Brain transplants reverse auditory brain-stem responses abolished by coagulation of the rat's inferior colliculi

$66 corded by non-invasive electrodes from the external auditory meatus (EAM) of 60 volunteers of both sexes, 12-67 years old, who had normal hearing for age. Quantitative data were obtained on: the number of ears displaying measurable SPs; the SP detection level; the SP onset, peak and rise times; the duration of the SP-AP complex; the SP amplitude and the S P / A P amplitude ratio. Previously unknown relationships were unveiled between the amplitude, but not the temporal, measures of the SP and laterality, sex, age, and audiometrically determined hearing thresholds to 4-8 kHz tones. The highest correlations were obtained with these last thresholds, which suggested that receptors in the basalturn of the cochlea played a dominant role in the generation of the EAM-detected SP. To improve on existing techniques for determining abnormal SP elevation as an indication of endolymphatic hydrops, a multiple regression method was devised that utilized sex, age, 4-8 kHz hearing thresholds and AP voltage to establish upper normal limits of SP amplitude for individual subjects and ears.

W16.02 MINUTE R H Y T H M S IN T H E STEADY-STATE R E S P O N S E S TO AUDITORY, VISUAL AND TACTILE STIMULI. S. Makeig, R. Galambos and D.R. Stapells (San Diego, CA, USA) A train of discrete stimuli (auditory clicks, visual flashes, tactile vibrations) delivered at rates near 40 per second evokes a corresponding train of roughly sinusoidal brain potentials (the 40 Hz ERP, the high-rates or steady-state response, etc.). Since the response train lasts as long as the stimuli continue (minutes, hours) it may be used as an ongoing probe of changes within the CNS. We have been studying the variations of response amplitude, phase, and phase coherence during threshold estimation, masking, sleep, and behavioral experiments. In quiet, waking subjects, quasi-rhythmic fluctuations in response amplitude and phase occur spontaneously, with periods of 20-120 seconds ('minute-rhythms') in all three modalities. Amplitudes may vary by 100 per cent or more, especially as the subject drifts into or out of sleep (in the auditory modality, at least). We have also observed these minute-rhythms in concurrent measures of performance and of EEG-band amplitudes obtained during a vigilance task. Evidently, minute-rhythms are a natural feature of brain electrophysiology. (Work supported by N.I.H.)

WI6.03 INTRACRANIAL S T U D I E S ON SURFACE AND DEPTH POTENTIAL GENERATORS IN MAN: 11. BIPOLAR SOMATIC, AUDITORY AND VISUAL VERTEXLIKE ACTIVITIES. M. Velasco and F. Velasco

(Mexico City, Mexico) Bipolar EEG responses correlated to somatic+ auditory and visual vertex activities (VA) were studied in different subcortical structures of patients with implanted electrodes used as an electrophysiological procedure for surgical treatment. Typical subcortical VA, evoked by somatic, auditory and visual stimuli, were recorded together from a single structure of a non-specific system including: reticulo, rnedial, dorsal thalamic, orbitofrontal, limbic and striatal structures. Typical subcortical VA were formed by four consecutive components O, A, B and C correlated to PI, N1, P2 and N2 of the surface VA. Polarity of component A was inverted between reticulo and medial-thalamic while that of component B was inverted between medial and dorsal thalamic and between dorsal thalamic and fronto-limbic structures. Typical subcortical VA were absent within specific thalamic nuclei (VPL, MG and LG) and pathways (medial and lateral lemniscus and optic radiations) in presence or absence of short latency responses evoked by specific sensory stimuli.

W16.04 BRAIN TRANSPLANTS REVERSE AUDITORY BRAIN-STEM R E S P O N S E S ABOLISHED BY COAGULAT I O N OF T H E RAT'S INFERIOR COLLICULI. M. Shkurovich, G. Cornejo, F. Fernfindez, B. Fernfindez and R. Drucker-Col]n

R.

Aguilar,

(Mexico City, Mexico) There is little information on the functional and electrical recovery of adult Central Nervous System (CNS) lesions in the central auditory system with intra-cephalic implants of embryonic CNS tissue. Auditory brain-stem evoked potentials (BAEP) were recorded from adult male Wistar rats during intraperitoneal chloral hydrate anaesthesia. Under stereotaxic surgery, bilateral collicular coagulation was performed through trephine holes, and embryonic collicular grafts dissected from 17-day old foetuses were stereotaxically implanted into the cavities after the lesion. A group of rats were not grafted. The animal's BAEPs were recorded 4, 8, 12 and 16 weeks after surgery. The animals were killed and their brains processed for histological studies. Bilateral collicular lesions abolished waves 5 and 6 of the BAEP, grafted animals showed recovery of waves 5 and 6 of the BAEP from the fourth week after transplantation, while control animals did not recover. It is concluded that embryonic grafting is capable of correct

$67 reinnervation and restoration of denervated central auditory system in the adult rat.

W16.05 SOURCES OF BAER WAVES I! AND III AS L O C A L I Z E D BY CLINICAL O B S E R V A T I O N S . H.C. Hopf

latencies of waves Pa and Nb increased and their amplitudes decreased. Only the inhalation agents produced significant changes in the brain-stem responses. (The latencies of waves Ill and V increased). The early cortical section of the auditory evoked response is the most promising for monitoring depth of anaesthesia as it is affected by all the agents tested. We are currently examining whether this signal is modified by surgery.

(Mainz, West Germany) Brain-stem auditory evoked responses (BAER) were studied in 9 patients, four with facial myokymia (FM), four with the tonic type of Brissaud's phenomenon (TBP) and one with both conditions. Continuous or rhythmic discharges were observed in each of the five patients with FM and additional spontaneous fibrillation in three. Normal BAER were found in one patient but in four wave iI was markedly abnormal with or without involvement of the following waves. Of the five patients with TBP one had normal BAER; four, however, showed involvement of wave IlI. At re-examination of the patient with F M on one side and TBP on the other side BAER waves lI and Ill, respectively, were restored in size and latency. In conclusion, involvement of wave iI in FM appears to be incompatible with the suggested generation in the extrapontine acoustic nerve segment (Moller et al 1981). Involvement of wave lit in TBP as well as in patients with impairment of the crossed acousticreflex (Kofler et al 1984) points to the trapezoid body as the site of its generation.

W 16.06 T H E E F F E C T O F GENERAL A N A E S T H E T I C S ON T H E BRAIN-STEM AND EARLY C O R T I C A L C O M P O N E N T S OF T H E A U D I T O R Y EVOKED R E S P O N S E .

C. Thornton, C.P.H. Heneghan, M. Navaratnarajah, P.E. Batem a n and J.G. Jones (Harrow, UK) The present methods available to assess depth of anaesthesia are unreliable, particularly when used with muscle relaxants and the newer intravenous agents. For clinical use we require a signal, not affected by muscle relaxants, that is proportional to depth of anaesthesia and shows dose-related changes with all general anaesthetic agents. We have chosen to look at the brain-stem and early cortical sections of the auditory evoked response. Using an essentially similar protocol we have examined the effect of 5 commonly used general anaesthetic agents, the inhalation agents halothane, enflurane, isoflurane and the intravenous agents etomidate and althesin. A total of 31 patients were studied. Each patient received one of the 5 agents at 5 concentrations. All the agents produced statistically significant changes in the early cortical response. With increasing concentration the

W16.07 EVALUATION OF BAEP TRAOPERATIVE MONITORING.

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C. Fischer, V. lbanez and F. Maugui+re (Lyon, France) Brain-stem auditory evoked potentials (BAEP) were monitored during 48 neurosurgical operations (acoustic neurinomas with preoperative good speech discrimination, microvascular decompression of cranial nerves, cerebellopontine angle tumors, brain-stem gliomas and posterior circulatory surgery). The type and the timing of alterations were evaluated and were correlated with the ongoing surgical manoeuvre, the patients neurological outcome and the postoperative auditory function. The abrupt obliteration of all waves resulted in post-op deafness. Two patients with no post-operative hearing had a preserved wave I which disappeared within the two weeks following surgery. Whenever BAEPs remained normal or had presented transient and reversible alterations, hearing was preserved. BAEP were found to be poorer predictors of post-operative neurological findings and outcome. Monitoring of evoked potentials can be useful for alerting that a critical situation is happening but may seldom help in preventing its development. Nevertheless, it is a simple technique which provides useful information on functional states of brain-stem and auditory pathways under neurosurgical anesthesia.

W16.08 LATERALIZED, N O N DEGENERATIVE, INTRINSIC BRAIN-STEM LESIONS: E L E C T R O P H Y S I O L O G I CAL STUDY. R. Koutlidis and M.C. Lavallard-Rousseau (Paris, France) An electrophysiological study was performed in 8 patients suffering from localized vascular or tumoral lesions, previously assessed by clinical and radiological investigations. They were tested by brain-stem auditory evoked potentials (BAEP) and by somatosensory evoked potentials (SEP). The data obtained were compared with an age-matched control group of 15 healthy subjects. BAEP showed two patterns of abnormality: in bulbar lesions, conduction time 1-11I was lengthened when evoked by stimulation of the ear ipsilateral to the lesion. In pontine and