Harvard Psychological Studies, Volume 1 eBook

This eBook from the Gutenberg Project consists of approximately 678 pages of information about Harvard Psychological Studies, Volume 1.

It is of course perfectly conceivable that the feeling of succession is an illusion (which will itself then need to be explained), and that the streak is seen continuously, its spacial reference only undergoing an instantaneous substitution.  If this is the case, it is singular that the correctly seen streak seems to enter consciousness so much reduced as to intensity below that of the false streak when it was eclipsed.  Whereas, if a momentary anaesthesia could be demonstrated, both the feeling of succession and the discontinuity of the intensities would be explained (since during the anaesthesia the after-image on the retina would have faded).  This last interpretation would be entirely in accordance with the observations of McDougall,[17] who reports some cases in which after-images are intermittently present to consciousness, and fade during their eclipse, so that they reappear always noticeably dimmer than when they disappeared.

   [17] McDougall, Mind, N.S., X., 1901, p. 55, Observation II.

Now if the event of such an anaesthesia could be established, we should know at once that it is not a retinal but a central phenomenon.  We should strongly suspect, moreover, that the anaesthesia is not present during the very first part of the movement.  This must be so if the interpretation of Schwarz is correct, for certainly no part of the streak could be made before the eye had begun to move; and yet approximately the first third was seen at once in its original intensity, before indeed the ‘innervation-feelings’ had reached consciousness.  Apparently the anaesthesia commences, it at all, after the eye has accomplished about the first third of its sweep.  And finally, we shall expect to find that movements of the head no less than movements of the eyes condition the anaesthesia, since neither by Schwarz nor by the present writer was any difference observed in the phenomena of falsely localized after-images, between the cases when the head, and those when the eyes moved.

III.  THE PERIMETER-TEST OF DODGE, AND THE LAW OF THE LOCALIZATION OF AFTER-IMAGES.

We have seen (above, p. 8) how the evidence which Dodge adduces to disprove the hypothesis of anaesthesia is not conclusive, since, although an image imprinted on the retina during its movement was seen, yet nothing showed that it was seen before the eye had come to rest.

Having convinced himself that there is after all no anaesthesia, Dodge devised a very ingenious attachment for a perimeter ’to determine just what is seen during the eye-movement.’[18] The eye was made to move through a known arc, and during its movement to pass by a very narrow slit.  Behind this slit was an illuminated field which stimulated the retina.  And since only during its movement was the pupil opposite the slit, so only during the movement could the stimulation be given.  In the first experiments nothing at all of the illuminated field was seen, and Dodge admits

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