LETTER

TO THE EDITORS

THE MOVEMENT AFTER-EFFECT AND EYE MOVEMENTS (Receiced 3 January 1975)

Illusory movement of a stationary target is seen after observing prolonged unidirectional movement of a patterned target. This illusory movement is known as the movement after-effect (MAE) or waterfall effect. A number of hypothetical explanations of this effect have been based on movements of the image of the test stimulus across the retina due to eye movements. The significance of such movements in the production of the MAE has been examined directly by using a stabilised retinal image. The stimulus was a square-wave grating (an array of lines separated by their width) of spatial frequency 25 lines,‘deg, seen against a field of 10” dia and 104td retinal illuminance. The grating was mounted on a piezo-electric element connected to a contact lens. (This apparatus will be described in a subsequent paper.) It was brought into focus by a high quality 40D lens. Thus, when the contact lens was on the observer’s eye, the image of the grating was stabilised. To ensure the contact lens fitted tightly, reducing slip to negligible proportions, a suction of 200mm of water was applied. The stability was crudely tested by sucking the lens onto a spherical glass surface and applying step movements of about 24’. No slip was observable, indicating an upper limit of 2% slip, or 05’, despite the much reduced adhesion as compared with the lens being on the eye. The target was moved by applying a suitable voltage to the piezo-electric element. The inducing stimulus was thus known more precisely than in normal vision, where the effect of eye movements is added to the imposed movement. Unfortunately the magnitude of movement was restricted to about 20’ by the voltages which were considered safe to use. A suitable stimulus was obtained by using a saw-tooth waveform with frequencies in the range of l-10 Hz. giving velocities in the range of 20--2(X’set-’ across the retina in one direction (the slow phase) and of the order of 2O.OCO’ set-’ in the other. The movement looked jerky but continuous, and seemed to be in the direction of the slow phase. Induction periods of 6Osec in duration were used. The test stimulus was obtained by switching off the applied voltage. As a control, the apparatus was held up to the eye (but not attached). The target was seen with one eye, the other eye was occluded. Under these conditions an MAE of about 10 set duration was observed. The contact lens was then inserted into the eye and allowed to settle firmly into position. When movement was imposed the grating was seen clearly. When movement was stopped, the image momentarily looked very sharp, and stationary (025-05 set). It

then appeared to move, much more convincingly than in unstabilised vision. As in other stabilised image experiments (e.g. Ditchbum, 1973) the image faded within a few seconds of movement being stopped. It was repeatedly made to reappear by interrupting the incident light. The MAE was observable in this way up to about 10 set after the target movement was stopped. The direction of apparent movement was constant during this time. No displacement of the target was observed as a result of the imposed movement after this movement was stopped. These observations were repeated by a second observer, who also reported seeing the MAE with a stabilised test stimulus. He did not. however, report the illusion as being much more convincing than in normal vision. This experiment seems to show that the MAE is independent of eye movements. But at least one other explanation is possible: that the observed after-effect was peculiar to the observation of a stabilized image and of different origin to the MAE seen in unstabilised vision. For example, movements of the eyes produce illusory movement of a stabilised image (Ditchburn and Ginsborg, 1952). The inducing stimulus probably produced nystagmoid eye movements persisting for several seconds after target movement was stopped (Helmholtz, 1910). This nystagmus could have produced illusory movement. This alternative hypothesis would predict movement in the same direction as the slow phase of the inducing stimulus. The observed direction of movement was in the opposite direction, making this alternative hypothesis unlikely. In conclusion, the MAE appears to be independent of movements both of the test stimulus and of the eyes. It may have been enhanced by eliminating the effect of eye movements on the image of the inducing stimulus. Dept. of Engineering and Cybernetics, University of Reading, Whiteknights, Reading RG6 ZAL, Berks., England

A. E. DRYSDALE

REFERENCES

Ditchburn R. W. (1973) Eye-Movementsand VisualPerception. Clarendon, Oxford. Ditchbum R. W. and Ginsborg B. L. (1952) Vision with a stabilised retinal image. Norare, Land. 170, 36-37. Helmholtz H. von (1910) Han&u& der Physiologischen Opt& Vol. III. pp. 247-250 (from the English translation by the Optical Society of America (192S), Dover, New York. 1962).

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The movement after-effect and eye movements.

LETTER TO THE EDITORS THE MOVEMENT AFTER-EFFECT AND EYE MOVEMENTS (Receiced 3 January 1975) Illusory movement of a stationary target is seen after...
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