It has been shown that when a stimulus figure exists in the visual field, the figure exhibits the so-called figure-effect in the form of changes in the light threshold for a small luminous point presented at places near the figure on the same front parallel plane. The finding leads to the following possibility: An effect similar to this will also occur in the depth of the visual space.
For this reason, in the present study, several experiments were performed with the following two objects in view. The first object is to examine whether such an effect can be shown to exist at all. The second is to examine what tendency the effect has, if it does exist.
The experimental conditions and the method of the measurement were as follows: The stimulus objects consisted of two parallel lines which were lit to form a pair of light stimuli, with three kinds of interior interval distance with visual angles of 12′, 18′, and 48′. They were constantly presented on the front parallel plane at a distance of 592.3mm from the observer. In one experiment, only one of them was presented. When the figure was presented, a small luminous point was simultaneously placed at various positions in front and the rear of the figure as shown in Fig. 2 (A), and light threshold for the point, value
t, was measured. Then the threshold for the same point was measured again at the same places when the figure was not presented, as shown in Fig. 2 (B). This threshold, called
t0, is a criterion value to check the degree of the elevation of the threshold
t. That is, the figure-effect was examined by the difference between
t and
t0, and the value of (
t-
t0)/
t0.
The results obtained in the experiments were as follows:
(1) It was found that the threshold values, as a whole, were higher in the depth than in the same frontal plane to the figure.
(2) The threshold value was gradually elevated as the distance between the figure and the point was increased, and at a certain place far from the figure, a marked elevation was seen. Such a tendency of the elevation was observed both in the front and the rear visual spaces of the figure.
(3) Such an elevation of the threshold with the increase of the distance were more pronounced as the interval distance between the two parallel lines became shorter.
In view of the above results, it is concluded that the figure-effects are present also in the depth, i.e., the third dimension, of visual space, and that they also cause the elevation of the threshold as the distance from the stimulus figure increases. Such a tendency seems to be dependent upon prevailing stimulus configuration.
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