Binocular summation within the nervous pathways of the pupillary light reflex

Abstract

The question 'Does simultaneous stimulation of the retina of both eyes give rise to a greater response than stimulation of one eye only?' has interested investigators since the last century. Lythgoe & Phillips (1937), who give references to previous authors, measured the change in light threshold during dark adaptation for right, left and both eyes and found that the binocular threshold was lower than the monocular. In both foveal and peripheral vision the binocular threshold was approximately the same as the monocular threshold for a test patch of twice the area, a finding which showed that a moderate degree of summation had occurred when both eyes had been used. Crawford (1940) has repeated the work of Lythgoe & Phillips with substantially similar results and, in addition, has measured the minimum detectable difference of brightness at various brightness levels for both eyes and for a single eye. These measurements showed only a small amount of summation at all brightness levels when the central fovea was used. With the parafovea (80) two eyes were about 30 % more sensitive than one eye to differences of brightness, except at low brightness levels when only a small amount of summation was obtained. Clearly the brightness threshold behaves in a different manner from the light threshold. Crawford gives no explanation of this difference. Pirenne (1943), using a new method of light threshold measurement, has found that there is a small degree of summation in the peripheral retina (200). He presented several series of 4 m./sec. flashes of light to the eye and for each series determined the number of flashes seen by the observer. The 'frequency of seeing' with both eyes was somewhat greater than that obtained with one eye only, but doubling the intensity of the flashes and observing with a single eye produced a much larger increase in the 'frequency of seeing', and he concluded that under the conditions of his experiment binocular summation was small in amount. Thus it seems that the degree of summation of afferent impulses derived from two eyes depends upon the way in which the effect of these afferent impulses is to be measured. The object of the present experiments was to find out if summation of afferent impulses occurs when a purely objective effect, such as contraction of the pupil, is measured. Since the nervous pathways of the pupillary light reflex do not ascend higher than the mid-brain, the results might help to localize the site of summation within the nervous system.

The action of tetanus toxin on the rabbit's iris

“…distance from the eye, was also measured on both sides. In addition, a number of photographic records of pupils was obtained by a method essentially the same as that described by Thomson (1947), to whom we are indebted for the loan of the necessary apparatus. This consisted of a Cine-Kodak special camera fitted with a device for synchronizing electrically the flash from a photo-flood lamp with the opening of the camera shutter; this flash was produced by the opening of an electromagnetic shutter fitted to the front of a light-tight box containing the photo-flood lamp (750 W.), and lasted 0-01 sec.…”

The psychophysical inquiry into binocular summation

“…But it is known that pupil diameter during binocular viewing is smaller than during monocular viewing (Reeves, 1918;Thomson, 1947), and, since the amount of light enterning the eye is proportional to the square of the diameter of the pupil, this difference in pupil size could differentially affect the monocular and binocular thresholds and consequently vitiate any test for summation. Specifically, the large pupil size during monocular viewing could lower the monocular threshold and mask any summation effect.…”

The effect of pupil size on binocular summation at suprathreshold conditions

“…[4][5][6] Pupils react by contracting in response to light and near viewing, and by dilating in response to darkness and far viewing. 4,[7][8][9] Moreover, pupil size is smaller in binocular than in monocular vision. 7,8,10 A variety of visual stimuli can be used to study the binocularsummation paradigm with natural and artificial pupils.…”

“…4,[7][8][9] Moreover, pupil size is smaller in binocular than in monocular vision. 7,8,10 A variety of visual stimuli can be used to study the binocularsummation paradigm with natural and artificial pupils. However, studies with artificial pupils offer the additional advantage that changes in pupil size are prevented, and consequently the action of the near-response mechanisms are limited.…”