Evoked Cortical Activity from Auditory Cortex in Anesthetized and Unanesthetized Cats

Topographic analysis of epidural pure-tone-evoked potentials in gerbil auditory cortex

This study investigated the tonotopic organization of pure-tone-evoked middle latency auditory evoked potentials (MAEPs) recorded at the auditory cortical surface in unanesthetized gerbils. Multielectrode array recording and multiple linear regression analysis of the MAEP demonstrated different degrees of tonotopic organization of early and late MAEP components. The early MAEP components P1 and N1 showed focal topography and clear dependence in location and size of cortical area covered on pure-tone frequency. The later components P2 and N2 showed a widespread topography which was largely unaffected in location and size of cortical area covered by pure-tone frequency. These results allow delimitation of the neural generators of the early and late MAEP components in terms of the spectral properties of functionally defined neural populations.

The effects of low-pass filtering on the primary cortical auditory potential of the rat

The effects of low-pass filtering on the primary cortical auditory evoked potential (CAEP) were studied using the rat as subject. The CAEP is the potential which is generated by the arrival of the afferent volley in the primary auditory projection area. Recordings were made from animals which were totally awake and those which were anaesthetised with pentobarbital. A total of 9 recordings were obtained from each subject. The high-pass (low-frequency) filter remained fixed at 3.2 Hz while the low-pass (high-frequency) filter was set at 32, 80, 160, 320, 800 Hz and 1.6, 3.2, 8 and 16 kHz. The CAEP recorded from the awake animal consisted of a primary positivity (P1) followed by a later secondary positivity (P2). In the anaesthetised subjects, only the P1 potential was present. As the bandpass was progressively opened, there was at first a quite steep decline in latency associated with a gradual increase in amplitude. After the low-pass filter setting had been raised to 320 Hz, the amplitude of components P1 and P2 when awake and of P1 when anaesthetised had stabilized and thereafter there was no additional increase. Likewise, the latency of P2 for the awake subjects subsequently remained constant. In contrast, the latency of P1 recorded from both awake and anaesthetised subjects showed a continuing small decline as the bandpass was extended to 3.2-16 kHz. It is probable that this phenomenon did not represent a further genuine decrease in the latency of P1 but was more likely an artefact caused by the distorting effects of a cluster of late high-frequency components of the brainstem auditory evoked potential generated temporally contiguous to P1. It was concluded that a bandpass of 3.2-320 Hz is optimal for recording both early and late components of the CAEP and that low-pass filtering had an essentially uniform effect on the waveform irrespective of the subject's state of arousal.

The effect of upper pontine transections on normal cochlear responses and on the protective effects of contralateral acoustic stimulation in barbiturate-anaesthetized normal-hearing guinea pigs

In barbiturate-anaesthetized guinea pigs with normal cochlear neural sensitivities, upper pontine transections were made to totally isolate the cell bodies of the olivocochlear neurons in the lower brainstem from all higher centres. The effects of this procedure were examined at the cochlea on normal compound action potential (CAP) thresholds and amplitudes, on the temporary threshold shifts (TTS) in CAP sensitivity caused by monaural loud sound exposures, and on the protective effects of low-level contralateral acoustic stimulation (Cody and Johnstone, 1982; Rajan and Johnstone, 1983a, 1988). The transection had no effects on any of these responses. These results suggest that centres above the metencephalon do not exert any tonic effects on the cell bodies of the olivocochlear pathways that result in tonic effects at the cochlea. Further, these results also suggest that the protective effects of contralateral acoustic stimulation are exercised solely through lower brainstem pathways.

Middle latency response: frequency and intensity effects

Auditory middle latency responses (MLR) and auditory brainstem responses (ABR) were measured with epidural electrodes in unanesthetized gerbils. Response thresholds of simultaneously recorded MLRs and ABRs, and latencies and amplitudes of MLR peaks were analyzed with respect to stimulus intensity (10-80 dB SPL) and frequency (0.5, 1, 2, 4, 8 and 16 kHz). Only minor changes in the latencies of the MLR were associated with increases in stimulus intensity. Changes in latencies were more apparent for waves A and B as compared to wave C, and were significant only at low intensities. Latencies did not change significantly as a function of stimulus frequency. Amplitudes of the MLR were highly variable between animals, particularly waves B and C, and showed complex changes with intensity. In general, wave amplitudes were inversely related to stimulus frequency. The gerbil MLR resembles MLRs recorded under similar conditions in guinea pig, cat, and rat. Some qualitative similarities between gerbil and human MLRs are apparent. Results indicate that the MLR is a less sensitive measure of hearing threshold relative to the fast waves of the ABR at frequencies above 1 kHz. However, clearly defined MLRs are elicited with a wide range of stimulus frequencies. Because the surface recorded MLR reflects activation of central auditory pathways, including the cortex, it may provide an electrophysiological measure which can be utilized to study central components of normal and pathological auditory function.

Midline and temporal lobe MLRs in the guinea pig originate from different generator systems: a conceptual framework for new and existing data

In the guinea pig and gerbil, individual components within the MLR time frame differ in optimal recording location. Specifically, MLR components obtained from the midline differ from those obtained over the temporal lobe. In the present paper midline and temporal lobe components were shown to differ not only in scalp topography but also in response to the following experimental manipulations: intracortical injection of neural inactivating agents (lidocaine and kainic acid), temporal lobe ablation, electrolytic lesions, systemic anesthesia, stimulation rate and course of development. Since midline and temporal lobe components respond differently to experimental manipulations, it can be concluded that the midline and temporal lobe responses are mediated by different generator sources. The particular orientation of the generators responsible for the MLR in the guinea pig and gerbil facilitates the identification of individual components. Results from simultaneous recordings of these components during experimental manipulations support the hypothesis of multiple MLR generators in laboratory animals and provide insight into the generators and developmental aspects of the MLR in humans.

Effects of chloral hydrate, pentobarbital, ketamine, and curare on the auditory middle latency response

Changes in threshold, latency, and amplitude of the auditory middle latency response (MLR) with anesthesia and neuromuscular paralysis were studied in guinea pigs. Although each component of the surface-recorded MLR was altered by barbiturate and nonbarbiturate agents, the early positive wave (wave A) was always present, and the later waves were generally identifiable at moderate levels of anesthesia. MLR threshold was not affected by anesthesia or curare. Pentobarbital, chloral hydrate, and ketamine each caused an increase in the latency of all MLR components, with increases progressively marked for later waves. Amplitude changes were more complex. Wave A increased in amplitude with anesthesia, while wave C decreased or disappeared temporarily. Wave B showed mixed amplitude changes. Changes in MLR associated with anesthesia were generally more pronounced at a stimulation rate of 10/sec as compared to 4/sec. No changes in waveform morphology or latency were seen with neuromuscular paralysis. The results provide evidence in support of separate neurogenic substrates for the different components of the guinea pig temporal lobe MLR. The presence of the MLR with moderate levels of anesthesia indicates that this animal is an appropriate model for studying the MLR in experiments requiring anesthesia and immobilization.

The effects of high-dose barbiturates on the acoustic reflex and auditory evoked responses. Two case reports

The effects of high-dose barbiturates (pentobarbital) on the acoustic reflex, and the auditory brainstem (ABR) and middle-latency (AMR) responses, are illustrated with two case reports. Auditory electrophysiologic data were recorded serially during recovery from therapeutic barbiturate coma. ABR latency remained within normal limits in barbiturate coma, but amplitude of the wave I component was abnormally augmented. Contralateral and ipsilateral acoustic reflex activity, and the Pa component of the AMR, were not observed in barbiturate coma, and reappeared with the emergence of brainstem neurologic signs. These findings suggest a fundamental difference in the neurophysiologic substrate of the ABR vs. acoustic reflex and AMR. Possible mechanisms for the differential influence of barbiturates on these three auditory electrophysiologic measures are offered.

Brain-stem auditory evoked potentials in rats with high-dose pentobarbital

Brain-stem auditory evoked potentials (BAEPs) are relatively resistant to alteration by barbiturate drugs, but the effects of the high doses that are used clinically to produce deep barbiturate coma for the treatment of intracranial hypertension and postischemic anoxic encephalopathy on BAEPs are unknown. We gave high-dose pentobarbital infusions to mechanically ventilated rats while recording serial BAEPs from the scalp. Pentobarbital progressively suppressed and then abolished all peaks. First the later waves, then all but the first wave, and finally all waves were abolished at intravenous doses of 120, 220, and 260 mg/kg, respectively, in addition to the initial anesthetic dose of 60 mg/kg i.p. The changes were at least partially reversible; peaks returned in reverse order of their disappearance. Peak latencies increased with dose. The results show a significant effect of pentobarbital on BAEPs in the rat, suggesting that BAEPs may be useful in assessing the depth of and recovery from barbiturate coma.

Auditory middle latency responses in the guinea pig

Auditory middle latency responses (latencies 6 to 50 msec in guinea pigs) were recorded from eight awake, restrained guinea pigs. Before recording, screw electrodes were implanted in the skull in a coronal plane in line with the bregma. Another electrode, which served to monitor auditory brainstem responses, was placed 1 cm posterior to the bregma. All electrodes were referenced to a lead positioned 2 cm anterior to the bregma. During the recording session, click stimuli of various repetition rates and intensity levels were delivered monaurally in a closed sound system. Auditory brainstem responses were monitored to ensure normal functioning of the peripheral auditory system. Responses from electrodes at the midline and over the temporal area ipsilateral to the stimulus ear were greatly attenuated or absent. From an electrode over the temporal area contralateral to the stimulus ear, two positive peaks occurred at latencies of approximately 12 and 27 msec. A negative trough was identified at approximately 17 msec. Latency and amplitude functions for this waveform were determined for various stimulus levels. Response amplitude increased as stimulus repetition rate was decreased. Anesthesia greatly altered waveform structure and prolonged peak latencies. These effects were more marked at stimulus repetition rates faster than 10/sec than at slower rates. Properties of the guinea pig middle latency response are compared with those previously reported for cats and humans.

Evidence for a primary cortical origin of a middle latency auditory evoked potential in cats

The origin of the scalp-recorded auditory evoked potential, Pa, was examined in cats anesthetized with chloralose-urethane and immobilized with gallamine triethiodide. This potential is a prominent positive wave which peaks approximately 12--15 msec following click stimuli. Mapping revealed that Pa is distributed on the scalp in the region overlying cortical area AI, contralateral to the stimulated ear. The cortical potential recorded from AI was a surface-positive wave, restricted to the anterior portion of AI. Laminar analysis of the cortical evoked potentials demonstrated the existence of a dipole generator at that area. The onset of this potential coincided with the onset of the scalp-recorded Pa. Comparison of the scalp and the cortex-recorded potentials showed that both the amplitude-intensity function and the amplitude-rate function for the scalp-recorded potential closely paralleled those recorded from AI. Acute and chronic lesion studies showed that extirpation of AI (particularly the anterior part) almost completely abolished the Pa response. This evidence indicates that the scalp-recorded Pa of cats is generated almost entirely from the anterior part of the contralateral AI.