Sixteen Experimental Investigations from the Harvard Psychological Laboratory | Page 9

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 an?sthesia could be demonstrated, both the feeling of succession and the discontinuity of the intensities would be explained (since during the an?sthesia 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 an?sthesia 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 an?sthesia 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 an?sthesia 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 an?sthesia, 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 an?sthesia 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 an?sthesia, 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 (ibid., p. 461) that this fact 'is certainly suggestive of a central explanation for the absence of bands of fusion under ordinary conditions.' But "these failures suggested an increase of the illumination of the field of exposure.... Under these conditions a long band of light was immediately evident at each movement of the eye." This and similar observations were believed 'to show experimentally that when a complex field of vision is perceived during eye-movement it is seen fused' (p. 462).
[18] Dodge, PSYCHOLOGICAL REVIEW, 1900, VII., p. 459.
Between the 'failures' and the cases when a band of light was seen, no change in the conditions had been introduced except 'an increase of the illumination.' Suppose now this change made just the difference between a stimulation which left no appreciable after-image, and one which left a distinct one. And is it even possible, in view of the extreme rapidity of eye-movements, that a retinal stimulation of any considerable intensity should not endure after the movement, to be then perceived, whether or not it had been first 'perceived during the movement'?
Both of Dodge's experiments are open to the same objection. They do not admit of distinguishing between consciousness of a retinal process during the moment of stimulation, and consciousness of the same process just afterward. In both his cases the stimulation was given during the eye-movement, but there was nothing to prove that it was perceived at just the same moment. Whatever the difficulties of demonstrating an an?sthesia during movement, an experiment which does not observe the mentioned distinction can never disprove the hypothesis.
[Illustration: Fig. 2.]
For the sake of a better understanding of these bands of light of Dodge, a perimeter was equipped in as nearly the manner described by him (ibid., p. 460) as possible. Experiments with the eye moving past a very narrow illuminated slit confirmed his observations. If the light behind the slit was feeble, no band was seen; if moderately bright, a band was always seen. The most striking fact, however, was that the band was not localized behind the slit, but was projected on to that point where the