Vaughan and Ritter (1970) have recently proposed that all components of the human, average, auditory evoked response (AER) to repetitive stimulation, including the long latency vertex potentials, can be ascribed to a dipole source reversing polarity at the level of the sylvian fissure. This assertion brings to the fore the need for futher information about the spatial distribution of vertex potentials over the surface of the head as determined with a remote, non-cephalic reference. Since a true sylvian phase reversal of the late components of the AER would have far-reaching theoretical and clinical interpretive implications, we have been prompted to carry out studies especially relevant to this point.
SUMMARY Visually evoked cerebral responses (VERs) from the occipital and central areas were compared between 50 patients with multiple sclerosis and 50 control subjects. The Because multiple sclerosis frequently produces visual defects, it is natural to anticipate concomitant changes in visually evoked cerebral responses. However, a preliminary study of a small group of patients revealed that, while there were evident changes in some patients, they were not necessarily those having signs of involvement of the visual system. In the present study, an expanded sample of patients with multiple sclerosis was examined in order to ascertain (1) the effect of the disease upon the latencies and amplitudes of visually evoked waves, (2) the proportion of patients who have clearly abnormal responses, (3) the clinical correlates, if any, of the alterations, and (4) their relation to routine electroencephalographic findings.
SUJBJECTS AND METHODSVisually evoked responses (VERs) were obtained on 50 patients with multiple sclerosis (35 females, 15 males) and 50 normal subjects (35 females, 15 males). The two samples were matched with respect to age (averages of 40 2 years each). Control subjects were in good health, without neurological illness, and not on any medication. All patients were hospitalized during a period of exacerbation of their disease. The final diagnosis of multiple sclerosis was made in each case by the Neurology Service. Routine EEGs were run on a Grass Model III Electroencephalograph. The evoked responses were summated with the Mnemotron Computer of Average Transients. Three computer runs were obtained. The averaging period for the first two runs was 250 msec and for the third was 125 msec. Light flashes were randomized and given one every 4 to 6 sec to a total of 50 for the first two runs and one every 0 5 to 1 sec to a total of 300 for the third run.During the first run, data were obtained from the following electrode positions: left and right frontal, central, occipital, and temporal areas. The reference was joined-ears. Occipital electrodes were placed 3 cm above and 3 cm lateral to the inion. Central electrodes were placed 4 cm from the midline in the interaural plane. Data from the second and third computer runs were used to substantiate the initial observations and further to assess possible ocular and myogenic artefacts. The summated responses were displayed on a Tektronix cathode ray oscilloscope and photographed with a Hathaway oscilloscope camera.Individual components of the evoked responses from the occipital and central regions were used for analysis. Both left and right-sided measurements were obtained with the left-sided values being used for statistical analysis (Kooi, Guvener, and Bagchi, 1965). The first five major occipital waves in either a negative or positive direction after the stimulus were studied. The vertex sharp wave 275
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