Tone Burst Stimulation Research Articles

Characterization of age-related changes in vestibular evoked myogenic potentials

A tone-burst stimulation of 500 Hz seems to be clinically most appropriate to elicit vestibular evoked myogenic potentials (VEMPs) because those VEMPs can be recorded at the lowest stimulus intensity possible. However, little is known about gender and age-related changes of the amplitude in tone-burst (500 Hz) evoked VEMPs. The aim of the present paper was therefore to investigate the influence of gender and age on VEMP amplitude in relation to the tonic muscle activity. VEMPs of 64 healthy subjects were recorded ipsilaterally during air- or bone-conducted tone burst stimulation. The EMG of the tonically activated sternocleidomastoid muscle was recorded ipsilaterally with surface electrodes. Averages were taken for P1/N1 amplitudes of male and female volunteers within 3 different age groups. Although the amplitude decreased with increasing age the tonic activity was not significant different between the age groups. Consequently the relation between VEMP amplitude and tonic muscle activity decreased with increasing age. The normative values of the age-dependent relation between VEMP amplitude and tonic muscle activity were described by the 90% confidence interval of the individual values. Normative thresholds were calculated. Normal saccular receptor function could be diagnosed if the VEMP amplitude is above (or equal to) the normative value at a given tonic muscle activity and age. Normative data as described above are required to diagnose isolated saccular defects, which are indicative of a vestibular disorder.

Eye movements induced by short tone burst stimulation: A video-oculography study

Eye movements induced by short tone burst stimulation: A video-oculography study

Vestibular-Evoked Myogenic Potentials in Patients With Otosclerosis Using Air- and Bone-Conducted Tone-Burst Stimulation

Otosclerosis is a progressive disease with a remodeling process causing ossicular malformation and conductive hearing loss. The aim of this study was to investigate whether vestibular-evoked myogenic potential (VEMP) correlates with the progression of otosclerosis. Fifteen patients with otosclerosis (21 ears) without operation and 10 healthy subjects (20 ears) underwent VEMP test using air-conducted (AC) and bone-conducted (BC) tone-burst stimulation. Tertiary referral university hospital. In 21 unoperated otosclerotic ears, 5 ears (24%) showed present AC-VEMPs, and 16 ears had absent AC-VEMPs. Conversely, 16 ears (76%) displayed present BC-VEMPs and 5 ears with absent BC-VEMPs. In those with both AC- and BC-VEMPs, none of them showed air-bone gap greater than 30 dB; in those with absent AC-VEMPs but present BC-VEMPs, 27% of the ears had air-bone gap greater than 30 dB; and in those with absence of both AC- and BC-VEMPs, 80% of the ears revealed air-bone gap greater than 30 dB. Thus, a significant relationship existed among the presence of AC-VEMPs, BC-VEMPs, and magnitude of conductive hearing loss. The presence of an AC-VEMP may indicate an earlier stage of otosclerosis, although absent BC-VEMP infers a later stage. Restated, AC-VEMPs may complement the results obtained with BC-VEMPs to classify the stage of otosclerosis.

Vestibular evoked myogenic potentials in middle ear effusion

Conclusion. The vestibular evoked myogenic potential (VEMP) responses were significantly affected by middle ear effusion (MEE). The VEMP was either non-responsive or significantly delayed. However, after the conductive hearing loss (CHL) was significantly reduced with tympanic aspiration, the VEMP latencies and asymmetry ratio returned to the range of healthy controls. In addition, the recovery of VEMP response was prompt and immediate. Objective. To evaluate the VEMP in various degrees of CHL resulting from MEE and to clarify the responses after the CHL was reduced. Materials and methods. VEMPs were collected from 21 unilateral subjects with MEE (8 male, 13 female, aged 21–79 years) using unilateral tone-burst stimulation. The pure tone audiograms and tympanograms were recorded. The effusion was then cleared with tympanocentesis. The VEMP responses were compared to those of the healthy volunteers and the responses after the MEE was reduced by tympanic aspiration. Results. The pure tone hearing thresholds decreased significantly with tympanic aspiration. Before tympanic aspiration, the VEMP response rate was 67% and increased significantly to 95% (p<0.05) after tympanic aspiration. The latencies of p13 and n23 were significantly prolonged before tympanic aspiration (p<0.01), and shortened to the normal range after tympanic aspiration. The VEMP asymmetry ratio was significantly decreased after tympanic aspiration (p<0.05).

Comparison of the Head Elevation Versus Rotation Methods in Eliciting Vestibular Evoked Myogenic Potentials

Because active and tonic sternocleidomastoid (SCM) muscle contraction is essential for recording the vestibular evoked myogenic potential (VEMP), false-negative VEMPs are sometimes encountered in those who cannot sustain SCM muscle contraction by head elevation. Hence, the goal of this study was to investigate whether the effortless head rotation method can replace the head elevation method in eliciting VEMP responses. Twenty healthy volunteers underwent VEMP testing, using monaural tone burst stimulation. First, the subject was instructed to keep the head elevated in the pitch plane for recording, followed by rotating the head sideways toward one shoulder as head down in the yaw plane for another recording (elevation-rotation sequence). On the next day, VEMP testing was performed in reverse order (rotation-elevation sequence). Twelve patients with cochleo-vestibular disorders were also enrolled in this study and underwent VEMP testing, using the two methods in random order. In the elevation-rotation sequence for subjects without cochleo-vestibular disorders, the response rate for the elevation method (100%) was significantly higher than the response rate for the rotation method (70%). In contrast, no significant difference existed in the response rate between the two methods in the rotation-elevation sequence (85% versus 88%). Comparison of the response rates for the initial elevation (100%) and initial rotation methods (85%) revealed a significantly lower response rate for the initial rotation method. Mean latencies of the onset waveform and peaks p13 and n23 showed significant differences between the two methods when using the elevation-rotation sequence but no differences on the rotation-elevation sequence. Furthermore, the rotation method displayed significantly smaller amplitude than the elevation method when using the elevation-rotation sequence but no significant difference in amplitude between the two methods when applying rotation-elevation sequence. In 12 patients with cochleo-vestibular disorders, the response rates for the elevation method (67%) and rotation method (58%) were significantly reduced compared with the rates for subjects without cochleo-vestibular pathology. However, when either the elevation or the rotation method response was considered, VEMPs were present in 11 (92%) of the 12 patients with cochleo-vestibular disorders. The head rotation method may serve as an alternative for eliciting VEMPs in those who cannot sustain SCM muscle contraction by head elevation. However, the lower response rate with smaller amplitude prevents the use of the head rotation method as an initial screening test for VEMPs. We therefore recommend that when VEMP responses cannot be elicited by the head elevation method, the head rotation method should be utilized to reduce false-negative results.

Eye movements induced by short tone burst stimulation: a video-oculography study

Eye movements induced by short tone burst stimulation: a video-oculography study

Normative data for P1/N1-latencies of vestibular evoked myogenic potentials induced by air- or bone-conducted tone bursts

Objective The response characteristics of acoustically elicited vestibular evoked myogenic potentials (VEMPs) largely depend on the stimuli applied. A tone-burst stimulation of 500 Hz seems to be clinically most appropriate because those VEMPs can be elicited at the lowest stimulus intensity possible. The aim of the present paper was to describe normative data for tone-burst evoked VEMPs. Methods VEMPs of 64 healthy subjects were recorded ipsilaterally during air- or bone-conducted tone burst stimulation. The EMG of the tonically activated sternocleidomastoid muscle was recorded ipsilaterally by surface electrodes. Averages were taken for P1/N1-latencies and -amplitudes of male and female volunteers within 3 different age groups. Results The latencies did not show any significant differences between female and male volunteers or between air- and bone-conducted stimulation. The latencies did also not show any significant difference among the 3 age groups. The limits for normal latencies (mean+2 SD) are, therefore, 20.3 ms for P1 and 28.0 ms for N1. Although the P1/N1-amplitudes were decreased with increasing age, the tonic muscle activity was not significantly different between the age groups. Conclusions The present findings strongly suggest the evaluation of VEMP latencies by using normative values obtained exactly with the same stimulus parameters. Significance Normative data as described in the present study are required to detect isolated saccular defects which are indicative of a vestibular disorder.

Radiation-induced otitis media—study of a new test, vestibular-evoked myogenic potential

Purpose Excluding the radiation history, current physical examination and audiovestibular function tests fail to differentiate radiation-induced otitis media (ROM) from chronic otitis media (COM). This study applied the newly developed vestibular-evoked myogenic potential (VEMP) test to investigate whether the VEMP test can be of help in differentiating between them. Methods and materials Fourteen irradiated nasopharyngeal carcinoma (NPC) patients with ROM (18 ears) and 14 age-matched, and gender-matched patients with COM (18 ears) were enrolled. Each patient underwent stimulation with a short tone burst initially, and then by tapping the forehead using a tendon hammer. To define the effect of the neck soft tissue on the VEMP response further, an additional two groups (10 non–NPC patients with ROM but no neck fibrosis/edema and 10 non–NPC patients without ROM but with neck fibrosis/edema) were included for comparison. Results The occurrence of normal VEMPs in the ROM ears (33%) did not differ significantly from that in the COM ears (56%) by tone-burst stimulation. However, using tapping evocation, most (89%) COM ears revealed normal VEMPs, and most (61%) ROM ears demonstrated delayed VEMPs. Furthermore, most (90%) non–NPC patients with ROM but no neck fibrosis/edema revealed delayed or absent VEMPs. In contrast, all non–NPC patients with neck fibrosis/edema but no ROM demonstrated normal VEMPs bilaterally, implying that the effect of neck soft tissue on the VEMP response is less. Conclusion Through tapping evocation, most COM ears revealed normal VEMPs, and most ROM ears demonstrated delayed VEMPs, indicating that ROM is different from COM because of the larger affected areas such as retrolabyrinthine or brainstem involvement. This result can explain why grommet insertion or tympanoplasty is not beneficial to postirradiated ears, possibly because both operations spare the inflammation outside the middle ear cavity.

Common sensory inputs and differential excitability of segmentally homologous reticulospinal neurons in the hindbrain.

In the hindbrain of zebrafish and goldfish, reticulospinal (RS) neurons are arranged in seven segments, with segmental homologs in adjacent segments. The Mauthner cell (M-cell) in the fourth segment (r4) is known to trigger fast escape behavior. Its serial homologs, MiD2cm in r5 and MiD3cm in r6, are predicted to contribute to this behavior, which can be evoked by head-tap stimuli. However, little is known about their input-output properties. Therefore, we studied afferent projections from the auditory posterior eighth nerve (pVIIIn) and firing properties of MiD2cm and MiD3cm for comparison with the M-cell in adult goldfish. Labeling of RS neurons and the pVIIIn afferents with fluorescent tracers showed that the pVIIIn projected to r4-r6. Tone burst and electrical stimulation of the pVIIIn evoked EPSPs in the M-cell, MiD2cm, and MiD3cm. Stepwise depolarization typically elicited a single spike at the onset in the M-cell but repetitive spiking in MiD2cm and MiD3cm. This atypical property of the M-cell was mediated by dendrotoxin-I (DTX-I)-sensitive voltage-gated potassium channels together with recurrent inhibition, because combined application of DTX-I, strychnine, and bicuculline led to continuous repetitive firing in M-cells. The M-cell but not MiD2cm or MiD3cm expressed Kv1.2, a DTX-I-sensitive potassium channel subunit. Thus, the M-cell and its segmental homologs may sense common auditory information but send different outputs to the spinal circuits to control adaptive escape behavior.

Galvanic and acoustic vestibular stimulation activate different populations of vestibular afferents.

To deduce whether similar or distinct populations of vestibular afferents are activated by acoustic and galvanic vestibular stimulation by comparing the effectiveness of 'matched' stimuli in eliciting vestibulospinal reflexes. Twelve subjects (5 men, 7 women) underwent individual 'matching' of 2 ms tone burst and galvanic stimuli, using vestibulocollic reflexes so that corrected reflex amplitudes to tone burst and galvanic stimuli were within 10% of each other. These same intensities were then administered using 20 ms durations to determine whether they were equally effective in evoking vestibulospinal responses. Corrected reflex amplitudes for vestibulocollic responses to tone burst and galvanic stimulation were not significantly different for the right (P=0.45) or left (P=0.68) sides. All subjects had vestibulospinal responses to galvanic stimulation (average intensity 4.0 mA for both sides). The short latency (SL) and medium latency (ML) components of the vestibulospinal reflexes were larger after galvanic compared to tone burst stimulation in 11 of 12 subjects (P<0.01). Despite evoking equal-sized vestibulocollic reflexes, there was a clear dissociation between the magnitude of tone burst and galvanic-induced vestibulospinal reflexes. Galvanic stimulation evoked SL and ML reflexes in all subjects. Tone burst stimuli evoked only small SL reflexes and, in most cases, no ML reflexes. Acoustically-evoked vestibulocollic reflexes are likely to be due to saccular excitation. The limited effectiveness of longer tone burst stimuli to evoke ML vestibulospinal reflexes suggests that saccular afferents have, at most, only a minor role in the production of these reflexes. We conclude that galvanic stimulation is more effective in eliciting vestibulospinal reflexes than tone burst stimulation, and that the two methods activate different populations of vestibular afferents.

Auditory cortical response patterns to multiple rhythms of AM sound.

Multiple amplitude modulation (AM) of a tone results in a time series with a complex structure of envelope fluctuations. In this study the cortical responses elicited by such an auditory stimulus were investigated. It was hypothesized that the auditory cortex responds simultaneously with different patterns of activity corresponding to the different rhythm of periodical stimulus fluctuations. The relations of these response patterns to either steady-state responses (SSR) in the 40 Hz range or the transient slow auditory evoked responses were investigated. Auditory evoked magnetic fields were recorded from the left temporal cortex of nine healthy humans in response to 250 Hz tones that were amplitude modulated with the sum of two sinusoids of 38 and 40 Hz. The stimulus was presented continuously to the subject's right ear at the intensity of 70 dB sensation level. In addition, the responses to 250 Hz tone-burst stimuli with duration of 20, 50, 100, and 200 msec, respectively, were recorded. The tone-bursts were presented at an inter-stimulus interval (ISI) of 0.5 sec, which was equivalent to the slowest rhythm of the multiple-AM stimulus. Magnetic source analysis was applied to all elicited waveforms. From all subjects SSR at 38 and 40 Hz were clearly recognizable and were superimposed by a periodical response pattern corresponding to 2 Hz stimulus envelope fluctuations. A positive peak with latency of 70 msec was the mostly pronounced response to tone-burst stimulation at the ISI of 0.5 sec and resembles the low-frequency response pattern to the AM stimulus. The N1-P2 complex showed an almost complete suppression at 0.5 sec ISI. In contrast, it was the most prominent evoked response activity when using a longer ISI of 3 sec. Source locations in overlapping areas were estimated for the SSR in the 40 Hz range, the prominent peak of low-frequency response to the AM stimulus, and the P1 peak at tone-burst stimulation, respectively. These were separated from the N1 source location by about 0.5 cm in anterior and 0.5 cm in medial direction. As a result of multiple AM the stimulus signal envelope fluctuates with frequencies of 2 Hz and around 40 Hz. The auditory cortex responds to this stimulus with different response patterns. Both modulation signals at 38 and 40 Hz were almost linearly represented as steady-state responses. The evoked response patterns to stimulus fluctuations at low-frequency rhythm resemble the slow cortical auditory evoked responses. The 2 Hz stimulus rhythm is perceived as loudness fluctuation, and 40 Hz modulation is perceived as a rough sound. It is assumed that the different cortical representations correspond to both types of perception of the complex AM sound.

Vestibular evoked myogenic potentials are intact after sudden deafness.

To evaluate vestibular evoked myogenic potentials (VEMPs) in cases of sudden deafness, and to confirm the noncochlear origin of the VEMPs. Prospective study. VEMPs, which were evoked by short tone burst (95 dB nHL) stimulation, were recorded in 20 patients with unilateral, idiopathic sudden deafness. The results of the deaf ears were compared with those of the contralateral healthy ears and the normal control ears. The relations between VEMPs and the hearing level or caloric response were then investigated. All 20 of the deaf ears displayed normal biphasic VEMPs. The mean latencies of p13 and n23, as well as mean amplitude p13-n23, were 15.1 +/- 2.8 msec, 20.7 +/- 3.3 msec and 25.2 +/- 12.6 microV, respectively, not significantly different to either the contralateral healthy ears (p > 0.05) or the normal control ears (p > 0.05). Five deaf ears displayed canal paresis or absent caloric response, whereas the remaining 15 ears revealed normal caloric response. All the lesioned ears of patients with idiopathic sudden deafness exhibit normal biphasic VEMPs. Neither the hearing level nor the caloric response correlated to the VEMPs.

Click and Low‐, Middle‐, and High‐Frequency Toneburst Stimulation of the Canine Cochlea

A method was developed to deliver tonebursts ranging in frequency from 1 to 32 kHz for frequency‐specific assessment of the canine cochlea. Brainstem auditory‐evoked responses (early latency responses, 0–10 ms) to a click (CS) and to 1‐, 2‐, 4‐, 8‐, 12‐, 16‐, 24‐, and 32‐kHz toneburst stimulations (TS) were compared at 80‐dB sound pressure level stimulus (SPL) intensity in 10 adult dogs. All stimulations yielded a 5–7 positive wave pattern, with the exception of the 1‐kHz TS, which evoked a frequency‐following response (FFR). Thresholds were lowest for the CS and the 12‐ and 16‐kHz TS. All individual peak latencies for TS were significantly (P≤ .05) longer than for CS. Peak I latencies were significantly (P≤.05) shorter for the 12‐ and 16‐kHz TS than for the other TS. Interpeak latencies I‐V were significantly (P≤ .05) longer for the 4‐ to 32‐kHz TS than for CS. Differences in interpeak latencies I—III were not significant. Amplitudes of waves I and V were significantly (P≤ .05) lower for TS than for CS, except for higher wave V amplitude (P≤ .05) at 2‐ and 32‐kHz TS. Peak I‐V amplitude ratios were significantly (P≤ .05) higher for the 2‐, 4‐, 16‐, 24‐, and 32‐kHz TS and lower for the 8‐ and 12‐kHz TS, compared to CS. We conclude that reproducible information on frequency specificity of the canine cochlea can be obtained by TS. This report provides a normative database for parameters needed to evaluate frequency‐specific hearing loss in dogs.

Central auditory onset responses, and temporal asymmetries in auditory perception

Historically, central auditory responses have been studied for their sensitivity to various parameters of tone and noise burst stimulation, with response rate plotted as a function of the stimulus variable. The responses themselves are often quite brief, and locked in time to stimulus onset. In the stimulus amplitude domain, it has recently become clear that these responses are actually driven by properties of the stimulus’ onset transient, and this has had important implications for how we interpret responses to manipulations of tone (or noise) burst plateau level. That finding was important in its own right, but a more general scrutiny of the available neurophysiological and psychophysical evidence reveals that there is a significant asymmetry in the neurophysiological and perceptual processing of stimulus onsets and offsets: sound onsets have a more elaborate neurophysiological representation, and receive a greater perceptual weighting. Hypotheses about origins of the asymmetries, derived independently from psychophysics and from neurophysiology, have in common a response threshold mechanism which adaptively tracks the ongoing level of stimulation.

Click and low-, middle-, and high-frequency toneburst stimulation of the canine cochlea.

A method was developed to deliver tonebursts ranging in frequency from 1 to 32 kHz for frequency-specific assessment of the canine cochlea. Brainstem auditory-evoked responses (early latency responses, 0-10 ms) to a click (CS) and to 1-, 2-, 4-, 8-, 12-, 16-, 24-, and 32-kHz toneburst stimulations (TS) were compared at 80-dB sound pressure level stimulus (SPL) intensity in 10 adult dogs. All stimulations yielded a 5-7 positive wave pattern, with the exception of the 1-kHz TS, which evoked a frequency-following response (FFR). Thresholds were lowest for the CS and the 12- and 16-kHz TS. All individual peak latencies for TS were significantly (P < or = .05) longer than for CS. Peak I latencies were significantly (P < or = .05) shorter for the 12- and 16-kHz TS than for the other TS. Interpeak latencies I-V were significantly (P < or = .05) longer for the 4- to 32-kHz TS than for CS. Differences in interpeak latencies I-III were not significant. Amplitudes of waves I and V were significantly (P < or = .05) lower for TS than for CS, except for higher wave V amplitude (P < or = .05) at 2- and 32-kHz TS. Peak I-V amplitude ratios were significantly (P < or = .05) higher for the 2-, 4-, 16-, 24-, and 32-kHz TS and lower for the 8- and 12-kHz TS, compared to CS. We conclude that reproducible information on frequency specificity of the canine cochlea can be obtained by TS. This report provides a normative database for parameters needed to evaluate frequency-specific hearing loss in dogs.

Rate influences on tone burst summating potential amplitude in electrocochleography: clinical a and experimental b data

Electrocochleographic recordings of action and summating potentials are widely used in the electrophysiological assessment of endolymphatic hydrops (ELH). Increased amplitudes of the summating potential (SP) in response to tone burst stimuli are indicative of positive ELH. This study reports the effect of repetition rate of tone burst stimulation on the SP amplitude. Using transtympanic electrocochleography (ECochG), the SP in response to 1 kHz tone bursts was recorded in both a Ménière and a non-Ménière population. Absolute values of the SP were systematically higher in the Ménière group. Moreover, in the Ménière and non-Ménière groups, the response amplitudes of the SP at a repetition rate of 8.4 tone bursts/s were only 66 and 32%, respectively, of the maximal response amplitude which was obtained at the rate of 37.4 tone bursts/s. Additionally, in normal guinea pigs chronically implanted with a round window electrode, the SP was recorded to 0.5–16 kHz tone burst stimulations presented at 100 dB SPL with the same different repetition rates. Similar enhancement of the SP amplitude was observed from 8.4 to 37.4 stimuli/s, whatever the frequency. This effect is interpreted as an increased asymmetry of vibration of the cochlear partition, whose mechanical operating point would not return to the normal resting position at high repetition rates, since it is permanently shifted in ELH.

Tonotopic cortical changes following stapes substitution in otosclerotic patients: a magnetoencephalographic study.

The aim of the study was to investigate and follow up the tonotopic organization of the primary auditory cortex in otosclerotic patients before and after corrective surgery. The characteristics of primary auditory cortex activation were studied in ten otosclerotic patients (i.e., subjects suffering from a conductive hearing loss, prior to and following stapes substitution). Magnetoencephalographic recordings of auditory evoked fields by tone-burst stimulation at octave frequencies between 250 and 2000 Hz were performed during monaural stimulation. The brain topography of the main cortical response (N100m) generators at different tones was studied in patients and compared with ten healthy controls; pre- post-surgical changes were also correlated to their clinical outcome following corrective surgery. A significant decrease of the tonotopic extension in the cortical region responsive to the four explored frequencies was found in patients before surgery with respect to the control population. At the time of postsurgical follow-up, the tonotopic representation had enlarged and was approaching the dimensions seen in normal subjects, although with higher variability. The extent of the enlargement of the postoperative tonotopically organized area was directly correlated with the postsurgery period duration. Our findings indicate that auditory cortical areas of human adults undergo functional reorganization following peripheral alteration of the sensory input entering the CNS. The restriction of the cortical tonotopic region caused by the long-term reduction of acoustic input is followed by a reorganization within the usual boundaries following the recovery of auditory function; this process is taking place in a time scale of a few weeks.

Age-related changes in the brainstem auditory evoked potentials of the marmoset

Changes in brain-stem auditory evoked potentials (BAEPs) with age were recorded in common marmosets ( Callithrix jacchus) at the age of 1–2, 6–8 and 10–12 years. The auditory function was assessed by thresholds, latencies and amplitudes of BAEPs evoked by use of tone burst stimulations with audible frequencies ranging from 1 to 99 kHz. Prolongation of the latencies of later waves was observed in the animals at the age of 6–8 and 10–12 years at high frequencies, suggesting that aging in marmosets, as reported previously in humans and other animals, may cause earlier hearing loss at high frequency than at low frequency within the hearing range. At 10–12 years of age, the elevations of BAEP thresholds and the declines of BAEP amplitudes in older animals were also observed. As the differences in the parameters are small, it was suggested that only a moderate hearing loss occurred with onset late in life in common marmoset similar to that in CBA/Ca mice. Based on the results obtained in this study, BAEP latencies appear to be more sensitive indicators than BAEP thresholds and amplitudes for the early detection of hearing impairment.

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In vivo intracellular characteristics of inferior colliculus neurons in guinea pigs

Intracellular in vivo recordings of physiologically identified inferior colliculus central nucleus (ICc) auditory neurons ( n=71) were carried out in anesthetized guinea pigs. The neuronal membrane characteristics are described showing mainly quantitative differences with a previous report [Nelson, P.G. and Erulkar, S.D., J. Neurophysiol., 26 (1963) 908–923]. The spontaneous spike activity was consistent with the discharge pattern of most extracellularly recorded units. The action potentials showed different spike durations, short and long, and some of them exhibited hyperpolarizing post-potentials. There were also differences in firing rate. The ICc neurons exhibited irregular activity producing spike trains as well as long silent periods (without spikes). Intracellular current injection revealed membrane potential adaptation and shifts that outlasted the electrical stimuli by 20–30 ms. Both evoked synaptic potentials and the spike activity in response to click and tone-burst stimulation were analyzed. Depolarizing-hyperpolarizing synaptic potentials were found in response to contralateral and binaural sound stimulation that far outlasted the stimulus (up to 90 ms). When ipsilaterally stimulated, inhibitory responses and no-responses were also recorded. Although few cells were studied, a similar phenomenon was observed using tone-burst stimulation; moreover, a good correlation was obtained between membrane potential shifts and the triggered spikes (input–output relationship). These in vivo results demonstrate the synaptic activity underlying many of the extracellularly recorded discharge patterns. The data are consistent with the known multi-synaptic ascending pathway by which signals arrive at the ICc as well as the descending corticofugal input that may contribute to the generation of long duration post-synaptic potentials.