Abstract

The electric consensual response wlien the collateral eye underwent electroretinographic stimulation was studied. Results obtained from normal subjects and 17 patients with optic atrophy are discussed.

Introduction

Many studies have been conducted in humans and in other animals to verify whether the stimulation with light of one retina could in some way influence the electrical activity of the controlateral retina. Motokawa and Mita (1942), Karpe (1945), Monnier (1946), and Dodt (1956) observed a positive or negative electrical potential in the controlateral retina with a latency of 150-300 ms.

Uchermann (1955) and Bagolini (1959) could not demonstrate a consensual electroretinographic response in man, whereas Wirth (1951) noted that the b­wave of the ERG was significantly smaller with binocular stimulation than with monocular stimulation. Muller-Limmroth (1954) observed a positive electri­cal potential in the non-illuminated eye of the guinea pig with a delay of 50 ms. This potential did not appear after section of the optic nerve or destruction of the lateral geniculate body on the side of the non-stimulated eye. The author, therefore, hypothesized the presence of crossed pathways in the chiasma connecting the two retinae.

Auerbach et al. (1961) and Hosten et al. (1961) observed, in humans and animals, synchronized potentials of opposite polarity to the b-wave of the non­ stimulated eye. These authors, however, later hypothesized that such potentials could represent simple diffusion of electric current from the stimulated eye through the surrounding tissues.

Ponte and Monaco (1963), in a series of experiments in the rabbit, reached the same conclusion. In spite of the fact that there have been a series of anatomi­cal (Cajal 1894; Polyak 1941; Wolter 1960; Cragg 1962; Pfister 1963) and physiological demonstrations (Granit 1955; Dodt 1956; Hunt and Jakobson 1972; Dowlingetal 1976) of the presence of centrifugal nervous path ways in the optic nerves, this hypothesis is far from being clear.

Because of the controversy over such an important physiological problem, it was felt worthwhile to repeat this type of study in man. In particular, the existence of consensual potentials in normal subjects and in subjects with both unilateral and bilateral optic atrophy was verified. In the latter case, the presence of potentials in the non-stimulated eye was not possible without a simple diffu­sion through the tissues.

Subjects and Methods

This study was conducted with 20 normal subjects and in 17 subjects with optic atrophy of various types; eight cases were unilateral and ten bilateral. Unilateral stimulation was first applied to the right eye and then to the left. Such stimulation was carried out in scotopic adaptation in eyes in which mydriasis was obtained with Visumidriatic (1%) eye drops. A Vescovini model 481 stimulator using a Xenon lamp (3 J) with white light and a frequency of one flash per second was employed.

Unilateral stimulation was effected using a type of underwater goggle which fitted perfectly to the anatomy under which corneal electrodes (Henkes type) and cutaneous electrodes (Fig. 1) were placed. The corneal electrode was connected with the positive input of the pre-amplifier.

Fig. 1. Position of the patient during examination inside Faraday's cage

Fig. 2. Above: ERG of the stimulated eye. Below: Electric consensual response of the non-stimulated eye

The potentials thus obtained were amplified by a Vescovini pre-amplifier with a passing hand from 0.3-500 Hz/s fed into a Packard Averager (Model 3240) so that each tracing derived from the average of 16 stimulations.

Visual-evoked potentials with a unipolar derivation using an occipital electrode placed on the median line 3 cm above the inion were simultaneously recorded. The occipital electrode was connected with the negative input of the pre-amplifier, the indifferent electrode on the ear lobe being connected to the positive input.

Results

The data obtained are summarized in Table 1. In all the normal subjects it was possible to show, in the non-stimulated eye, a potential with an amplitude of 25-200 µV after stimulation of the controlateral eye (Fig. 2). This potential was composed of two positive deflections with amplitudes calculated by using the base line passing through the apex of the negative inferior deflection as a reference.

The average amplitude of waves I and II was 91.5 and 92.5 µV respectively. The results of the stimulation carried out in subjects with optic nerve pathology are shown in Table 2. In the more severe bilateral cases (patients 8, 9, 10 and 11), the consensual response appears to be completely abolished with the VER. In the unilateral, cases a net reduction or complete disappearance of the consensual potential was observed.

Discussion

The results obtained in normal subjects clearly demonstrate the presence of a recordable potential in the eye in relation to light stimulation of the consensual eye. With this type of experiment, it is not possible to demonstrate whether these potentials are due to biretinal interactions transmitted through the optic nerve or to a transmission through pathways other than nervous ones i.e. by simple diffusion through surrounding tissues.

The matter is clarified if the results obtained in subjects with optic atrophy are examined. In such cases, consensual potentials are drastically reduced in unilateral optic atrophy, or absent in several cases of bilateral optic atrophy. This would favour the hypothesis that the diffusion from one eye to the other occurs via structures contained in the optic nerve.

This assumption can be confirmed by the behaviour of VECP which, though not parallel, is quite similar to the behaviour of the electric consensual response. In fact, a reduction or absence of the consensual potential corresponds in most cases to a reduction, or to an absence, of VECP.

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