Multichannel neuromagnetic measurements in focal epilepsy

$34 Subjects (age range: 50-65; 10 controls and 10 depressed in-patients with high scores at the Hamilton depression and the Widlocher retardation scales, seen before anti-depressant treatment onset) were presented with series of 2 tones of different pitches, in various conditions (passive listening, divided attention, focused attention either to the right or to the left ear, requiring a motor response after each target-stimulus). ERPs (time-locked to stimuli and motor responses) were recorded from 5 equidistant electrodes (from Fz to Pz, linked ear-lobes reference). Even though they performed the task with no more errors than the controls, the depressed, compared to the controls, were characterized by the very low amplitude (or absence) of N200 and P300 for the targets in the attended ear and by slower, more variable RTs. These results will be discussed with respect to attention disorders at various levels of information processing and to their effect on psychomotor retardation.

- D I A G N O S I S AND P R O G N O SIS WITH THE HELP OF CNV. W 5.12 SCHIZOPHRENIA

P. Abraham and C. Churcher-Brown (Alresford, UK) The combination of low amplitude CNV and long duration of the Spiral After Effect (SAE) illusion of movement characterizes patients with florid schizophrenic symptom atology. The SAE returns to normal with remission of symptoms thus qualifying as a marker of the schizophrenic state. In the same patients pre-S2 CNV amplitude remains abnormal in spite of medication and remission and, in general, patients with the lowest CNVs do not remit during their two to three month admission. This indicates a role for CNVs as a trait marker of schizophrenic illnesses with a capacity for predicting outcome at least in the short term. A ten year follow-up of 20 of the 93 subjects who yielded the above data gave necessary support for the diagnostic assessments arrived at with the help of the Present State Examination when the recordings were made during the initial admission. The status of CNVs as a trait marker was reinforced by the observation that the half with lower amplitude CNVs had a very much poorer clinical and social outcome than the others with CNVs in the normal range.

MEG, CBF AND T O P O G R A P H I C EEG DISPLAYS W 6.01 REFRACTORY TEMPORAL LOBE EPILEPSY: MAGNETIC RESONANCE IMAGING (MRI) AND SURGICAL PATHOLOGY. R.S. McLachlan, R.L. Nicholson, S. Black, T. Carr, W.T. Blume, J.P. Girvin, J.C.E. Kaufmann and H. Vinters (London, Canada) MRI (Technicare, 0.15 Tesla Imager) was done on 16 patients with refractory TLE and no known gross structural lesions who were being considered for surgical management. Blind interpretation of the patients' MRIs, compared to those of a normal control group, revealed atrophic changes (temporal lobe, mesial temporal or hemisphere) and increases in spin-spin relaxation time (T2) which were found to correlate with the electrographically determined seizure focus in 11 of 14 patients with these abnormalities. Eight patients have had temporal lobectomies (3 right, 5 left). Atrophic changes in the temporal lobe as demonstrated by MR1 were confirmed at surgery in 7 of 8 patients, the other patient having no signs of atrophy either by MRI or direct observation in the operating room. However, the degree of atrophy seen with MRI did not correlate with that found by the surgeon nor was a correlation found with the histological abnormalities. Four patients had increased T 2 in the mesial temporal region on the resected side and one on the non-resected side. Increases in T 2 did not correlate with histological change or the maximal area of focal spiking on electrocorticography. Atrophic changes in the temporal lobes seen by MRI correlate qualitatively but not quantitatively with the gross findings at surgery but the significance of focal increases in T 2 in the mesial temporal region remains unclear.

W 6.02 M U L T I C H A N N E L NEUROMAGNETIC S U R E M E N T S IN FOCAL EPILEPSY.

MEA-

G.B. Ricci, G.L. Romani, I. Modena, S. Buonomo, R. Leoni and M. Peresson (Rome, Italy) A systematic investigation carried out in several cases of focal epilepsy since 1981 has shown that the neuromagnetic method provides three-dimensional localization of the focal activities. By comparing it with anatomical localization (x-ray a n d / o r surgical findings) we have demonstrated the reliability of this approach. Until now measurements have been performed by means of a single-channel magnetic sensor. Consequently the mapping of magnetic activity over the scalp could only be performed sequentially and its interpretation was dependent on the correlation - when present with the corresponding EEG signals. Here we present a new set of measurements on focal epilepsies obtained with a four-channel neuromagnetic sensor

$35 specifically designed for operation in an unshielded environment. Our first results confirm that simultaneous detection of magnetic activity at four different sites of the scalp provides a more complete and dynamic view of interictal focal epileptic activity. Moreover, the drastically reduced duration of record-. ing sessions is going to be a significant improvement for clinical use.

W 6.03 N E U R O M A G N E T I C D E T E R M I N A T I O N S OF EVOKED R E S P O N S E S IN HUMAN S! AND Sll ASSOCIATED WITH MECHANICAL STIMULATION OF THE INDEX FINGER.

used to reveal sources of evoked potentials in the following ways: (1) the tangential part of the current source can be located on the basis of the magnetic field pattern; (2) the potential distribution generated by these tangential sources can be calculated, and, (3) subtracted from the measured potential distribution. The 'residual' distribution is mainly due to radial and deep sources, these being easier to interpret than the original distribution. Examples will be given about the usefulness of the MEG in differentiating between simultaneously active sources.

W 6.05 ATTENTION RELATED DIFFERENCE WAVES IN T H E MEG.

Y.C. Okada. C. Paulsen, L. Kaufman and S.J. Williamson L. Kaufman, Sarah Curtis and N.J. Williamson (New York, USA) (New York. USA) The somatic magnetic field (SEF) evoked by mechanical stimulations of the right index finger was measured over the left hemisphere of three normal male volunteers to study activities elicited in the primary and secondary somatic areas of the human cerebral cortex. The stimulus was a periodic train of 200-msec long brush strokes, repeated every 1536 msec and applied to the ventral surface of the finger. The waveform of the SEF, recorded with a band-width of 0.5-45 Hz, was typically triphasic with latencies of 30-50, 70-100 and 150-200 msec for the three components. The amplitude of the second component was maximal over four areas: i) (13-17, 0-4), i.e., 13-17 cm above the ear canal, 0 - 4 cm in front of the vertical axis passing the ear canal and perpendicular to the horizontal axis connecting the ear canal and the corner of the left eye; ii) (9 12, 2-6); iii) (8 12, 0-4); and iv) (3-6, 2-6). The polarities of regions 1 and 3 were the opposite of regions 2 and 4. The locations of the extrema and their polarities are consistent with the hypothesis that they represent activities in two distinct areas of the cortex. The locations of the field reversal suggest that these are in SI an SII, in agreement with earlier neuromagnetic measurements obtained with electrical stimulations of somatic afferents. The longer latencies (by about 25 msec) of responses in regions 3 and 4 are consistent with the hypothesis.

W 6.04 LOCATING BRAIN EVENTS WITH MAGNETIC MEASUREMENTS.

Pseudorandom sequences of tone bursts of either 1000 Hz or 1050 Hz were presented at a repetition rate of 3 Hz to one ear, and of 3000 Hz or 3050 Hz at 3.5 Hz to the other ear. The subject shadowed one of the two repeating pseudorandom sequences to find the beginning and end of the repeat (about 40 bursts), while ignoring the stimulus to the other ear. We measured the component of the magnetic field normal to the side of the head contralateral to the attended sequence at more than 60 different positions with a movable five-channel neuromagnetometer. Average responses, recorded with a bandwidth of 1-45 Hz, were obtained for both the 3 Hz and 3.5 Hz stimuli. The differences between the responses to the attended stimuli and comparable unattended stimuli were computed. The difference between the left hemisphere's response to the attended and comparable unattended stimuli was highly significant. This was not true for the right hemisphere. Since our bandpass was above 1 Hz, this difference wave could not show the attention related negativity difference observed by others. The ac magnetic difference wave contains an early component (peak latency of about 60 msec), a component having a latency about 20 msec shorter than the magnetic 'NI00', and another with the same as the 'P200'. The short latency of the early component of the difference wave, together with the fact that its source appears to be in or near the primary auditory cortex, suggests a very early selective filtering of shadowed periodic stimuli. This research was supported by the U.S. Air Force of Scientific Research.

Riitta Hari, M. Hamalainen and R. llmoniemi (Espoo, Finland) In interpreting magnetic fields produced by cerebral activity, we face the non-uniqueness of the inverse problem. Physiological and anatomical information can, however, be used to restrict the number of solutions. The present attempts to locate brain activity assume a spherical head shape with concentric inhomogeneities and are based on simple source models. Sinmltaneous magnetic and electrical recordings may be

W 6.06 LOCALIZATION OF CONTRAST EVOKED RES P O N S E S IN MAN USING MEG AND EEG,

CJ. Stok, F.H. Lopes da Silva, H. Spekreijse and M.J. Peters (Leiden, The Netherlands) The localization of contrast evoked transient responses within the visual cortex was studied in 6 subjects, The stimulus