D. Pupukostopoulos, ed. / Psychophystologv Society abstracts. I 981 299

rejection of artefact, sampling rate and duration of epoch, number of trials, calibration, etc.

(3) Errors introduced by signal enhancement techniques. Effects of variability of response in time and amplitude (habituation), superimposition of averages, etc.

(4) Datu arm’ysis. Measurement of evoked potentials, establishment of base line, integral measures, spatial distribution. principal component analysis.

CLINICAL VALUE OF FLASH EVOKED POTENTIALS

G.F.A. HARDING

Clmcal Neurophysiologv Department, Untoersit~~~ of Aston, Birnmghum, U.K.

Visual evoked potentials to flash stimuli were demonstrated as early as 1934. The advent of modern averaging techniques have allowed the develop- ment of this phenomenon and its clinical utilisation.

Although flash stimulation is an experimentally crude technique, clinically it is robust and unaffected by many variables which cannot be controlled in a clinical situation. This technique, like all electrodiagnostic techniques, is used to supplement other clinical information. For this reason the technique is often used in conditions where subjective responses to visual stimuli are non-existent or are for one reason or another unreliable. It is therefore of great value in the very young, the handicapped, those who do not have English as a language and in so-called ‘functional blindess’. It is of value as a prognostic tool in conditions where the media of the eye is opaque. It is of use in conjunction with the EEG in direct and indirect optic nerve trauma and also cortical insults.

By combining the responses to flash and those obtained by pattern reversal it is of diagnostic value in a variety of toxic optic neuropathies, hereditary optic atrophy of both the dominant and Lebers type and surprisingly in Primary Pre-Senile Dementia. In this latter condition it appears to have better diagnostic potential than either CT Scan or EEG.

The flash stimulus is also capable of eliciting subcortical evoked potentials from a postchiasmal site (Harding and Rubinstein, 1981). Although we have only recently identified this component and succeeded in separating its be- haviour from that of the electroretinogram and visual evoked cortical potential it may well have future clinical value.

D. Papakostc>poulos, ed. / Psychophysiology Society ubstructs, 19x1

Reference

Harding, G.F.A. and Rubinstein, M.P. (1981). Early components of the visual evoked potential in man are they of subcortical origin? In: Spekreijse, H. and Apkarian. P.A. (Eds.). Documenta

Ophthalmologica Series NO. 27. Junk Publishers: The Hague/Boston/London, 49-65.

CLINICAL APPLICATION OF THE VEP

A.M. HALLIDAY

MRC External Staff, The National Hospital. Queen Square, London WCI, U.K

Pattern EP amplitude correlates much more closely with the visual acuity level than does the flash EP and abnormalities of the response closely parallel the degree of visual impairment, whether uniocular or binocular. However, clinically ‘silent’ abnormalities are frequently seen in multiple sclerosis (indi- cating unsuspected demyelination of the visual pathways) and also occur in the early stages of compressive lesions. These clear indications of clinically occult lesions have proved particularly valuable in diagnostic neuro-ophthalmology. With lesions behind the optic nerves, the character of the abnormality changes radically. Half-field stimulation in the healthy subject allows one to evoke the VEP from one hemisphere and this has a characteristically asymmetric distri- bution over the occipital scalp. Similar asymmetries are seen in patients with honomymous hemianopia and also in human albinos, where the asymmetry is associated with the anomalous retino-cortical projections which come to each hemisphere predominantly from the contralateral eye. Within the half-field, it is possible to distinguish independent components of the response evoked by macular and paramacular stimulation, and these studies in healthy subjects allow one to understand the characteristic changes seen in patients with central scotomata (e.g. in association with tobacco or alcohol amblyopia). There are also late cognitive components of the visual evoked response which are currently being explored.