2OP Society pmceedings

Over the past few years, there has been an increasing awareness of the usefulness of dermatomal (segmental) evoked potentials (DERMs) in the diagnosis of radiculopathies, confirming the results of our long experience in this field (V. DiCarlo - Clin. EEG 20:69; 1989). With proper technique, these tests are painless and require absolutely no sedation. Technical details may be important and have a bearing on the quality of the results. In our lab, reproducibility of responses on normal volunteers and patients has been very good, even when the test repeti- tions were spaced at intervals of months or years. Here we are present- ing data obtained on 200 trauma patients on whom neurological exami- nation, SEPs DERMs, and MRls were obtained and the results ana- lyzed. Briefly, DERMs appear to correlate satisfactorily with MRls and neurological exams, even as far as level is concerned, both in cervical and in lumbosacral spine. On 30 additional trauma cases in which EMGs were performed, our results indicate that in every radiculopathy case, an abnormal EMG exam was accompanied also by abnormal DERMs at the same or approximately the same levels.

E122: Comparison of EEG and MEG recorded event-related po- tentials - Ralf Siedenberg, Douglas S. Goodin, Michael J. Aminoff, Howard A. Rowley, Timothy P. Roberts (Departments of Neurology and Radiology, UCSF, San Fran- cisco, CA 94143)

MEG may be better than EEG for source localization of the brain activity underlying event-related potentials (ERPs). Any such advan- tage depends upon the reliability and stability of signals recorded with these technics. We simultaneously recorded the EEG and MEG in 6 subjects (aged 27-46 yrs) during a choice reaction-time task. Each listened to a pseudorandom sequence of 420 tones (86% at 1000 Hz; 14% at 2000 Hz) and responded to each tone by extending the right or left middle finger depending upon condition. EEG responses were recorded from electrodes placed at C2, P2, C3, and C4 referenced to linked mastoids. MEG responses were recorded from these same gen- eral locations. EEG responses to frequent (1000 Hz) tones consisted of the WI and P2 components of the auditory vertex potential. Responses to rare tones consisted of Nl, apparent P2, N2, and P3 components of the ERP. These EEG responses were best seen in the midline (at C2 and P2) and were reproducible for the same subject on different trials for both rare and frequent tones. Nl and P2 components in the MEG were stable but were best seen at C3 and C4, and were out of phase between these two electrode locations. The timing of this phase reversal did not correspond consistently with the timing of the Nl and P2 peaks in the EEC. By contrast, the later responses in the MEG were consistently less stable and less consistent than the simultaneously recorded EEG re- sponses. Thus, long latency ERPs are more consistent between subjects and more stable within subjects when recorded with EEG than with the MEG.

E123: Adequate scalp sampling density for the human visual event- related potential - *Michael Murias, *Don M. Tucker, and Margaret Sereno (University of Oregon and *Electrical Geo- desics, Inc.)

The visual event-related potential was examined in four subjects studied individually over repeated recording sessions with the 128. channel Geodesic Sensor Net (Tucker, 1993). This device performs a regular tessellation of the head surface in a geodesic geometry (icosahedron), with AG/AGCI sensors in pedestals at each geodesic vertex. The elastic contraction of the tension network adapts the geo- desic geometry to the individual’s head shape. Checkerboard stimuli subtending 3.5 degrees visual angle were presented at five locations at 2.5 second intervals. Subjects were instructed to remain tixated on a central point on the screen for the duration of the recordings. Analysis of the waveform plots showed clear delineation of the focal events of the visual ERP, including the PI and Nl components, between adjacent channels, indicating that additional spatial information was yielded by

the increase in sampling density from 64 to 128 channels. Dr. Murias and Dr. Tucker are employees and major stock shareholders of Electri- cal Geodesics, Inc.

E124: Joint time frequency analysis of event related EEG pertur- bations - Ramesh Srinivasan and Don M. Tucker (Department of Psychology, University of Oregon, Eugene, OR 97403. Electrical Geodesics, Inc., Eugene, OR 97403)

Transient evoked potentials (ERP) remain the principal tool in clinical and cognitive studies of event related EEG perturbations. By contrast, the dynamics of the EEG are best understood in the frequency domain, where basic rhythms such as alpha have been identified. The joint time frequency analysis estimates the evolution of magnitude and phase in frequency bands (alpha, beta, gamma, etc.) over the temporal window of cognitive processing. This approach is applied to several basic ERP studies suggesting additional dynamical perturbations masked in the evoked potentials.

Dr. Srinivasan and Dr. Tucker are employees and major stock shareholders of Electrical Geodesics, Inc.

E125: Intraoperative motor and somatosensory evoked potentials monitoring: a review of 116 cases - Keith Nagle, Ronald Em- erson, David Adams, Eric Heyer, Edward Gallo, Kathrine Dowling, Paul McCormick, David Roye, John Pile-Spellman, Bennet Stein, Mark Weidenbaum (Department of Neurology, Anesthesiology, Orthopedic Surgery and Neurosurgery, Neu- rological Institute, Columbia-Presbyterian Medical Center, New York, NY 10032)

We monitored motor (MEPs) and somatosensory evoked potentials (SEPs) in 1 I6 patients undergoing surgery for scoliosis/kyphosis (45), spinal cord tumor (28), tethered cord (16), vascular malformation (1 I), spinal stenosislspondylolisthesis (7), pseudoarthrosis (2). vertebral fracture (2), and vertebral tumor (5). MEPs were monitored by electri- cally stimulating the spinal cord an(1 recording compound muscle ac- tion potentials. SEPs were monitored by stimulating median and/or posterior tibia1 nerves and recording using cephalic-cephalic and ce- phalic-noncephalic derivations. Anesthesia was maintained with a N20/02/opioid technique supplemented with potent inhalational agents. Partial neuromuscular blockage was maintained using a vecuronium infusion. Both MEPs and SEPs were monitorable in 97 (84%) cases. Neither MEPs nor SEPs were recorded in 9 (8%) patients, each of whom had a clinical myelopathy. In 3 cases only SEPs could be re- corded and in 7 cases only MEPs were obtained. Deterioration occurred during 9 operations (8%). In 8 cases both SEPs and MEPs deteriorated. In 1 case, only MEPs deteriorated. We believe that optimal monitoring during spinal surgery entails concurrent recording of both SEPs and MEPs. This provides independent verification of spinal cord integrity using two parallel but independent systems, and also allows detection of the rare insults that affect selectively either motor or sensory sys- tems.

E126: Somatosensory P300 ERP: a new tool for coma assessment - Peggy S. Gott, Orna Newfield, Adrian L. Rabinowicz, Jorge D. Correale, Thomas D. Smith, Raja B. Boutros and Christopher M. DeGiorgio (Department of Neurology, University of South- ern California School of Medicine, Los Angeles, CA 90033)

Interest in auditory P300 (AP3) ERPs for coma prognosis is increas- ing. However, hearing loss may preclude the recording of an AP3 in the elderly or after anoxia. In a pilot study we recorded somatosensory P300 (SP3) ERPs with an ‘oddball’ paradigm analogous to the AP3 in 10 non-traumatic coma patients. Stimuli were electrical pulses deliv- ered unilaterally to the median nerve: frequent stimulus duration 0.1 ms; rare stimulus duration 0.2 ms, probability = 20%. N = 12. The montage was Fz, Cz, and Pz referenced to Al+A2. EOG was recorded