Harvard Psychological Studies, Volume 1 eBook

This eBook from the Gutenberg Project consists of approximately 757 pages of information about Harvard Psychological Studies, Volume 1.
through the whole of the first group of experiments as to just how much disturbance was being caused in the estimation of the distance by visualizing.  I therefore made a series of experiments to determine what effect was produced upon the illusion if in the one set of judgments one purposely visualized and in the other excluded visualizing as far as possible.  In my own case I found that after some practice I could give very consistent judgments, in which I felt that I had abstracted from the visualized image of the arm almost entirely.  I did not examine these results until the close of the series, and then found that the illusion was greater for those judgments in which visualization was excluded; that is, the filled space seemed much larger when the judgment was made without the help of visualization.  It is evident, therefore, that the tactual illusion is influenced rather in a negative direction by visualization.

In the second group of experiments, where the judgments were obtained through the localization of the points, it would seem, at first sight, that the judgments must have been very largely influenced by the direct vision used in localizing the points.  The subject, as will be remembered, looked down at a card of numbered points and named those which were directly over the contacts beneath.  Here it should seem that the optical illusion of the overestimation of filled spaces, filled with points on the card, would be directly transmitted to the sensation on the skin underneath.  Such criticism on this method of getting at the illusion has already been made orally to me.  But this is obviously a mistaken objection.  The points on the card make a filled space, which of course appears larger, but as the points expand, the numbers which are attached to them expand likewise, and the optical illusion has plainly no influence whatever upon the tactual illusion.

A really serious objection to this indirect method of approaching the illusion is, that the character of the cutaneous sensation is never so distinctly perceived when the eyes are open as when they are closed.  Several subjects often found it necessary to close their eyes first, in order to get a clear perception of the locality of the points; they then opened their eyes, to name the visual points directly above.  Some subjects even complained that when they opened their eyes they lost track of the exact location of the touch points, which they seemed to have when their eyes were closed.  The tactual impression seems to be lost in the presence of active vision.

On the whole, then, I feel quite sure in concluding that the overestimation of the filled cutaneous spaces is not traceable to the influence of visualization.  Parrish has explained all sporadic cases of overestimation as due to the optical illusion carried over in visualization.  I have already shown that in my experiments visualization has really the opposite effect.  In Parrish’s experiments the overestimation occurred in the case of those collections of points which were so arranged as to allow the greatest differentiation among the points, and especially where the end-points were more or less distinct from the rest.  This, according to my theory, is precisely what one would expect.

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Harvard Psychological Studies, Volume 1 from Project Gutenberg. Public domain.
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