1. Gregory, R. L., Nature, 238, 51–52 (1972).
2. Osgood, C. E., Method and Theory in Experimental Psychology, 232 (Oxford University Press, New York, 1951).
3. Coren, S., Psychol. Rev., 79, 359–367 (1972).
4. Kanizsa, G., It. J. Psychol., 1, 93–113 (1974).
5. Lawson, R. B., and Gulick, W. L., Vision Res., 7, 271–297 (1967).