Auditory sensitivity of the diencephalon of the leopard frogRana p. pipiens

Summary Evoked potentials were recorded from the posterocentral nucleus in the dorsal diencephalon of leopard frogs (Rana p. pipiens) in response to acoustic stimulation. This electrophysiological study confirms the anatomical study by Neary (1974) of the existence of an auditory area within this nucleus.The response of the auditory thalamic area showed a selectivity for stimuli that simultaneously excited both the amphibian and the basilar papillae in the inner ear. The magnitude of the evoked potential to the combination of either low (300 Hz) and high (1 700 Hz) or mid (600 Hz) and high (1700 Hz) frequency tones was much greater than the sum of the responses to the component tones individually (Fig. 5). This selective convergence is not seen in the torus semicircularis: in this midbrain center the sum of the responses to the individual tones is approximately equal to the magnitude of the response to the combination tone (Fig. 7).The selectivity of the thalamic center for stimuli with patterned energy distributions is compared to the spectral combinations occurring within several of this species' vocal signals. This comparison indicates that the extraction of spectral patterns involves a hierarchical organization within the anuran's auditory system which probably plays a major role in processing complex sounds.This research was supported by the U.S. Public Health Service (NIH Research Grant NS-09244). We would like to thank Anne Moffat for her assistance in collecting data on the tuning characteristics of the VIIIth nerve units.     

P300-like potentials in the normal monkey using classical conditioning and an auditory ‘oddball’ paradigm

Endogenous components of evoked potentials resembling P300 in humans were sequentially studied in 3 cynomolgus monkeys (Macaca fascicularis) using an auditory ‘oddball’ paradigm. The two different auditory stimuli were 500 Hz and 4000 Hz tones, designated as the ‘frequent’ and ‘rare’ stimuli, respectively. The probability of ‘rare’ tone presentation was initially 0.2. We further used probabilities of 0.1, 0.3 and 0.5. The ‘rare’ stimulus was reinforced by electrical stimulation, which followed the onset of the high tone by 700 msec. After 3–5 training sessions, a late positive wave was observed following the ‘rare’ tone. The latency of this P300-like signal was 314±16.2 msec, and teh amplitude 23.6±3.14 μV. The amplitude of this potential was modified by changes in stimulus presentation probability and by withholding reinforcement.     

Voltage responses to tones of outer hair cells in the basal turn of the guinea-pig cochlea: significance for electromotility and desensitization.

Voltage responses were recorded from outer hair cells (OHCS) in the basal coil of the guinea-pig cochlea in response to tones at frequencies above the characteristic frequency (CF) presented together with a 100 Hz tone at 80 dB or 85 dB sound pressure level (SPL). The amplitude and polarity of voltage responses to a 100 Hz, 85 dB SPL tone were altered when presented together with tones at frequencies above CF according to the frequency and level of the high-frequency tone, OHC phasic (ac) (greater than 500 microV) but not tonic (dc) voltage responses were elicited by the high-frequency tone. Thus the responses of OHCS to low-frequency tones can be altered when presented together with a high-frequency tone without an apparent dc change in membrane potential. Recordings were made from an OHC during cochlear desensitization through exposure to an intense tone. The maximum voltage response to high-level low-frequency tones remained unchanged, although the OHC response to high-frequency tones became less sensitive to low-level stimuli and more linear as a function of level. It is suggested that desensitization is associated with a change in the mechanical properties of the cochlea, possibly associated with the OHCS themselves, and not with inactivation of the transducer channels. The amplitude of the OHC ac voltage response was measured at neural threshold, and the consequences of these measurements on hair cell electromotility are considered.     

Neuromagnetic evidence for early auditory restoration of fundamental pitch

Background

Understanding the time course of how listeners reconstruct a missing fundamental component in an auditory stimulus remains elusive. We report MEG evidence that the missing fundamental component of a complex auditory stimulus is recovered in auditory cortex within 100 ms post stimulus onset.

Methodology

Two outside tones of four-tone complex stimuli were held constant (1200 Hz and 2400 Hz), while two inside tones were systematically modulated (between 1300 Hz and 2300 Hz), such that the restored fundamental (also knows as “virtual pitch”) changed from 100 Hz to 600 Hz. Constructing the auditory stimuli in this manner controls for a number of spectral properties known to modulate the neuromagnetic signal. The tone complex stimuli only diverged on the value of the missing fundamental component.

Principal Findings

We compared the M100 latencies of these tone complexes to the M100 latencies elicited by their respective pure tone (spectral pitch) counterparts. The M100 latencies for the tone complexes matched their pure sinusoid counterparts, while also replicating the M100 temporal latency response curve found in previous studies.

Conclusions

Our findings suggest that listeners are reconstructing the inferred pitch by roughly 100 ms after stimulus onset and are consistent with previous electrophysiological research suggesting that the inferential pitch is perceived in early auditory cortex.     

Cross-correlation and latency compensation analysis of click-evoked and frequency-following brain-stem responses in man

Cross-correlation (CC) and latency compensation (LC) analyses were applied to the human click-evoked brain-stem auditory evoked response (BAER) and the brain-stem frequency-following response (FFR). FFRs were elicited by pure tone stimuli (230 Hz and 460 Hz) or by complex tones derived from the sum of 3rd (920 Hz), 4th (1150 Hz), and 5th (1380 Hz) harmonics of the missing 230 Hz fundamental. The lower and upper harmonics always began in sine phase, while the middle harmonic varied in starting phase, resulting in harmonically complex stimuli with differing amplitude and phase patterns.Cross-correlations were computed between individual trials and a wave form t emplate (smoothed wave V for BAER, pure tone stimulus sinusoids for FFR). Trials were included in the analysis only if values of r² exceeded 0.5 (negative values of r were thus included, which controlled for the chance occurrence of positive correlations). Although brain-stem recordings are noisy, requiring as many as 1000 stimuli/average, correlation analysis consistently identified more positive than negative trials (approximately 2:1 ratio). Trials were also deleted if the lag associated with the selected r² was at the maximum shift position (‘extreme lag’).Averaging trials that satisfied the correlation and lag criteria led to sizeable enhancement of BAER (mean = 114%) and FFR (mean = 68% for 230 Hz stimulus) amplitudes. LC analysis resulted in additional, albeit smaller, increases in amplitude (approximately 10%). FFRs to harmonically complex stimuli were characterized by a clear periodicity at the missing fundamental frequency (230 Hz). However, amplitudes varied according to the modulation depth of the stimulus and, in certain cases, actually exceeded that of the FFR response to a 230 Hz pure tone.The results demonstrate the effectiveness of cross-correlation and, to a lesser degree, latency compensation analysis, applied to two classes of brain-stem potentials. It is anticipated that such techniques will prove useful in the study of auditory signal processing at the level of the brain-stem.     

P300 in young and elderly subjects: Auditory frequency and intensity effects

Auditory event-related potentials (ERPs) were assessed in young and elderly subjects when stimulus intensity (40 vs. 60 dB SL) and standard/target tone frequency (250/500 Hz and 1000/2000 Hz) were manipulated to study the effects of these variables on the P3(00) and N1, P2 and N2 components. Auditory thresholds for each stimulus type were obtained, and the stimulus intensity was adjusted to effect perceptually equal intensities across conditions for each subject. Younger subjects demonstrated larger P3 amplitudes and shorter latencies than elderly subjects. The low frequency stimuli produced larger P3 amplitude and shorter latencies than the high frequency stimuli. Low intensity stimuli yielded somewhat smaller P3 amplitudes and longer peak latencies than high intensity stimulus tones. Although additional stimulus intensity and frequency effects were obtained for the N1, P2 and N2 components, these generally differed relatively little with subject age. The findings suggest that auditory stimulus parameters contribute to P3 measures, which are different for young compared to elderly subjects.     

Mismatch field to tone pairs: neuromagnetic evidence for temporal integration at the sensory level

The mismatch field (MMF) to minor pitch changes in two experimental conditions was studied. Standard tones of 1000 Hz and deviant tones of 1050 Hz both of 50 ms duration were delivered in single tone condition. Paired tones of the same duration were used in the paired tone condition. The standard tone pair consisted of two 1000 Hz tones, whereas the deviant tone pair was composed of a 1000 Hz tone in the first position and a 1050 Hz tone in the second position with a silent interval of 15 ms between the two. Standards of 90% and deviants of 10% probability were presented in random order and with a randomized interstimulus interval between 600 and 900 ms. The source analysis showed a more lateral location for the MMF obtained in the paired tone condition (MMF.P) compared to the MMF elicited by the single deviants (MMF.S). The source location of both the MMF.P and MMF.S turned out to be significantly anterior relative to the sources of the M100. The increased stimulus repetition in the paired tone condition (two times more stimuli than in the single tone condition) lead to a strong suppression of the field amplitude and of the dipole moment of the M100, while this effect could not be seen for the MMF. The data demonstrate a fundamental difference between the processes reflected by the M100 and the MMF: while the M100 represents the processing of every individual tone, the MMF reflects the change detection of the paired stimuli as unitary events, forming a perceptual group. The different sources of the MMF.P and MMF.S also support an integrated processing of the paired stimuli.     

Information processing at a central synapse suggests a noise filter in the auditory pathway of the noctuid moth

1. The central projections of the A1 afferent were confirmed via intracellular recording and staining with Lucifer Yellow in the pterothoracic ganglion of the noctuid moths, Agrotis infusa and Apamea amputatrix (Fig. 1). Simultaneous recordings of the A1 afferent in the tympanal nerve (extracellularly) and in the pterothoracic ganglion (intracellularly) confirm the identity of the stained receptor as being the A1 cell. 2. The major postsynaptic arborizations of interneurone 501 in the pterothoracic ganglion were also demonstrated via intracellular recording and staining (Fig. 2). Simultaneous recordings of the A1 afferent (extracellularly) and neurone 501 (intracellularly) revealed that each A1 spike evokes a constant short latency EPSP in the interneurone (Fig. 2Bi). Neurone 501 receives only monaural input from the A1 afferent on its soma side as demonstrated by electrical stimulation of each afferent nerve (Fig. 2Bii). EPSPs evoked in neurone 501 by high frequency (100 Hz) electrical stimulation of the afferent nerve did not decrement (Fig. 2Biii). These data are consistent with a monosynaptic input to neurone 501 from the A1 afferent. 3. The response of neurone 501 to a sound stimulus presented at an intensity near the upper limit of its linear response range (30 ms, 16 kHz, 80 dB SPL) was a plateau-like depolarization, with tonic spiking activity which continued beyond the end of the tone. The instantaneous spike frequency of the response was as high as 800 Hz, and was maintained at above 600 Hz for the duration of the tone (Fig. 3). 4. The relationship between the instantaneous spike frequency in the A1 afferent and that recorded simultaneously in neurone 501 is linear over the entire range of A1 spike frequencies evoked by white noise sound stimuli (Fig. 4). Similarly, the relationship between instantaneous spike frequency in the A1 afferent and the mean depolarization evoked in neurone 501 is also linear for all A1 spike frequencies tested (Fig. 5). No summation of EPSPs occurred for A1 spike frequencies below 100 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)     

Correlation between auditory evoked responses in the thalamus and species-specific call characteristics

This evoked potential study of the bullfrog's auditory thalamic area (an auditory responsive region in the posterior dorsal thalamus) shows that complex processing, distinct from that reported in lower auditory regions, occurs in this center. An acoustic stimulus consisting of two tones, one which stimulates either the low-frequency or the mid-frequency sensitive population of auditory nerve fibers from the amphibian papilla and the other the high-frequency sensitive population of fibers from the basilar papilla, evoked a maximal response. The amplitude of the response to the simultaneous stimulation of the two auditory organs was, in some locations, much larger than the linear sum of the responses to the individual tones presented separately. Bimodal spectral stimuli that had relatively long rise-times (greater than or equal to 100 ms) evoked much larger responses than similar sounds with short rise-times. The optimal rise-times were close to those occurring in the bullfrog's mating call. The response was dependent on the waveform periodicity and harmonic content, with a fundamental frequency of 200 Hz producing a larger response than those with fundamentals of 50, 100 or 300 Hz. Six of the natural calls in the bullfrog's vocal repertoire were tested and the mating call and warning call were found to evoke the best responses. Each of these calls stimulate the two auditory organs simultaneously. The evoked response had a long refractory period which could not be altered by lesioning the efferent telencephalic pathways. The type of spectral and temporal information extracted by the auditory thalamic area suggests that this center is involved in processing complex sounds and likely plays an important role in the bullfrog's detection of some of its vocal signals.     

Fast habituation of the long-latency auditory evoked potential in the awake albino rat

Fast habituation of the long-latency, vertex-recorded auditory evoked potential (AEP) peaks in humans was first described by Callaway (1973) as a reduction in AEP amplitude that occurs to the second of a pair of acoustic stimuli when both stimuli are presented with an interstimulus interval (ISI) of no more than 10 sec. When acoustic stimuli are presented in pairs with an ISI of 2 sec and an interpair interval (IPI) of approximately 10 sec, reduction in amplitude to the second tone occurs by as much as 30–50%. Fast habituation may depend somewhat on a subject's anticipation of the stimulus and on other factors related to attention and orienting. Studies in our laboratory have demonstrated this amplitude decrement to the second tone of a pair in human infants, children and adults and have explored the implications of this finding with respect to attentional processes and the allocation of cerebral resources. In the present investigation we describe an animal model of fast habituation. Here, vertex-recorded AEPs were obtained to paired tone stimuli delivered to awake adult male Sprague-Dawley rats chronically implanted with skull electrodes. Findings showed: (a) an AEP wave form with 8 distinct peaks, (b) for one component there was a marked decrement in amplitude from tone 1 to tone 2 in recordings obtained from an electrode placed slightly to the right of midline, and (c) that there were no significant differences in peak latencies across tones. This methodology may further our understanding of fast habituation in humans and may prove useful for studies of attention, orienting, and resource allocation using techniques that are not possible for use with human subjects.     

Auditory responses in the torus semicircularis of the cane toad, Bufo marinus. I. Field potential studies

Field potentials have been recorded in the torus semicircularis of the toad, Bufo marinus, in response to brief tones presented in the free field. The amplitude of the potentials varied with the frequency of the stimulus and location of the electrode along the rostro-caudal axis of the torus. All frequencies in the auditory range evoked largest potentials when the stimulus was located in the contralateral auditory field. Potentials evoked by low to mid frequencies were largest when the stimulus was located near the line orthogonal to the long axis of the animal. For progressively higher frequencies, the optimal stimulus position was progressively more anterior in the contralateral field. In animals in which one eighth nerve had been sectioned, field potentials evoked by tones of low to mid frequency were less sensitive to changes in stimulus direction than in normal animals. However, the directional sensitivity of field potentials evoked by mid to high frequencies was similar in monaural and normal animals. These observations suggest that binaural neural integration is important in determining the directional sensitivity of field potentials in the torus evoked by low to mid frequencies but not for potentials evoked by mid to high frequencies.     

Properties of auditory brainstem responses evoked by intra-operative electrical stimulation of the cochlear nucleus in human subjects

Electrically evoked auditory brainstem response (EABR) testing can aid placement of the stimulating electrodes during surgical implantation of an auditory brainstem implant (ABI). To facilitate efficient testing, this study of EABR properties examined the effects of various stimulating and recording parameters on the magnitude and clarity of the EABRs obtained from 9 successive ABI patients during intra-operative monitoring. Both stimulus polarities elicited EABRs; the response waveforms were similar and no significant differences between the latencies were found. Stimulus-response relationships displayed thresholds and non-linear growth, characteristic of neural activity, and provided a stimulus amplitude that elicited readily detectable EABRs in all subjects. The stimulus rate could be increased without degrading the EABRs, but usually 50 Hz was used with a 10 ms sampling sweep so that muscle responses, which occurred later than EABRs, could be detected. When 3 stimulating electrodes in a line were tested, the pair with the largest separation consistently provided the largest response. A recording filter passband of 10–3000 Hz was useful for attenuating interference signals because there is negligible energy in the EABR at frequencies above 3 kHz, but there is some energy below 100 Hz.     

The habituation of event-related potentials to speech sounds and tones

We examined the short- and long-term habituation of auditory event-related potentials (ERPs) elicited by tones, complex tones and digitized speech sounds (vowels and consonant-vowel-consonant syllables). Twelve different stimuli equated in loudness and duration (300 msec) were studied. To examine short-term habituation stimuli were presented in trains of 6 with interstimulus intervals of 0.5 or 1.0 sec. The first 4 stimuli in a train were identical standards. On 50% of the trains the standard in the 5th position was replaced by a deviant probe stimulus, and on 20% of the trains the standard in the 6th position was replaced by a target, a truncated standard that required a speeded button press response.Short-term habituation (STH) was complete by the third stimulus in the train and resulted in amplitude decrements of 50–75% for the N1 component. STH was partially stimulus specific in that amplitudes were larger following deviant stimuli in the 5th position than following standards. STH of the N1 was more marked for speech sounds than for loudness-matched tones or complex tones at short ISI. In addition, standard and deviant stimuli that differed in phonetic structure showed more cross-habituation than did tones or complex tones that differed in frequency. This pattern of results suggests that STH is a function of the acoustic resemblance of successive stimuli.The long-term habituation (LTH) of the ERP was studied by comparing amplitudes across balanced 5.25 m stimulus blocks over the course of the experiment. Two types of LTH were observed. The N1 showed stimulus-specific LTH in that N1 amplitudes declined during the presentation of a stimulus, but returned to control levels when a different stimulus was presented in the subsequent condition. In contrast, the P3 elicited by the deviant stimuli showed non-specific LTH, being reduced across successive blocks containing different stimuli. P3s elicited by target stimuli remained stable in amplitude.     

P300, probability,and the three-tone paradigm

The effects of stimulus probability on P300 from a 3-tone paradigm were examined in two experiments. Experiment 1 manipulated the probability of the non-target tone as 0.10, 0.45, or 0.80, while the target tone probability was always 0.10. Experiment 2 manipulated the probability of 3 tones as 0.10, 0.30, or 0.60, with one of the infrequent tones assigned as the target in each condition. Subjects were required to press a button in response to the target stimulus in both experiments. The results indicated that the P300 to the target and the non-target were both affected by the probability of the eliciting stimulus, such that component amplitude was inversely related to probability; no reliable P300 latency effects were found. Target tones elicited larger P300 amplitude than the non-target tones at the same probability. The findings suggest that probability effects on P300 amplitude are independent of responding to a specific target stimulus and are discussed with reference to the clinical utility of the 3-tone paradigm.     

Spectral and temporal gating mechanisms enhance the clutter rejection in the echolocating bat,Rhinolophus rouxi

Summary Doppler shift compensation behaviour in horseshoe bats, Rhinolophus rouxi, was used to test the interference of pure tones and narrow band noise with compensation performance. The distortions in Doppler shift compensation to sinusoidally frequency shifted echoes (modulation frequency: 0.1 Hz, maximum frequency shift: 3 kHz) consisted of a reduced compensation amplitude and/or a shift of the emitted frequency to lower frequencies (Fig. 1).Pure tones at frequencies between 200 and 900 Hz above the bat's resting frequency (RF) disturbed the Doppler shift compensation, with a maximum of intererence between 400 and 550 Hz (Fig. 2). Minimum duration of pure tones for interference was 20 ms and durations above 40 ms were most effective (Fig. 3). Interfering pure tones arriving later than about 10 ms after the onset of the echolocation call showed markedly reduced interference (Fig. 4). Doppler shift compensation was affected by pure tones at the optimum interfering frequency with sound pressure levels down to –48 dB rel the intensity level of the emitted call (Figs. 5, 6).Narrow bandwidth noise (bandwidth from ± 100 Hz to ± 800 Hz) disturbed Doppler shift compensation at carrier frequencies between –250 Hz below and 800 Hz above RF with a maximum of interference between 250 and 500 Hz above resting frequency (Fig. 7). The duration and delay of the noise had similar influences on interference with Doppler shift compensation as did pure tones (Figs. 8, 9). Intensity dependence for noise interference was more variable than for pure tones (-32 dB to -45 dB rel emitted sound pressure level, Fig. 10).The temporal and spectral gating in Doppler shift compensation behaviour is discussed as an effective mechanism for clutter rejection by improving the processing of frequency and amplitude transients in the echoes of horseshoe bats.Abbreviations CF constant frequency - FM frequency modulation - RF resting frequency - SPL sound pressure level     

The mismatch negativity for duration decrement of auditory stimuli in healthy subjects

The amplitude and latency of the mismatch negativity (MMN) elicited by occasional shorter-duration tones (25 and 50 ms) in a sequence of 75 ms standard tones were studied in 40 healthy subjects (9–84 years). The replicability and age dependence of the MMN-responses were determined. The 25 ms deviant tone evoked a clear response in 39 of the subjects, while the 50 ms deviant tone evoked an observable MMN only in 32 of the subjects. The MMN peak amplitude for the 25 ms deviants was significantly larger than for the 50 ms deviants. There was no significant difference in the peak latencies (measured from stimulus offset). For the 25 ms deviant, the amplitude diminished with increasing age. The MMN curves for the 25 ms deviant, measured on separate days in 14 subjects, looked very replicable. As a result of noise and filtering effect, the product-moment correlations were poor. The results indicate that the signal-to-noise ratio for the MMN to 25 ms deviants, obtained even in a 25 min recording session, is large enough for clinical use and individual diagnostics when undetectable (or very low amplitude) MMN is used as a sign of pathology. However, judged from the low correlation coefficients, despite the good replicability in visual evaluation, better methods for MMN quantification have to be used for clinical follow-up.     

Steady-state auditory evoked potentials (SSAEPs) in the rabbit. Contribution of the inferior colliculus

Steady-state auditory evoked potentials (SSAEPs) were recorded in rabbits with both surface and depth electrodes. Surface recording from the bregma provided the largest and most typical SSAEPs as compared to other surface locations when a stimulus rate of 50 Hz was used. The medial geniculate body (MGB) showed no potential corresponding to the surface SSAEP. On the other hand, the latency of SSAEP in the inferior colliculus (IC) corresponded closely to that of the surface potential. Furthermore, the amplitude of the IC potential tended to become large with the stimulus rate of 50 Hz as compared with transient stimuli. Although other auditory nuclei in the brain-stem, the ventral nucleus of the lateral lemniscus, the trapezoid body and the auditory nerve responded to transient stimuli with an amplitude larger than that of the IC, no amplification occurred with 50 Hz stimuli in these nuclei. These findings suggest that the IC contributes to the generation of SSAEP to a great extent.     

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.     

Auditory event-related potentials to deviant stimuli during drowsiness and stage 2 sleep

Twelve subjects were tested using a 3-tone auditory oddball paradigm consisting of a standard 1000 Hz tone (P = 80%) and two deviants, namely, a 1200 Hz tone and a 2000 Hz tone (both P = 10%). Testing took place in 3 conditions: (1) attend, in which the subject had to count one of the deviant tones; (2) ignore, in which the subject read a book; and (3) sleep, in which the subject was encouraged to go to sleep during presentation of the tones.In the awake conditions stimulus deviance elicited mismatch negativity (MMN) and P3. During drowsiness, no separate mismatch negativity (MMN) could be detected, but the 2000 Hz tone evoked a broad fronto-central early negative deflection, suggesting an overlap of N₁ and MMN. In the same condition, P210, N330 and P430 appeared, all being sensitive to magnitude of deviance. During stage 2, the P210, N330 and P430 amplitudes increased, most notably to the large deviant.These data indicate that differential processing of auditory inputs is maintained during drowsiness and stage 2 sleep, but do not support the notion that MMN or P3 activity comparable to the waking state occurs to oddball stimuli during this stage. It is hypothesised that during light sleep, scanning of the environment is performed by a different system than in the awake state and that during drowsiness a gradual switch between these two systems takes place.     

Effect of stimulus rate on the cortical posterior tibial nerve SEPs: a topographic study

We performed topographical mapping of somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation delivered at 2, 5 and 7.5 Hz in 15 healthy subjects. P37 was significantly attenuated at 5 and 7.5 Hz and the N50 component attenuated only at 5 Hz, its amplitude remaining stable for further increases in stimulus frequency. Frontal N37 and P50 potentials showed no significant decrease when the stimulus repetition frequency was changed from 2 to 7.5 Hz. P60 showed an attenuation of the amplitude only at 7.5 Hz. Latency and scalp topographies of all cortical components examined remained uncharged for the 3 stimulus rates tested The optimal stimulus rate for mapping of tibial nerve SEPs was lower than 5 Hz. The distinct recovery function of the contralateral N37-P50 and ipsilateral P37-N50 responses suggests that these potentials arise from separate generators