Anomalies of Form Perception Resulting from Elements Moving within an Aperture or from an Aperture Moving over a Form

January 1971

Anomalies of Form Perception Resulting from Elements Moving within
an Aperture or from an Aperture Moving over a Form

J. J. Gibson, Cornell University


The World Wide Web distribution of James Gibson’s “Purple Perils” is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.

Psychology is plagued by what I call confusion-creating experiments. These are procedures that yield curious and repeatable data but that are worthless or worse since they promote and perpetrate confusion. One such is the experimental procedure described by Parks (1965) involving motion and an aperture. It was taken up by Haber (1968) and also by Hochberg (1968). The procedure as such goes back at least to Gibson (1947, p. 88 ff) but I do not think it has any bearing on ordinary visual perception.

The use of windows, apertures, frames, or holes in setting up experiments on visual perception is very tricky, and when relative motion is added to the situation it is even more tricky. E is interested in O’s perception of what is behind the aperture. But (as Hochberg and Haber ought to know) the more an aperture approximates to being a slot the more will the information in the array approximate to being motion along the slot and the less information will there be for the perception of whatever is behind the aperture. This is demonstrated by Michotte’s method of producing optical motions in an array, a method which entails no form perception whatever. When the aperture becomes a mere slit in the screen the only information in the optic array is for spots moving along the slit. The information in the light for spirals (or whatever) moving behind the screen is then nonexistent (references: unpublished study by Gibson and Flock of slit-motions).

Moreover the history of research on the apparent direction of the motion of moving stripes in a window, as this depends on the shape of the occluding edges of the window, the frame, should also make these investigators realize that you cannot expect to move an aperture over a picture, or move a picture behind an aperture, without providing anomalies in the perception of the picture. These anomalies have nothing whatever to do with the perception of form as such. Aperture-motions interfere with pattern perception, they do not simulate the sequential character of perception-with-scanning.

The exploratory sampling of the external optic array by head-movements and eye-movements is not at all comparable to an experiment in which the edges of a small window are made to cover and uncover the parts of a large figure. The eye of man is not a window of his soul, through which he looks out on the world, and neither is the bony orbit of his skull, nor the fovea of his retina.

There are a whole set of muddles that this mischief-making experiment encourages. For example the investigators have been led to distinguish between a mere “painting” of a figure on the retina (with fast motion) and a “post-retinal integration” of a figure (with slow motion) that is, perception genuinely dependent on. But this entails a whole tangle of conceptual confusions. It distracts us from considering the fact that a rapidly moving aperture looks transparent and makes the display visible (as with an episcotister) whereas a rapidly moving display tends to become invisible as the motion tends to become streaming.

The perception of aperture motion has been one of the worst muddles of experimental psychology since the work of J. F. Brown and before. Experiments of this sort are problems in themselves, not ways of getting useful evidence.


Parks, T. E. Post-retinal visual storage. Amer. J. Psychol., 1965 78, 145-147.

Haber, R. N. & Nathanson, L. S. Post retinal storage? Some further observations on Parks’ camel as seen through the eye of a needle. Percept. and Psychophys., 1968, 3, 349-355.

Hochberg, J. In the mind’s eye. In Haber (Ed.),Contemporary theory and research on visual perception. Holt, Rinehart, & Winston, 1968.

Gibson, J. J. Motion picture testing and research. U.S. Govt. Printing Office, 1947.