Clinical investigation of lesions of the visual pathway: a new objective technique.

There have been several previous reports of the use of the gross electrical brain potentials evoked by visual stimulation (evoked potentials) to provide A popular method has been to direct brief, very bright, flashes of light into one or both eyes, and to extract the scalp evoked potentials from the spontaneous EEG by computer averaging. The potentials evoked in this way are not easily quantified , and it is therefore difficult to define clearly the difference between 'normal' and 'abnormal' responses. It is also difficult to compare the integrities of foveal and extrafoveal pathways. The notorious variability of evoked potentials imposes a severe limitation on the precision with which relationships may rapidly be measured between responses to stimulations of different areas of the retina, when such stimulations are performed successively (Copen-This is a preliminary report of a new method of investigating visual field defects which is objective, fairly rapid, non-traumatic, and makes comparatively small demands on the patient. We have developed a method which enables two areas of the retina to be stimulated simultaneously and weakly. The results of this are to minimize the effect of evoked potential variability, to minimize the effect of light scattered within the eye, and to improve patient cooperation. Our method gives results in both visual and quantitative forms, and can distinguish between different parts of the visual field by using different kinds of stimuli. METHODS The nasal and temporal half-fields of one eye were simultaneously stimulated by weakly flickering light, or by chessboard patterns whose black and white squares alternated. The repetition frequencies of the stimuli falling on the left and right half-fields were sufficiently similar to be effectively identical for the evoked potential system, but sufficiently different for the analysing equipment to isolate and measure the individual responses to left and right half-field stimulation (Regan and Cart-wright, in press). A stable difference between the left and right stimulation frequencies of about 0-3 c/s-for example, 18 3 and 18-0 c/s-was found to be suitable. This method of simultaneously recording two signals had the effect of reducing the effect of variability and so enabling the difference between the potentials evoked by left and right half-field stimulation to be measured with precision. In comparison with the method of separately measuring the responses to half-field stimulation, the present technique reduces the effect of variability by a large amount. A reduction of roughly six times is typical, which …