Left and Right I N Science and Art

An asymmetric object can exist in two left-right mirror-image forms, enantiomorphs, a phenomenon which has fascinated philosophers, cosmologists and artists. Psychologists and neurophysiologists have been particularly puzzled by the extreme difficulty children and other animals have in learning to distinguish left-right mirror images. The authors propose an explanation of why mirror images are so confusing. In the natural world almost all mirror images are actually two aspects of the same object, for example, the two sides of a face or a silhouette viewed from the front and back. Therefore a perceptual mechanism that treats mirror images as equivalent would be adaptive. The perceptual equivalence of mirror images only becomes maladaptive or confusing under very special conditions. One of these is learning an orthography containing mirror images such as b and d. DifJiculty in learning to read, may, in part, be due to d i f fu l ty in overcoming the normal tendency to treat mirror images as the same stimulus. In thefinal portion of rhe paper rhe authors consider rhe eJfecrs of mirror-reversing apainring and, more generally, left and right in pictorial space. They suggest that some pictorial anisotropies, such as profile orientation, reflect the influence of lateralized brain functions, whereas others, such as the tendency to look at a picture,from lefi to right, are cultural conventions. How is the left hand different from the right hand? The paradox that an asymmetric object can exist in two mirror-image forms has fascinated philosophers, artists and scientists for a long time (Figs. 1 to 4) and still bewilders children trying to distinguish b from d and s from 2. This essay will consider two aspects of the perception of left and right. One is the difficulty of distinguishing enantiomorphs-of telling left from right mirror images. The second is the role of left and right in art. We will suggest that present-day ideas on brain function and its evolution may help elucidate both phenomena. I. LEFT, RIGHT AND COSMOLOGY Is there a 'left' and 'right' in the universe? Newton said yes, and Leibnitz said no. Newton thought that the coordinates of space were absolute and 'Godgiven'. Leibnitz attacked this view and argued that left and right were 'in no way different from each other' [I]. Kant was puzzled by enantiomorphs for decades and they led him to side with Newton [2]. For Kant the difference between left and right mirror images was literally 'inconceivable'. They could only be distinguished through intuition, through the a priori structure of the mind, and therefore for Kant, the apriori left-right structure of the universe [3]. Until very recently, however, cosmologists agreed with Leibnitz: the universe was symmetrical and right and left were arbitary human conventions. *Psychologist, Dept of Psychology, Princeton University, Princeton NJ 08540, U.S.A. (Received 28 Feb. 1977). The problem was of strictly theoretical interest until many astronomers came to believe that there must be millions of inhabitable planets and therefore probably other beings at least as intelligent as humans. Distinguishing between right from left then became of practical importance for developing a method of extraterrestrial communication. A digital code is clearly the best Fig. 1. Quartz crystals [from the Nobel prize address of V. Prelog, Chirality in Chemistry, Science 193, 17 (1976)l. Fig. 2. Horns of Marco Polo's sheep [from D. W . Thompson, On Growth and Form (Cambridge: Cambridge Univ. Press, 1969)l. 30 Charles G. Gross and Marc H. Bornstein