Gender differences in medium-latency acoustic evoked potentials

Medium-latency acoustic (auditory) evoked potentials (MLAEPs) were recorded in 30 men and 30 women. The MLAEPs recorded in the left and right mastoid derivations were found to be asymmetrical, the lateral differences depending on the sex: binaural stimulation and stimulation of the right ear yielded a higher total amplitude of the set of medium-latency components in the right derivation in men and in the left derivation in women. If the left ear was stimulated, there were no sex-related differences in MLAEP asymmetry. The data are discussed in terms of gender differences with respect to functional specialization of the cerebral hemispheres.     

Brain-stem auditory evoked potentials (BAEPs) from basal surface of temporal lobe recorded from chronic subdural electrodes

BAEPs were recorded from the basal surface of the temporal lobe by subdural electrodes chronically implanted in 6 patients who were evaluated for surgical management of intractable partial seizures. Near-field recordings were obtained by recording between the subdural electrode closet and most distant to the brain-stem. Far-field recordings were obtained by recording between the subdural electrodes and an indifferent electrode over the spinal process of the seventh cervical vertebrae. The recordings were compared with standard ear-vertex recordings.After ipsilateral ear stimulation, the subdural electrode closet to the brain-stem recorded large amplitude waves I and II, followed by less well-defined waves of longer latencies. Recordings to contralateral stimulation showed no clearly defined waves I and II and a large amplitude wave Vn. Waves III, IV, V, Vn and VI were of opposite polarity after ipsi- and contralateral stimulation. These findings indicate that waves I and II are generated ipsilaterally to the stimulation side, whereas wave Vn has a contralateral origin. Wave Vn may be generated in the brachium of the inferior colliculus, as suggested from latency and from dipole configuration studies. This conclusion is consistent with the classical anatomical observations that the supracollicular auditory pathways are predominantly crossed.     

Altered Neuronal Intrinsic Properties and Reduced Synaptic Transmission of the Rat's Medial Geniculate Body in Salicylate-Induced Tinnitus

Sodium salicylate (NaSal), an aspirin metabolite, can cause tinnitus in animals and human subjects. To explore neural mechanisms underlying salicylate-induced tinnitus, we examined effects of NaSal on neural activities of the medial geniculate body (MGB), an auditory thalamic nucleus that provides the primary and immediate inputs to the auditory cortex, by using the whole-cell patch-clamp recording technique in MGB slices. Rats treated with NaSal (350 mg/kg) showed tinnitus-like behavior as revealed by the gap prepulse inhibition of acoustic startle (GPIAS) paradigm. NaSal (1.4 mM) decreased the membrane input resistance, hyperpolarized the resting membrane potential, suppressed current-evoked firing, changed the action potential, and depressed rebound depolarization in MGB neurons. NaSal also reduced the excitatory and inhibitory postsynaptic response in the MGB evoked by stimulating the brachium of the inferior colliculus. Our results demonstrate that NaSal alters neuronal intrinsic properties and reduces the synaptic transmission of the MGB, which may cause abnormal thalamic outputs to the auditory cortex and contribute to NaSal-induced tinnitus.     

Electrophysiological investigation of conduction of afferent impulses through the medial geniculate body

Extra- and intracellular reactions of 280 neurons of the pars principalis of the medial geniculate body (MGB) and of 408 auditory cortical neurons in area AI to stimulation of the inferior brachium of the midbrain and geniculocortical fibers were studied in cats immobilized with D-tubocurarine. Single electrical stimulation of the inferior brachium was shown to evoke a long and complex neuronal response in MGB in the form of excitation of some and inhibition of other neurons. The initial component of this response lasted 13 msec. Excitation of 72% of neurons participating in the response took place during the first 3 msec after the beginning of stimulation. In the same period 84% of IPSP arose. The inferior brachium was shown to contain a certain number of descending fibers. Some of them are axons of MGB neurons. Many fibers of the inferior brachium reach the auditory cortex without synaptic relay in MGB. Of all cells of MGB excited by stimulation of the inferior brachium monosynaptically, 76% are thalamocortical relay neurons; the rest are interneurons. Of the relay neurons of MGB 90% are excited monosynaptically, the rest by impulses passing through two or three synaptic relays in MGB. During stimulation of the inferior brachium, responses consisting of EPSP-IPSP and primary IPSP are recorded in many neurons of MGB. About 20% of primary IPSP arise monosynaptically, evidently in response to stimulation of inhibitory fibers of the inferior brachium. Most IPSP arise disynaptically, with the participation of an inhibitory interneuron located at the entrance to MGB. Inhibition observed in this case is direct afferent in nature.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 6, pp. 515–523, November–December, 1979.     

Effect of repetition rate on middle latency auditory evoked potentials in humans

Middle Latency Auditory Evoked Potentials (MLAEPs) were recorded from 15 healthy subjects in order to evaluate the influence of different repetition rates on the latency and the amplitude of their main components Na, Pa and Nb. MLAEPs were obtained from Cz-ipsilateral ear lobe by averaging responses to 2000 monaural clicks delivered to both ears, at 65 dB SL of intensity, for each of 3 different repetition rates (1.1, 4.1, 8.1 Hz). Time base was 100 ms, analogical band-pass filter 5-1000 Hz (off-line digital bandpass: 20-100 Hz). The statistical analysis (repeated measures analysis of variance), demonstrated that, the latency and the amplitude of the Nb component were slightly influenced by repetition rate while Pa and Na were not. Moreover Nb showed the greatest interindividual variability (as already pointed out by other authors too); thus, we suggest that a stimulus rate of 8.1 Hz and the analysis of Na and Pa component only, can be regarded as the best assessment for MLAEPs evaluation when they are used for clinical purposes.     

Human middle-latency auditory evoked potentials: vertex and temporal components

We recorded middle-latency (20–70 msec) auditory evoked potentials (MLAEPs) to monaural and binaural clicks in 30 normal adults (ages 20–49 years) at 32 scalp locations all referred to a balanced non-cephalic reference. Our goal was to define the MLAEP components that were present at comparable latencies and comparable locations across the subject population. Group and individual data were evaluated both as topographic maps and as MLAEPs at selected electrode locations.Three major components occurred between 20 and 70 msec, two well-known peaks centered at the vertex, and one previously undefined peak focused over the posterior temporal area. Pa is a 29 msec positive peak centered at the vertex and present with both monaural and binaural stimulation, Pb is a 53 msec positive peak also centered at the vertex but seen consistently only with binaural and right ear stimulation. TP41 is a 41 msec positive peak focused over both temporal areas. TP41 has not been identified in previous MLAEP studies that concentrated on central scalp locations and/or used active reference electrode sites such as ears or mastoids.Available topographic, intracranial, pharmacologic, and lesion studies indicate that Pa, Pb and TP41 are of neural origin. Whether Pa and/or Pb are produced in Heschl's gyrus, primary auditory cortex, remains unclear. TP41 is probably produced by auditory cortex on the posterior lateral surface of the temporal lobe. It should prove of considerable value in experimental and clinical evaluation of higher level auditory function in particular and of cortical function in general.     

Dynamics of MLAEP changes in midazolam-induced sedation

This study aimed at assessing the effects of midazolam (MDZ) sedation on auditory brainstem (BAEP) and middle latency (MLAEP) evoked potentials in intensive care conditions. Ten ventilated comatose patients were receiving an intravenous MDZ bolus dose (0.2 mg/kg) followed by a 2 h continuous infusion (0.1 mg/kg/h). MLAEPs and BAEPs elicited by clicks (90 dB HL+masking) were simultaneously and continuously monitored during the first 6 h and for 30 min the next morning. We found no effect of MDZ sedation on BAEPs. Only MLAEP components were modified. However, none of the patients presented any total abolition of the MLAEPs. Bolus injection led to very early alteration of cortical responses, beginning after 5 min and lasting almost 1 h (maximum Pa latency increase, 3.1 ms; maximum Pa-Nb amplitude decrease, 46%). During continuous infusion, MLAEPs remained slightly, although significantly, altered (Pa latency, +1.3 ms; Pa-Nb amplitude, 27%). The Nb wave seemed to be modified earlier and to return to normality later than the Pa wave. These findings incite a careful interpretation of MLAEP tracings acquired during the first hour following MDZ bolus injection. If possible, MDZ should be administered as continuous infusion for reliable interpretation of evoked potential changes in intensive care unit, or during surgery.     

Specificity of neuronal responses evoked in the medial geniculate body by stimulation of its different main inputs:In vivo andin vitro investigations

Responses evoked in single neurons of the medial geniculate body (MGB) by electrical stimulation of auditory cortex and fibers of the brachium of the inferior colliculus (BIC) were investigated in vivo and in vitro. In vivo experiments were performed on cats anaesthetized by kalipsol. In vitro experiments were carried out on surviving slices of the rat brain using MGB intranuclear simulation. It has been found that the responses to cortical and nuclear local stimulations show similar peculiarity: an increase in stimulation rate is followed by potentiation and summation of slow EPSPs. At the same time, BIC stimulation evokes mainly fast EPSPs (both in vivo and in vitro) which are remarkably suppressed when rate and intensity of BIC stimulation increase. Distinct features of the MGB neuronal responses to activation of ascending pathways and corticofugal fibers are probably due to differences in anatomical structural among the tested inputs and in chemical mechanisms of the synaptic processes.Neirofiziologiya/Neurophysiology, Vol. 25, No. 3, pp. 203–211, May–June, 1993.     

Auditory evoked potentials in Down's syndrome

Short-, middle- and long-latency auditory evoked potentials (SAEPs, MAEPs and LAEPs) were examined in 12 subjects with Down's syndrome and in 12 age-matched normal subjects. In comparison with the normal subjects, Down subjects showed shorter latencies for SAEP peaks II, III, IV and V (and correspondingly shorter interpeak intervals I–II and I–III) so long as stimulus intensity was at least 45 dB SL. The MAEP peak Na had a longer latency in Down subjects than in normal subjects, but not the Pa latency. In passive oddball experiments for LAEPs, the latencies of all components from N1 to P3 were progressively longer in Down subjects, and the N2-P3 amplitude increased slightly between the first and fourth blocks of stimuli (whereas in the normal subjects it decreased). These alterations in auditory evoked potentials, which may correlate with cerebral alterations in organization and responsiveness responsible for deficient information processing, may constitute an electrophysiological pattern that is characteristic of Down's syndrome.     

Effect of stimulus mode on middle latency auditory evoked potentials in humans

Middle Latency Auditory Evoked Potentials (MLAEPs) were recorded in 35 healthy subjects; all underwent monaural stimulation and 18 of them additionally underwent binaural stimulation. The aim of the study was to determine the effect of stimulus mode on MLAEP Na, Pa and Nb components and to assess normative data for clinical purposes. MLAEPs were respectively obtained from Cz-ipsilateral ear lobe (monaural mode) and from Cz-A1 and Cz-A2 (binaural mode) by twice averaging 1000 responses to 65 dBHL alternating clicks delivered at a repetition rate of 8.1 Hz. Time base was 100 msec; analogical band-pass filter setting was 5-1000 Hz (off-line digital badpass: 20-100 Hz). The statistical analyses (paired t-test, repeated measures analysis of variance) were not able to demonstrate any differences that derived from differing sides of stimulation (monaural mode) or from differing recording derivations (binaural mode); on the contrary, we demonstrated a slight increase in waveform amplitudes when the binaural mode was employed. In particular, we observed an increase in Na-Pa peak-to-peak amplitude, whereas Pa-Nb amplitude was unmodified. This finding is explicable in terms of a binaural interaction effect. Finally, we propose some guidelines for the correct performance and evaluation of MLAEPs in clinical practice.     

Role of auditory centers in echolocation tracking of a moving target by greater horseshoe bat

The role of the auditory cortex and inferior colliculus in echolocation detection and tracking of a moving target and also the role of these parts of the brain in regulation of the vocal apparatus were studied by behavioral and bioacoustic methods in the greater horseshoe batRhinolophus ferrum-equinium. Total bilateral blocking of the auditory cortex was shown to cause significant and irreversible changes in tracking a moving target. Meanwhile destruction of the auditory cortex had no appreciable effect on activity of the bat's vocal apparatus. Total bilateral destruction of the inferior colliculi in the greater horseshoe bat led to disappearance of the response to a moving target (either an artificial target or a natural prey — an insect). In animals with destruction of the inferior colliculi drastic changes were observed in the spectra of the location signals: Numerous low-frequency and high-frequency spectral components appeared. It is concluded from the results that the inferior colliculi in the midbrain participate directly in echolocation detection of moving targets and also in coordination of the reception-emission system of the echolocator in bats. Participation of the auditory cortex in echolocation detection of moving targets is manifested as optimization of the working of the echolocation system.A. A. Ukhtomskii Physiological Research Institute, A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 16, No. 6, pp. 737–745, November–December, 1984.     

Reaction of neurons of the medial geniculate body to acoustic stimulation

Responses of medial geniculate body (MGB) neurons to pure tones and clicks were studied in acute experiments in immobilized cats, preliminary operations being performed under calypsol anaesthesia. MGB units were identified by their reactions to cortical zone AI and brachium of inferior colliculus stimulations. When tonal stimuli were applied relay neurons of pars principalis of MGB usually demonstrated either unimodal tuning curves with narrow frequency band or fragmental ones with several narrow bands. On-response with subsequent inhibition of the background activity or without such an inhibitory period was most frequent type of the reaction (66.6%) of relay MGB neurons to tonal stimulation. The group of relay neurons with the tonic type of reaction (9.1%) was classified for which the duration of tonic response depends on the duration of tonal stimulus. Change of the excitatory reaction to the inhibitory one when the characteristic tone frequency is changed by non-characteristic++ ones is supposed to be a mechanism supplying sharpness of tuning at relay MGB neurons. It is concluded that responses of acoustic cortical neurons to sound stimulation depend to a great extent on the pattern of impulsation that comes from MGB relay units.     

Distributed current analyses of bi-hemispheric magnetic N1m responses to ipsi/contralateral monaural stimuli from a single subject

Magnetoencephalographic (MEG) responses of both auditory cortices to simple auditory stimuli presented monaurally to either ear were recorded from a single subject. A distributed current model and a current dipole model were used to analyse the responses at the latency of the dominant N1m complex. At the N1m the current density was localised to a single area and was consequently well modelled by a single current dipole close to the peak current density. In the left hemisphere, the contralateral response (as identified by the peak current density) preceded the ipsilateral response by 3 msec. This value was 7 msec for the right hemisphere. Evidence was found in the right hemisphere of a posterior-anterior movement along the sylvian fissure. Also, the left hemisphere N1m sources were all represented more posterior than the right hemisphere N1m sources.     

Facial motor responses to microstimulation of the superior colliculi in the white mouse

Facial motor responses to microstimulation of different zones of the superior colliculi have been investigated in the albino mice craniotomized under thiopental anaesthesia. Local responses of the mystacial vibrissae, upper lip and eyelids were initiated by microstimulation of the rostral parts of the inner layers of the colliculus superior (high-frequency volleys of 5-7 pulses with a current limit of 35 microA). Sequential changes in the pattern of facial responses were observed within microelectrode traces indicating vertical orientation of facial motor representations in the superior colliculus. Some differences in the localization and pattern of facial responses in the right and left superior colliculi were revealed: 1) vibrissae and lip representations in the right superior colliculus occupy more extensive zone (vertical distribution from 300 to 2,300 microns) as compared to those in the left one (700-2,000 microns); 2) microstimulations of the right superior colliculus produce both uni- and bilateral vibrissal motor responses, whereas stimulation of the left superior colliculus evokes only unilateral responses. The duration of the latent period of the vibrissal and lip motor responses to stimulation of the right superior colliculus varied from 10 to 26 ms (16.1 +/- 2.4 ms; n = 199), to stimulation of the left one-from 10 to 18 ms (mean 14.9 +/- 1.8 ms; n = 55). It is suggested that polysynaptic motor responses to microstimulation of the superior colliculi are realized via the reticular and other premotor nuclei of the brain stem which have direct inputs from the superior colliculus and direct projections to the facial motor nucleus.     

Distribution of auditory motion-direction sensitive neurons in the barn owl's midbrain

Barn owls have neurons sensitive to acoustic motion-direction in the midbrain. We report here that acoustic motion-direction sensitive neurons with receptive-field centres in frontal auditory space are not randomly distributed. In the inferior colliculus and optic tectum in the left (right) brain, the responses of about two-thirds of the motion-direction sensitive neurons were sensitive to clockwise (counter-clockwise) motion. The midbrain contains maps of auditory space that represent about 15 degrees of ipsilateral and all of contralateral space. Since a similar bias in motion-direction sensitivity was observed for neurons with receptive-field centres in ipsilateral as well as for neurons with receptive fields centres in contralateral auditory space, the brain side at which a motion-direction sensitive neuron was recorded was a more important predictor for the preferred direction of a cell than the spatial direction of the centre of the receptive field. Within one dorso-ventral electrode pass motion-direction sensitivity typically stayed constant suggesting a clustered or even a columnar-like organization. We hypothesize from these distributions that the right brain is important for orientating movements to the left hemisphere and vice versa.     

Evoked potentials recorded from brain-stem nuclei in awake cat: interaction of tone bursts and continuous tone

Previous work indicated that components of the auditory thalamocortical potential evoked by a brief binaural tone burst could be enhanced by certain stimulus combinations, e.g., a brief tone burst in the presence of a continuous tone. The principal questions of the present study were whether enhaced components could be obtained caudal to thalamocortex and whether monaural stimuli would be effective in producing enhancement. Eight cats received electrodes in cochlear nucleus and the nucleus of the inferior colliculus. Custom earmolds were made for each ear of each animal. The median attenuation produced by the earmolds was 35 dB and the use of a single earmold approximated monaural stimulation. Auditory evoked potentials were recorded from the electrodes while the animals were unanesthetized but comfortably restrained. Brief 6.25 kHz tone bursts were presented against a background of silence or of a 4.96 kHz continuous tone. In the presence of the continuous tone, enhanced components were obtained from a majority of the electrodes in inferior colliculus but from none of the electrodes in cochlear nucleus. The late negative component in the colliculus potential was increased in amplitude while other components were reduced in amplitude by the continuous tone. The latencies of all components from all electrodes were increased by the presence of the continuous tone. It was concluded that enhancement effects could be obtained at the level of inferior colliculus, and that binaural stimulation does not appear to be necessary to produce enhanced components.     

Alpha-neo-endorphin-like immunoreactivity in the cat brain stem

This paper examines the distribution of fibers and cell bodies containing alpha-neo-endorphin in the cat brain stem by using an indirect immunoperoxidase technique. A high or moderate density of immunoreactive cell bodies was found in the superior central nucleus, nucleus incertus, dorsal tegmental nucleus, nucleus of the trapezoid body, and in the laminar spinal trigeminal nucleus, whereas a low density of such perikarya was observed in the inferior colliculus, nucleus praepositus hypoglossi, dorsal nucleus of the raphe, nucleus of the brachium of the inferior colliculus, and in the nucleus of the solitary tract. The highest density of immunoreactive fibers was found in the substantia nigra, dorsal motor nucleus of the vagus, nucleus coeruleus, lateral tegmental field, marginal nucleus of the brachium conjunctivum, and in the inferior and medial vestibular nuclei. These results indicate that alpha-neo-endorphin is widely distributed in the cat brain stem and suggest that the peptide could play an important role in several physiological functions, e.g., those involved in respiratory, cardiovascular, auditory, and motor mechanisms.     

Electrophysiologic and behavioral changes in cats following section of the brachia of the inferior colliculi]

Evoked potentials to acoustic stimuli were recorded in the temporal cortical area, the medial geniculate body and the posterior lateral thalamic nucleus in acute experiments on anaesthetized cats. Section of the brachia of the inferior colliculi in an acute experiment resulted in the disappearance of potentials in the examined structures. A distinct correlation has been revealed between the recovery of evoked potentials in the cortico-thalamic auditory structures (in four to six weeks) and the possible elaboration of conditioned reactions within this time period after lesion of the inferior colliculi brachia. The involvement of the temporal area in the general brain activity appears to be one of the major conditions for the formation of new conditioned connections. Possible ways of restoration of afferent input to the temporal cortical area after lesion of the inferior colliculi brachia are discussed.     

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.     

Evidence of neuronal plasticity within the inferior colliculus after noise exposure: a study of evoked potentials in the rat

Recent investigations have implicated that the central nervous system has a role in the changes that occur in auditory function following acoustic trauma caused by noise exposure. These investigations indicate that the inferior colliculus may be the primary anatomical location in the ascending auditory pathway where noise-induced neuronal plasticity occurs, thereby resulting in changes in the neuronal processing of auditory information. In the present investigation, we show that the amplitudes of all peaks in the click-evoked response from the external nucleus of the inferior colliculus decrease during a 30 min exposure to a tone (104 dB sound pressure level (SPL) at 4 kHz and 8 kHz). After tone exposure, the amplitudes of two of the peaks of the response from the external nucleus of the inferior colliculus that reflect the input from more caudal structures slowly returned to baseline levels, whereas the amplitudes of the two peaks reflecting neuronal activity in the inferior colliculus increased above baseline levels and remained at the increased levels for at least 90 min following exposure to the tone.We also show that exposure to a 4 kHz tone at 104 dB SPL causes changes in the neuronal processing of tonebursts in the form of changes in the temporal integration function for one of the peaks of the response from the external nucleus of the inferior colliculus that originates in the inferior colliculus. Before tone exposure the amplitude of this peak decreased with increasing stimulus duration, but after tone exposure the amplitude of this peak was independent of the duration of the toneburst stimulus.We interpret these changes as evidence that noise exposure (tone exposure) causes changes in the excitability of the inferior colliculus that are not seen in more caudal structures, and these changes are probably a result of a change in the balance between inhibition and excitation in the inferior colliculus.