Subcortical correlates of the auditory brainstem potentials in the monkey: referential responses

Brainstem auditory evoked potentials (BAEPs) in the cynomolgus macaque monkey. Equivalence with human BAEPs and proposal of a new nomenclature

Several groups have studied brainstem auditory evoked potentials (BAEPs) in non-human primates. However, the nomenclature of the waves elicited and their correspondence with human waves I-V differ among authors. BAEPs were recorded from six anaesthetised young cynomolgus macaques (Macaca fascicularis), using different sound stimuli parameters. A constant pattern of four main waveforms was present in all the animals with stimulus intensities over 60 dB SPL, although up to four smaller waveforms were observed in some of the individuals. Latency values increased with decreasing stimulus intensities and with increasing repetition rates. These results were similar to the BAEPs observed in other species of macaques. Although an approximate equivalence between human and monkey BAEPs is possible, some discrepancies suggest that there may be generators which contribute to different waves in both species. This is the reason for our proposal of a new nomenclature for BAEP waveforms in monkeys, following a descriptive order with Arabic numerals preceded by the letter M.

Effects of lidocaine on salicylate-induced discharge of neurons in the inferior colliculus of the guinea pig

Using the extracellular recording method, the effects of lidocaine (a local anesthetic known to relieve tinnitus) on discharge of inferior colliculus (IC) neurons of the guinea pig were studied before and after salicylate (200 mg/kg) administration. The salicylate-induced discharge was inhibited by intravenous injection of lidocaine at a concentration (1 mg/kg) clinically used for treating tinnitus. IC neurons could be classified into two groups according to the difference in sensitivity to lidocaine: (1) weakly-sensitive neurons and (2) highly-sensitive neurons. In weakly-sensitive neurons, the duration of the lidocaine effect lasted for less than 5 min, and the inhibitory action on the discharge of neurons was greater when the latency to sound stimulus became longer. In highly-sensitive neurons, on the other hand, the activity of neurons was almost completely inhibited for longer than 30 min, irrespective of the latency to sound stimulus. The clinical relevance of these types of neurons is discussed.

Tectal plate gliomas. Part III: Apparent lack of auditory consequences of unilateral inferior collicular lesion due to localized glioma surgery

The authors present one of their cases operated on for intrinsic tectal plate glioma. The complete resection of the right inferior colliculus (I.C.) had no apparent auditory consequences. The pre- and post-operative tonal and vocal auditory tests were normal. The brain-stem auditory evoked potentials (BAEPs) and middle latency potentials (MLPs) were recorded pre-, post- and intraoperatively. At the end of surgery all waves were present with a marked delay of wave V and a slight delay of the Pa component. The dichotic test showed a significant right ear extinction but admittedly much less important than expected. The role of inferior colliculus (I.C.) in hearing is discussed.

Temporary latency shifts in auditory evoked potentials by injection of lidocaine in the rat

Temporary latency shifts in auditory evoked potentials by intravenous injection of lidocaine were investigated in albino rats. Auditory brainstem responses (ABR) were recorded with 4000Hz short tone bursts. There was no significant difference of latencies in wave Pl of the ABR between drug-administered group (6 mg/kg body weight of lidocaine) and control group (0.6 ml/kg of 0.9% saline). As for waves P2. P3 and P4 of ABR, the latencies of the waves were significantly prolonged after the lidocaine injection (p < 0.05. U-test). The prolongation of latency of wave P4 was the most prominent in the fast waves and dependent on the dose of lidocaine. The latencies of the slow positive wave of the evoked potentials on the inferior colliculus (wave S) with clicks and the slow positive wave (wave P) and the negative wave following ABR (wave NA) with 4000Hz short tone bursts were also studied. The latencies of waves S. P and NA showed significant temporal prolongation (p < 0.05, U-test) after lidocaine injection. The present study revealed that a relatively small dose of lidocaine can cause temporary prolongation of latencies of auditory evoked potentials not only in the fast components but also in the slow components including wave NA which is a large slow negative wave following the ABR. The effects are considered to be caused by the action of lidocaine which reduces the excitabilities on the auditory nervous system.

The effects of experimental brain-stem ischaemia on brain-stem auditory evoked potentials in primates

We related intracranial auditory brain-stem evoked potentials (BAEPs) to the surface BAEP using a model of focal brain-stem ischaemia. In 17 baboons anaesthetised with alpha-chloralose, BAEPs were recorded bilaterally at the mastoids and in the caudal lateral lemniscus (LL) and inferior colliculus (IC), in response to monaural click stimulation. Electrodes at these sites were each connected to the positive input of a differential amplifier, and one other electrode, placed at the vertex, was connected to all the negative inputs. Measurements of local cerebral blood flow (CBF) by hydrogen clearance were made at the LL and IC sites. The LL wave form contained 5-7 positive peaks, the second (B wave) being dominant and coinciding with the negative wave II of the surface BAEP. Following graded ischaemia, produced by basilar artery occlusion and controlled hypotension, the latency changes of these two peaks were significantly correlated, as were those of the third wave (C wave) of the LL response and the surface wave III. In the IC, the contralateral wave form contained 4 positive waves (A-D) and a later, dominant, slow negative wave; changes in its peak latency and those of the slow negative surface wave were similarly correlated. The thresholds of local CBF for increases in latency of waves B and C in the LL were similar (12-15 ml/100 g/min), but in the IC the thresholds were 20, 30-35 and 20-24 ml/100 g/min for the B, C and slow negative waves, respectively. Our data indicate that a gradient of sensitivity to ischaemia is present along the brain-stem auditory pathways; this could explain the earlier change of the late, rather than early, BAEP components as reported in clinical cases involving brain-stem lesions.

Wakefulness-sleep modulation of the surface and depth auditory evoked potentials in man

Amplitude and latency changes of early and late components of surface and depth auditory evoked potentials were determined during wakefulness-sleep steady state shifts in epileptic patients, with implanted electrodes used as an electrophysiological procedure for surgical treatment of temporal lobe seizures. Early surface (I and V) and depth (N8 and N15) components of the auditory brainstem potentials and late surface (P2 and N2) and depth (B and C) components of the auditory evoked potentials were produced by either 8/s or single clicks, delivered monoaurally and simultaneously recorded from the vertex and contralateral thalamic (lateral geniculate thalamic nucleus) and frontotemporal (amygdala, hippocampus and orbitofrontal cortex) regions, while patients spontaneously shifted from initial wakefulness (W1) to slow wave sleep (SWS I, II and IV), to paradoxical sleep (PS) and to final wakefulness (W2). Amplitude of late surface (P2 and N2) and depth (B and C) components significantly decreased when patients shifted from SWS IV to PS and increased from PS to W2. Latency of components P2 and B increased while that of components N2 and C decreased from SWS IV to PS. No latency changes in late components were found from PS to W2. In addition, amplitude and latency of P2 and B components significantly decreased while those of N2 and C increased from W1 to SWS IV. Polarity of all late components remained unchangeable during all wakefulness-sleep state shifts, with the exception of that of component C which reversed from W1 to SWS IV. In contrast, early surface (I and V) and depth (N8 and 15) components showed no systematic changes in amplitude and latency during all consecutive wakefulness-sleep shifts, with the exception of a significant increase amplitude but no latency of component V from PS to W1.

Subcortical correlates of the P300 potential complex in man to auditory stimuli

Bipolar and referential responses correlated to the surface P300 auditory potential complex (components N2, P3 and N4) were studied in different subcortical structures of parkinsonian and epileptic patients with implanted electrodes used as an electrophysiological procedure for surgical treatment. Bipolar and referential subcortical responses were recorded from a wide subcortical region which included subthalamo-thalamic (Sth-Th), dorsothalamic (DTh), orbitofronto-hippocampal (OFH) and striatal (STR) structures. They were formed by 3 consecutive components C, D and E correlated to components N2, P3 and N4 of the surface responses. Component D correlated to surface P3 was either absent or very small in all subcortical bipolar responses and in referential responses recorded from OFH structures. Polarity of bipolar component C was inverted between DTh and OFH while that of bipolar component E was inverted between Sth-Th and DTh, between OFH and STR and between Sth-Th and STR. Polarity of the referential component C was inverted between STh-Th and DTh and between OFH and STR while that of referential components D and E remained unchanged along all subcortical structures. Referential components C-E and D attained maximal amplitudes and minimal latencies at the subthalamic and at the medial geniculate thalamic nucleus regions respectively. From here, their amplitudes decreased and latencies increased with distance in other structures rostrally and caudally located, with the exception of those at the orbitofrontal (components C-E) and dorsal thalamic and striatal regions (component D) where no amplitude-latency gradients were observed.

Short-latency auditory evoked potentials in the monkey. I. Wave shape and surface topography

The components and topography of the short-latency auditory evoked potentials (SLAEPs) in the monkey were analyzed using a combination of surface and epidural electrodes. Nine positive peaks, and a slow negativity following wave 7, are identified in the SLAEP recorded at the vertex referred to the mastoid ipsilateral to a monaural stimulus. Subcomponents of waves 1 and 3 are consistently observed, and other waves are probably compound as well. Barbiturate anesthesia results in latency prolongations which are cumulative across components but does not alter the component sequence. With binaural stimulation there is simple summation of the monaural responses for the components preceding wave 7, but a binaural interaction for wave 7. The polarity and clarity of the various components depend on the recording configuration selected. No single configuration is optimal for the demonstration of all components, and the use of montages with multiple electrodes may increase the information gained from clinical SLAEP recordings in man. The monkey SLAEP components can be grouped based on similarities of their surface topographies, which suggest similar anatomic orientations of their underlying generators. Probable homologues between the components of the simian and human SLAEP, based on temporal sequence, topography, and the effects of variation of the stimulus parameters, are: Jewett's waves I through VII correspond to monkey waves 1, 3, and 5 through 9. Due to differences in the size and shape of the heads, the monkey waves 2 and 4 correspond to negativities in the human SLAEP, labeled IN and IIN after the components which they follow. Another peak, with latency between waves 7 and 8 in the monkey and V and VI in man, is only clearly seen in the mastoid-to-mastoid linkage, contralateral negative.

Short-latency auditory evoked potentials in the monkey. II. Intracranial generators

The generators of the short-latency auditory evoked potentials (SLAEPs) in the monkey have been defined by intracranial mapping from cochlea to auditory cortex. SLAEP components other than 1a and the slow negativity (SN) following wave 7 derive from compound action potentials propagated in subcortical auditory pathways. The component generators are complex due to the presence of two bursts of activity in the eighth nerve, to the fact that the ascending auditory fibers both synapse on and bypass specific relay nuclei, and to the differences in orientation of segments of the auditory pathways. Most SLAEP components recorded at the surface reflect the summation of activity from multiple generators. However, much of the activity seen within subcortical structures cannot be traced to the surface of the brain. Component 1a is identified with the cochlear summating potential, while 1b reflects the initial afferent volley in the distal portion of the eighth nerve. Component 2 represents the initial depolarization of the eighth nerve terminals within the ipsilateral cochlear nucleus. Component 3h reflects the second volley of activity in the distal portion of the eighth nerve and the outflow of the cochlear nucleus which decussates in the trapezoid body. Component 3v represents the initial cochlear nucleus outflow volley ascending the lateral lemniscus. Component 4 principally reflects the second volley of activity within the eighth nerve terminals, and outflow from the ipsilateral superior olivary complex ascending in that lateral lemniscus, with a possible contribution from activity in the contralateral CNC. Component 5 represents the outflow of the contralateral superior olivary complex ascending in that lateral lemniscus. Component 6 reflects another volley from the ipsilateral superior olivary complex ascending in that lateral lemniscus, as well as outflow from both inferior colliculi propagating in their brachii. The generators of component 7 are the most complex encountered, representing volleys in both lateral lemnisci, activity of the contralateral inferior colliculus, and activity in both auditory radiations. A component that follows wave 7, seen best in mastoid-to-mastoid recording linkage, represents outflow from both inferior colliculi propagating in their brachia. Components 8 and 9 principally reflect propagated action potentials in the auditory radiations bilaterally, with an additional contribution from activity of both inferior colliculi. The SN mainly represents volume-conducted postsynaptic potentials from both inferior colliculi and cochlear nuclei.

Subcortical correlates of the somatic, auditory and visual vertex activities. II. Referential EEG responses

A systematic analysis of polarity, amplitude and latency of the referential EEG responses correlated to surface somatic (SVA), auditory (AVA) and visual (VVA) vertex activities (VA) was done in various subcortical structures of a non-specific system of the same group of patients with electrodes implanted and reported in a previous work. These structures included the caudal and rostral mesencephalic reticular formations (cttc and rttc), the subthalamic region (Sth), the centro-median (Ce), parafascicular (Pf), ventrolateral (VL), dorsolateral (DL) and reticularis (Rt) thalamic nuclei; the orbito-frontal cortex (GO), the anterior commissure (Acm), the pallidum medialis and lateralis (Pm and Pl), the putamen (Pt) and the amygdala (Am). Subcortical SVA, AVA and VVA were formed by 4 consecutive components O, A, B and C correlated to components P1, N1, P2 and N2 of the surface VA. Components A and B of the subcortical SVA reversed polarity 4 times when recorded in structures arranged in a caudo-rostral order: between rttc-Ce and VL, between Rt and GO, between GO and Acm and between Acm and Am. Components A and B of subcortical AVA and VVA reversed polarity two times each: AVA between cttc and rttc and between GO and Acm; while VVA between GO and Acm and between Acm and Pm. Components A and B of SVA and VVA attained maximal amplitudes and minimal latencies at GO. From here, their amplitude decreased and latency increased with distance along other structures rostrally and caudally located. Components A and B of AVA attained maximal amplitudes and minimal latencies at cttc.(ABSTRACT TRUNCATED AT 250 WORDS)

Subcortical correlates of the auditory brain stem potentials in the monkey: bipolar EEG and multiple unit activity responses

Bipolar EEG and multiple unit activity (MUA) responses correlated to the vertex auditory brain stem potentials (ABSP) were recorded in different brain stem and diencephalic primary auditory pathways and other anatomically related structures of large monkeys under barbiturate anesthesia. Bipolar EEG responses were recorded bilaterally to monaural stimulation and were formed by 3 or more of 6 consecutive components labeled A, B, C, D, E and F (peak latencies of 3.8, 4.2, 4.6, 5.0, 7.8 and 11.8 msec) correlated in latency with waves III, IV, V, VI, VII and SP3 of the ABSP, respectively. Component B was prominent and showed clearcut reverse polarity at the trapezoid body (TB) and superior olivary complex (SOC), while components C and F inverted and everted polarity at the mesencephalic reticular formation (MRF) and medial geniculate nucleus thalami (MG). Subcortical MUA peaks A, B, C, D and E time locked to the stimulus presentation correlated in latency to those of the bipolar EEG responses. In addition, a significant correlation was found between percentage amplitude of the subcortical EEG response components and MUA peaks in different structures contra- (r = 0.847) and ipsilateral (r = 0.973) to the stimulated ear. Although a single wave of the vertex ABSP correlated in latency with more than one response component in different subcortical loci, amplitude of component A was significantly larger (P less than or equal to 0.02) at TB ipsilateral, B at TB and SOC bilateral, and C and F at MRF, and D and E at MG contralateral to the stimulated ear.