The significance of luminance on visual evoked potentials in diagnosis of MS

Visual evoked potentials in multiple sclerosis: frequency response shows reduced alpha amplitude

Visual evoked potentials were measured in a group of 16 multiple sclerosis (MS) patients and in a control group of 20 subjects. With respect to vertex and occipital recordings, latencies of main peaks were prolonged and response amplitudes were reduced in the MS group. As a result of frequency domain analysis we found that the amplitude reduction was not uniform in all frequency ranges: alpha (7-12 (Hz) components of EPs were markedly reduced whereas theta (4-7 Hz) responses were not altered. It is remarkable that the frequency components were altered to a different degree--this may shed some light on the physiological roles of the frequency components: As MS is frequently associated with optic neuritis, our interpretation of this frequency-dependent pattern is based on regarding MS as a model of impaired sensory input to the brain: the fact that in this situation alpha responses are markedly reduced hints at a link between alpha responses and primary sensory processing. Theta responses turned out to be unaltered--i.e., less dependent on sensory inputs--and might thus reflect associative sensory processing. This conclusion for functional roles of EP frequency components has also been drawn from previous investigations of topographic differences of EP frequency components.

Auditory evoked potentials in multiple sclerosis: alpha responses are reduced in amplitude, but theta responses are not altered

Auditory evoked potentials (EPs) were measured in a group of 16 multiple sclerosis (MS) patients and in a control group of 20 subjects. In vertex recordings, response amplitudes were reduced in the MS group. Remarkably, EP frequency components computed from the averaged EPs showed different degrees of amplitude reduction in different frequency channels: alpha (7-12 Hz) components were reduced whereas theta (4-7 Hz) responses were not altered. Our interpretation takes into account results of our companion paper (Başar-Eroglu et al., this issue) on similar results in the visual modality and is based on considering MS as a disease with disturbed sensory input to the brain. The fact that in this disorder alpha responses are reduced while theta responses are not altered can be interpreted as follows: alpha components might be mainly dependent on sensory input and thus reflect primary sensory processing. Theta responses, being unaltered in MS, might mainly reflect associative processing. The results are in accordance with conclusions drawn from investigations of topographic differences of evoked electric and magnetic brain responses.

Pattern visual evoked potential luminance and multiple sclerosis

Pattern visual evoked potentials (PVEPs) were recorded from 111 patients classified as having possible, probable or definite multiple sclerosis. Patients were stimulated with a checkerboard pattern using high and low luminances in order to test the hypothesis that an attenuated pattern luminance increases the detection rate of PVEP abnormalities. With increasing certainty of diagnosis, there was a concomitant increase in the incidence of PVEP abnormalities. However, there was no evidence that stimulating with a lower luminance pattern enhanced the sensitivity of the test. The same findings were also apparent when the patient data was analyzed according to the presence or absence of a history of optic neuritis or other visual symptoms. It is concluded that, within the luminance limits used in this study, the role of varied luminance in detecting demyelinating lesions in the optic nerves using the PVEP is minimal, although there was some limited evidence that a high level of luminance may be more appropriate than a low level.

Foveal interocular time thresholds and latency differences in multiple sclerosis

Nineteen patients with suspected multiple sclerosis (MS) and 28 control subjects were foveally stimulated by a small cross formed by rectangular red-light-emitting diodes. By means of crossed polarizers one eye was exposed only to the horizontal bar of the cross, the other to the vertical. Stimulus onset asynchrony ranged from 0 to +/- 300 ms (the horizontal bar preceding or following the vertical). The task was to indicate whether the horizontal or vertical bar had appeared first. Compared with normal subjects, MS patients exhibited much higher time thresholds (ranging from -150 to +130 ms) and had considerable interocular latency differences (up to 29 ms), indicating unilateral or asymmetrical impairment of the visual pathways. The psychophysical latency differences of the patients were compared to monocular latencies and interocular latency differences of the visually evoked cortical potential (VEP) obtained by foveal stimulation. Under the stimulus conditions chosen in this study, the diagnostic value of the psychophysical measurements was equal to or, for McAlpine's classes I and II of definite and probable MS patients, better than that based on VEP recordings.