抄録
This paper is the first report on the experimental studies which were designed to testify to the inhibitory effects of an additional stimulus preceding two stimuli to be compared with each other, in the successive comparison of brightness.
Three light-spots of 1cm in diameter were successively presented at an identical location in a perfectly dark room. The additional stimulus (e) always preceded the standard stimulus (N) and the latter preceded the comparative stimulus (V). The durations of three stimuli were 2sec. in length. The interval between e and N was always constant at 1sec., while that between N and V was varied. The light-value of e was made physically equivalent to that of N (6m.a.) to eliminate the so-called assimilation-contrast phenomenon. The effect of additional stimulus was measured according as the PSE, calculated by the method of complete series, was larger or smaller than that obtained without the presence of additional stimulus in a control series.
The main results in this study were as follows:
(1) When N and V werepresented without an additional stimulus, the normal p-function was obtained, i.e., the time-error gradually changed from positive to negative as the interval increased from 1 to 9sec., except that the error was a little more positive than normal at every interval (Exp. I, see Fig. 1).
(2) When the additional stimulus was presented, the time-error shifted towards a negative direction, i.e., underestimation of N occurred independently of whether the interval between N and V was short (1sec.) or long (6sec.) (Exp. II, & III. see Table 3, 5).
(3) Nevertheless the amount of the effect of additional stimulus was not constant throughout every interval. Although the time-error also shifted towards a negative direction at the 1-sec. interval, this shift decreased its amount at the 3-sec. interval. And at longer intervals than 3sec., the time-error gradually shifted towards a negative direction once more (Exp. IV, see Fig. 2).
(4) The above-mentioned tendency was also recognized when the light-value of e was so controlled as it was phenomenally equivalent to that of N perfectly to eliminate the assimilation-contrast mechanism between them. In this condition, however, the shift towards a negative direction was more remarkable throughout all the intervals (Exp. V, see Fig. 3).
(5) According to the observer's introspection, N was seen darker than e, especially at shorter intervals. But e had not a disturbing effect upon the comparative judgement of brightness between N and V at 1-or 3-sec. interval. On the contrary, the jud gement itself was disturbed at longer intervals, images of e and N having fused themselves together with the lapse of time (Exp. II-V).
(6) In addition, when two light-spots, placed side by side with each other in a horizontal line at the distance of 1cm, were presented twice successively, the magnitude of time-error did not differ significantly from that in the control series at the 1-sec. interval. On the other hand, the shift towards a negative direction was clearly noticed at the 6-sec. interval (Exp. II, & III, see Table 3, 5).
