Sixteen Experimental Investigations from the Harvard Psychological Laboratory | Page 5

light seen while the eyes were at rest. Neither my assistant nor I was able to detect any difference in brightness between them and the background when altogether unobstructed." Dodge finds that this experiment 'disproves' the hypothesis of an?sthesia.
If by 'an?sthesia' is meant a condition of the retinal end-organs in which they should be momentarily indifferent to excitation by light-waves, the hypothesis is indeed disproved, for obviously the 'three clear-cut round holes' which appeared as bright as the unobstructed background were due to a summation of the light which reached the retina during the movement, through three holes of the disc, and which fell on the same three spots of the retina as long as the disc and the eyeball were moving at the same angular rate. But such a momentary an?sthesia of the retina itself would in any case, from our knowledge of its physiological and chemical structure, be utterly inconceivable.
On the other hand, there seems to be nothing in the experiment which shows that the images of the three holes were present to consciousness just during the movement, rather than immediately thereafter. A central mechanism of inhibition, such as Exner mentions, might condition a central an?sthesia during movement, although the functioning of the retina should remain unaltered. Such a central an?sthesia would just as well account for the phenomena which have been enumerated. The three luminous images could be supposed to remain unmodified for a finite interval as positive after-images, and as such first to appear in consciousness. Inasmuch as 'the arc of eye movements was 4.7��' only, the time would be too brief to make possible any reliable judgment as to whether the three holes were seen during or just after the eye-movement. With this point in view, the writer repeated the experiment of Dodge, and found indeed nothing which gave a hint as to the exact time when the images emerged in consciousness. The results of Dodge were otherwise entirely confirmed.
II. THE PHENOMENON OF 'FALSELY LOCALIZED AFTER-IMAGES.'
A further fact suggestive of an?sthesia during movement comes from an unexpected source. While walking in the street of an evening, if one fixates for a moment some bright light and then quickly turns the eye away, one will observe that a luminous streak seems to dart out from the light and to shoot away in either of two directions, either in the same direction as that in which the eye moved, or in just the opposite. If the eye makes only a slight movement, say of 5��, the streak jumps with the eye; but if the eye sweeps through a rather large arc, say of 40��, the luminous streak darts away in the opposite direction. In the latter case, moreover, a faint streak of light appears later, lying in the direction of the eye-movement.
This phenomenon was probably first described by Mach, in 1886.[8] His view is essentially as follows: It is clear that in whatever direction the eye moves, away from its luminous fixation point, the streak described on the retina by the luminous image will lie on the same part of the retina as it would have lain on had the eye remained at rest but the object moved in the opposite direction. Thus, if the eye moves to the right, we should expect the streak to appear to dart to the left. If, however, the streak has not faded by the time the eye has come to rest on a new fixation point (by supposition to the right of the old), we should expect the streak to be localized to the left of this, that is, to the right of the former fixation-point. In order to be projected, a retinal image has to be localized with reference to some point, generally the fixation-point of the eyes; and it is therefore clear that when two such fixation-points are involved, the localization will be ambiguous if for any reason the central apparatus does not clearly determine which shall be the point of reference. With regard to the oppositely moving streak Mach says:[9] "The streak is, of course, an after-image, which comes to consciousness only on, or shortly before, the completion of the eye-movement, nevertheless with positional values which correspond, remarkably enough, not to the later but to the earlier position and innervation of the eyes." Mach does not further attempt to explain the phenomenon.
[8] Mach, Ernst, 'Beitr?ge zur Analyze der Empfindungen,' Jena, 1886.
[9] Mach, op. citat., 2te Aufl., Jena, 1900, S. 96.
It is brought up again by Lipps,[10] who assumes that the streak ought to dart with the eyes and calls therefore the oppositely moving streak the 'falsely localized image.' For sake of brevity we may call this the 'false image.' The explanation of Lipps can be pieced together as follows (ibid., S. 64): "The explanation presupposes that sensations