Take any portrait painted within the last 600 years and find a point halfway between the left and right sides of the frame. Then draw a vertical line straight down through the painting. According to a scientist who carried out this exercise (in preparing for a study on how the left and right sides of the brain perceive works of art), something quite unexpected will happen. One of the portrait subject's eyes will almost always fall on the center line. This holds true if the subject is looking straight ahead or to one side.
''I can't explain it,'' said Dr. Christopher Tyler of the Smith-Kettlewell Eye Research Institute in San Francisco, who described his finding in the April 30 issue of Nature magazine. A rule for positioning eyes in this manner does not appear in any book on art composition nor do art teachers and art critics seem to know about it, he said. Artists from Botticelli to Picasso appear to follow the rule unconsciously for esthetic reasons.
Dr. Tyler, an expert on vision and the brain, speculated that the center line might be a particularly salient region of the canvas for conveying the emotional impact of eyes. When the head is turned and the forward eye falls on this line, the person appears bold. But when the other eye falls at the center, he said: ''You get a sense of more intimate connection with the person; you connect with their shyness. Emotional effectiveness rises to a peak on that part of the canvas.''