Abstract
The present investigation was conducted to study the effect of an extra line upon the successive comparison of the lengths. of two lines. Three lines were the black ones on a white background in horizontal position and successively presented at an identical location for the same brief duration, but the time intervals between the lines were varied within a short period. A standard line (N) of 30mm. in length preceded the comparison line (V), and the extra line (i) preceded N and V or was interpolated between them. It was understood that V was underestimated or overestimated due to the effect of the extra line according to whether the PSE, obtained by the method of complete series, was larger or smaller than that obtained without the presence of the extra line in a control experiment. The results were as follows :
I. Proactive effect of i in the sequence i-N-V.
1) When i was 60, 45, or 30mm, in length, underestimation of V occurred regardless of the time intervals between the lines.
2) When i was 15mm. and both time intervals i-N and N-V were short, overestimation of V occurred. However, if either one of the intervals was much longer than the other, underestimation of V appeared.
II. Proactive-retroactive effects of i in the sequence N-i-V.
The effect of i was almost the same as in the above, except for a little differences in the amount of the effect and the way in which the effect varied with the time intervals. These differences were interpreted as due to the appearances of both proactive effect upon V and retroactive effect upon N of i which did not exist in the sequence i-N-V.
The effect of the extra line upon the comparison of other two lines, as mentioned above, were assumed to be ascribed to an assimilation-contrast phenomenon. The present investigation indicates that the assimilation-contrast effect varies not only with the extent of the extra stimulus, but with its temporal relations with the standard and comparison stimuli respectively.
