivity. Best sensitivity of all fibers fell between -40 and -70 dB re: 1m/s (2). 4. These results indicate that (a) behavioral thresholds are based on acceleration-sensitive endorgans--most likely lateral line canal (rather than superficial) neuromasts, (b) behavioral performance can be accounted for on the basis of information from a single population of fibers, and (c) sensitivity varies along the fish's body in a manner that corresponds to the size and distribution of neuromasts.     

Electrical Vestibular Stimuli to Enhance Vestibulo-Motor Output and Improve Subject Comfort

Electrical vestibular stimulation is often used to assess vestibulo-motor and postural responses in both clinical and research settings. Stochastic vestibular stimulation (SVS) is a recently established technique with many advantages over its square-wave counterpart; however, the evoked muscle responses remain relatively small. Although the vestibular-evoked responses can be enhanced by increasing the stimulus amplitude, subjects often perceive these higher intensity electrical stimuli as noxious or painful. Here, we developed multisine vestibular stimulation (MVS) signals that include precise frequency contributions to increase signal-to-noise ratios (SNR) of stimulus-evoked muscle and motor responses. Subjects were exposed to three different MVS stimuli to establish that: 1) MVS signals evoke equivalent vestibulo-motor responses compared to SVS while improving subject comfort and reducing experimentation time, 2) stimulus-evoked vestibulo-motor responses are reliably estimated as a linear system and 3) specific components of the cumulant density time domain vestibulo-motor responses can be targeted by controlling the frequency content of the input stimulus. Our results revealed that in comparison to SVS, MVS signals increased the SNR 3–6 times, reduced the minimum experimentation time by 85% and improved subjective measures of comfort by 20–80%. Vestibulo-motor responses measured using both EMG and force were not substantially affected by nonlinear distortions. In addition, by limiting the contribution of high frequencies within the MVS input stimulus, the magnitude of the medium latency time domain motor output response was increased by 58%. These results demonstrate that MVS stimuli can be designed to target and enhance vestibulo-motor output responses while simultaneously improving subject comfort, which should prove beneficial for both research and clinical applications.     

Formalin injection causes a coordinated spinal cord CO/NO-cGMP signaling system response

Transcutaneous sine-wave stimuli at frequencies of 2000, 250 and 5 Hz (Neurometer) are thought to selectively activate Aβ, Aδ and C afferent fibers, respectively. However, there are few reports to test the selectivity of these stimuli at the cellular level. In the present study, we analyzed action potentials (APs) generated by sine-wave stimuli applied to the dorsal root in acutely isolated rat dorsal root ganglion (DRG) preparations using intracellular recordings. We also measured excitatory synaptic responses evoked by transcutaneous stimuli in substantia gelatinosa (SG) neurons of the spinal dorsal horn, which receive inputs predominantly from C and Aδ fibers, using in vivo patch-clamp recordings. In behavioral studies, escape or vocalization behavior of rats was observed with both 250 and 5 Hz stimuli at intensity of ~0.8 mA (T5/ T250), whereas with 2000 Hz stimulation, much higher intensity (2.14 mA, T2000) was required. In DRG neurons, APs were generated at T5/T250 by 2000 Hz stimulation in Aβ, by 250 Hz stimulation both in Aβ and Aδ, and by 5 Hz stimulation in all three classes of DRG neurons. However, the AP frequencies elicited in Aβ and Aδ by 5 Hz stimulation were much less than those reported previously in physiological condition. With in vivo experiments large amplitude of EPSCs in SG neurons were elicited by 250 and 5 Hz stimuli at T5/ T250. These results suggest that 2000 Hz stimulation excites selectively Aβ fibers and 5 Hz stimulation activates noxious transmission mediated mainly through C fibers. Although 250 Hz stimulation activates both Aδ and Aβ fibers, tactile sensation would not be perceived when painful sensation is produced at the same time. Therefore, 250 Hz was effective stimulus frequency for activation of Aδ fibers initiating noxious sensation. Thus, the transcutaneous sine-wave stimulation can be applied to evaluate functional changes of sensory transmission by comparing thresholds with the three stimulus frequencies.     

Dynamic characteristics of motor responses evoked by stimulation of the motor cortex in unanesthetized cats

Temporal characteristics of motor responses evoked in unanesthetized cats by stimulation of the motor cortex through bipolar needle electrodes were investigated in chronic experiments. Isometric and isotonic contractions of the flexor muscles of the hip and knee joints of the limb contralateral to the point of stimulation were recorded. The latent period of response varied from 100 msec or more in the case of low-frequency (100–150 Hz) and low-threshold (1.1–1.2 thresholds) stimulation of the motor cortex to 30–35 msec in the case of "optimal" parameters of stimulation (300–400 Hz, 1.5–1.6 thresholds). If the intensity of stimulation was high enough the rising time constant of evoked contraction was 50–80 msec; values of the falling time constant of muscular contraction after cessation of stimulation were much greater, namely 150–300 msec. The rising time constant of contraction decreased with an increase in both the frequency and strength of motor cortical stimulation. The results are examined and discussed from the standpoint of methods of automatic control theory.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 5, pp. 451–458, September–October, 1980.     

Electrophysiological and psychophysical quantification of temporal summation in the human nociceptive system

Animal experiments have shown that the nociceptive reflex can be used as an indicator of central temporal integration in the nociceptive system. The aim of the present study on humans was to investigate whether the nociceptive reflex, evoked by repetitive strong electrical sural nerve stimuli, increased when summation was reported by the volunteers. The reflexes were recorded from the biceps femoris and rectus femoris muscles in eight volunteers following a series of stimulations at 0.1, 1, 2, and 3 Hz. Each series consisted of five consecutive stimuli. Using 0.1- and 1-Hz stimulation, the reflex was not facilitated in the course of the five consecutive stimuli. Following 2- and 3-Hz stimulation, the reflex size (root mean square amplitude) increased significantly during the course of the fifth stimulus. This reflex facilitation was followed by a significant increase (summation) in the pain magnitude when compared with 1- and 0.1-Hz stimulation. Furthermore, the threshold for psychophysical summation could be determined. This threshold (stimulus intensity) decreased when the stimulus frequency (1–5 Hz) of the five consecutive stimuli was increased. The nociceptive reflex and the psychophysical summation threshold might be used to clarify and quantify aspects of temporal summation within the human nociceptive system.     

Stability of rapidly adapting afferent entrainment vs responsivity

Spike discharge activity was recorded from low-threshold, rapidly adapting, skin mechanoreceptive afferents (RA afferents) dissected from the median (forelimb) or tibial (hindlimb) nerves in anesthetized monkeys and cats. The spike activity was evoked by delivery of controlled sinusoidal vertical skin displacement ("flutter") stimuli to the receptive field (RF). The stimuli (15-30 Hz; 30-400 microm peak-to-peak amplitude; duration 0.8-15 s) were superimposed on a static skin indentation (0.5-1.0 mm) which was either maintained continuously throughout the run or applied trial-by-trial. The neural activity and the analog signal of the position of the stimulator probe were digitized at 10 kHz resolution and stored for off-line analysis. The main goal was to determine whether changes in the RA afferent response to skin flutter stimulation may be responsible for the enhanced capacity to discriminate stimulus frequency that accompanies a relatively brief (approximately 1 min) pre-exposure to such stimulation in humans. To this end, the spike train data were evaluated using methods that enabled independent measurement of entrainment and responsivity. Responsivity (response intensity) was measured as the average number of spikes/stimulus cycle, while entrainment (the degree to which evoked spike train activity is phase-locked to the stimulus) was quantitatively assessed using statistical techniques developed for the analysis of "circular" (directional) data, supplemented by methods based on the calculation of power spectra from point process data. The methods are demonstrated to enable quantification of RA afferent entrainment over a range of stimulus durations and amplitudes substantially greater than reported in previous studies. While RA afferent responsivity was found to decline to a minor extent (10-20%) both across and within stimulus trials, entrainment remained consistently high and stable, and exhibited no temporal trends or dependence on any other measured factor. The average phase angle of the entrained RA afferent response also remained stable both within and across trials, showing only a tendency to increase slightly during the initial 100-500 ms after stimulus onset. The results imply that the improved capacity to discriminate stimulus frequency that develops in response to an exposure to cutaneous flutter stimulation is not attributable to a change in RA afferent entrainment per se.     

Stability of rapidly adapting afferent entrainment vs responsivity

Spike discharge activity was recorded from low-threshold, rapidly adapting, skin mechanoreceptive afferents (RA afferents) dissected from the median (forelimb) or tibial (hindlimb) nerves in anesthetized monkeys and cats. The spike activity was evoked by delivery of controlled sinusoidal vertical skin displacement ("flutter") stimuli to the receptive field (RF). The stimuli (15-30 Hz; 30-400 mum peak-to-peak amplitude; duration 0.8-15 s) were superimposed on a static skin indentation (0.5-1.0 mm) which was either maintained continuously throughout the run or applied trial-by-trial. The neural activity and the analog signal of the position of the stimulator probe were digitized at 10 kHz resolution and stored for off-line analysis. The main goal was to determine whether changes in the RA afferent response to skin flutter stimulation may be responsible for the enhanced capacity to discriminate stimulus frequency that accompanies a relatively brief (approximately equal to 1 min) pre-exposure to such stimulation in humans. To this end, the spike train data were evaluated using methods that enabled independent measurement of entrainment and responsivity. Responsivity (response intensity) was measured as the average number of spikes/stimulus cycle, while entrainment (the degree to which evoked spike train activity is phase-locked to the stimulus) was quantitatively assessed using statistical techniques developed for the analysis of "circular" (directional) data, supplemented by methods based on the calculation of power spectra from point process data. The methods are demonstrated to enable quantification of RA afferent entrainment over a range of stimulus durations and amplitudes substantially greater than reported in previous studies. While RA afferent responsivity was found to decline to a minor extent (10-20%) both across and within stimulus trials, entrainment remained consistently high and stable, and exhibited no temporal trends or dependence on any other measured factor. The average phase angle of the entrained RA afferent response also remained stable both within and across trials, showing only a tendency to increase slightly during the initial 100-500 ms after stimulus onset. The results imply that the improved capacity to discriminate stimulus frequency that develops in response to an exposure to cutaneous flutter stimulation is not attributable to a change in RA afferent entrainment per se.     

Neuronal responses to turtle head rotation in vitro

Extracellular recordings were made during vestibular stimulation from an in vitro turtle brain stem in which the temporal bones remained attached. Under visual control, microelectrodes were slowly advanced into the vestibular nucleus (VN) while we rotated the brain and searched for a single isolated unit whose spike activity was modulated by the lateral semicircular canals. In some experiments, responses were shown to be due to stimulation of the lateral canals, either by positioning the brains in forward or backward pitch during horizontal rotation or by plugging the vertical canals with wax. VN neurons usually had low spontaneous activity and rectified sinusoidal responses to sinusoidal stimulation. Spike response histograms were averaged from many stimulus cycles and were then fit to a sine function. The fitted phase and amplitude parameters were plotted relative to stimulus frequency and amplitude. The sample of VN cells were quite heterogeneous. Using stimuli at 1 Hz, however, each cell's response phase was weakly correlated with the slope of the plots of response amplitude versus frequency so that a cell could be categorized as sensitive to velocity or acceleration and as sensitive to ipsiversive or contraversive rotation, depending on whether its phase was near −180°, −90°, 0°, or 90°, and whether the gain exceeded 0.4 spikes/s per °/s. The properties of these VN cells suggest that there is substantial complexity in the vestibular responses at this first site of central vestibular processing. These data are compared to that of other species where such vestibular signals play an important role in oculomotor and spinal reflexes. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 99–177, 1997