抄録
The fact that the visual threshold of one stimulus figure is influenced by another one presented in spatio-temporal proximity to it has usually been regarded as products of some psychophysiological interaction or “induction”. The strength of the induction, or of the “field” influenced by one figure at a certain point around it, is usually determined in terms of the change, in general heightening, of the threshold of a small test spot (TS) presented there.
For instance, Yokose, following Köhler's isomorphism, has studied this effect systematically, and has proved that the properties of psychophysiological “potential field” could exactly be predicted in terms of the formula derived from these of electromagnetic field. On the other hand, some have insisted that this effect is to be attributed to the stray light in the eye, and have called it the “glare effect”.
In the present experiment, it was attempted to decide which of the two theories was correct. Four experiments were performed.
(1) The threshold of TS was measured under two conditions: First, TS was exposed at the center of a dimmly illuminated disc, and second, at the center of the enclosed dark area by a bright ring. (In the former, the stimulus situation was that of direct adaptation, and in the latter, of indirect adaptation.) A value of the illuminance of the disc and of the ring respectively was determined experimentally so that the thresholds of TS was the same under both conditions. The ratio of illuminance of the disc to the ring thus obtained being kept constant, five levels of illuminance of these two figures were set in equal logarithmic interval. Thresholds of TS were measured at each of the five levels of illuminance for two figures. If the glare effect theory was correct, the threshold of TS at each of the five levels should be identical under both conditions. Our results confirmed the prediction.
(2) Area-intensity interchangeability of glare effect was examined. As stimuli, contour figures of a ring and a square, or segmental parts of each were used. Illuminance of the stimuli was determined such as to increase in proportion to the decrease of the area. The test spot was always exposed at the center of the ring or square, or at the corresponding position for segment figures. No remarkable differences were found among these conditions.
(3) Boynton has recently studied the nature of stray light in an excised eye, and obtained a regular functional relationship between the strength of stray light and distance. We have tried to compare following two conditions. One was to vary the distance of the disc figure from the centrally fixated TS at the fovea (glare stimulation). The other was to vary the illuminance of the disc, presented always at the central retina (figure-ground relationship). Illuminance conditions in our experiment were determined by calculation from Boynton's result, in order to show the strength of stray light at each distance. If Boynton's result was right, it would be expected that in the two conditions the adaptative liluminance at the position of TS should be equal. Results showed that this expectation was correct and the threshold of TS was quite the same under two conditions.
(4) Optic disc is receptor-free. Even if light is casted there, no physiological changes occur. In this experiment, a light stimulus was casted to optic disc and to the neighbouring positions, while the thresholds of TS were measured always at the fovea. Results showed that the threshold of TS in this case was not lower than when a light stimulus was casted to any other positions. The threshold was merely a function of the distance from the light stimulus to TS.
It was concluded that these findings could. be explained more plainly in terms of stray light rather than of physiological induction.
