Dominguez, K. E. (1954). A study of visual illusions in the monkey. Journal of Genetic Psychology, 85, 105-127.

(Summary and conclusions.) The purpose of the present study was to test the monkey for certain static and dynamic illusions to which human beings are known to be susceptible. These included the breadth of rectangles illusion, the vertical-horizontal illusion, and the illusion of apparent motion. Three types of monkeys were used, the rhesus, mangabey, and cebus. An intensive training series preceded the tests for each illusion. The results seem to warrant the following general conclusions:

1. Vertical-horizontal illusion. This illusion was used in two forms, areas and lines. The illusion-effect was about 2 per cent for the areas and 3 per cent for the lines. This corresponds roughly to the amount of the illusion-effect found in man.
2. The breadth of rectangles illusion. The illusion-effect, as measured under our conditions, was 17 per cent. The corresponding index is not known for man. No explanation can be offered why this illusion is greater than the vertical-horizontal illusion.
3. Illusion of apparent motion. The presence of this illusion was demonstrated. Critical rates were determined and checked with two human subjects. The critical rate of speed at which the illusion occurs is approximately the same in man and monkey.
4. The three types of monkeys tested included samples from the old world stem (rhesus and mangabey) and the new world stem (cebus). The degree of the illusion-effect would seem to be about the same for all the types tested. On the whole, there is no consistent individual difference.