Pupillary Response and Behavior
The pupils of the eyes have been referred to as "windows to the soul." Although this romantic notion is untrue, scientists have dicovered some interesting things about pupillary size changes and behavior. Pupillometrics refers to the measurement of variations in the diameter of the pupillary aperture of the eye.
In his book on pupillometry, Janisse ( 1977) wrote that one of the earliest references to the pupil of the eye was made by Travisa ( 1495) in his translation of Bartholomaeus ( Angelica Palli , 1398), in which is stated that "the blacke of theye . . . is callyd Pupilla in latyn for small, ymages ben seen therin" ( Janisse, 1977, p. 1). The word pupil is derived from the Latin "pupilla," meaning "little girl," thus referring to the tiny reflections of persons that one can see when looking into the eyes of another. At least two different writers of the past ( Joshua Sylvester , 1591, and Guillaume de Salluste) have referred to the pupillary apertures as "windows to the soul," implying that they were observation points for a person's innermost thoughts. Scientific observation of pupils is much less romantic than implied by these early writers, because we now know that dilations and constrictions of the pupil are governed by the autonomic nervous system. As we shall see in this chapter, pupillometry has provided psychophysiologists with interesting information about pupil size changes in various situations. Among the issues that have attracted researchers in this field are whether pupils dilate with positive stimuli and constrict with unpleasant ones. Changes in pupil diameter that occur during information processing, perception, short-term memory, learning, and nonverbal communication are also topics of interest. After some basic information about the pupillary response has been presented, including anatomy and physiology and how it is measured, its value in explaining psychological phenomena is discussed.
The pupil is the opening at the center of the iris of the eye through which light passes. Thus, the pupil is merely a hole surrounded by the iris muscle. A major function of the iris is to increase pupillary diameter in dim light and to decrease it in bright light. This adjusts the amount of light allowed to enter the eye according to the surrounding environment. The pupil of the human eye can constrict to 1.5 mm in diameter, it dilates to about 8 to 9 mm, and can react to stimuli in 2 sec ( Guyton, 1977; Lowenstein & Loewenfeld, 1962). The constriction and dilation of the pupillary aperture is produced mainly through ANS control exerted on the muscles of the iris. More specifically, neurons of the PNS innervate circular fibers of the iris, causing pupillary constriction, whereas excitation by SNS neurons causes the radial fibers of the iris to produce dilation of the pupil.