EARLY EFFORTS IN PUPIL-BASED MEASUREMENT OF COGNITIVE WORKLOAD
The pupil has often been characterized as ‘a window into the mind’ because of the direct neural pathways from the brain to the pupil. The pathways between the brain stem and the pupil have been much studied for many years and are well understood.
Pupil size is governed by two muscles: the constrictor or sphincter muscle that directly encircles the pupil and the radial muscle that lie in the iris beyond the constrictor muscle. The radial muscle is part of the sympathetic nervous system while the sphincter muscle is part of the para-sympathetic system. Both muscles receive electrical impulses directly from the brainstem.
These two muscles interact to produce two reflexes, the light reflex and the dilation reflex. The light reflex is familiar to anyone who has ever stepped from a dark room into bright sun—the pupil immediately shrinks as the constrictor muscle receives activation and the radial muscle is inhibited. Physicians routinely use the light reflex to test for neurological problems because it is easy to observe and well understood.
The dilation reflex may be less familiar to the general population, but it is equally well understood by scientists. The dilation reflex is the process by which activity in the brain becomes evident in the pupil. It too follows the law of reciprocal innervation involving both muscles surrounding the pupil. The radial muscle is the agonist in the dilation reflex. When activated, it causes the pupil to open wider. The sphincter muscle is the antagonist. When inhibited, it relaxes the degree of pupil constriction. This sympathetic activation is fast and short lasting. The result is a small, fleeting increase in pupil dilation.
[For more information on the physiology of the pupil, a good reference is Loewenfeld, I. (1993). The Pupil, Volumes I and II. Ames, IA: Iowa State University Press. These volumes summarize more than 15,000 research studies on the topic.]
The relationship between pupil activity and the brain is well established, especially in the medical world. Physicians routinely check the pupils when abnormal brain functioning is suspected. Anesthesiologists monitor the pupil to judge the depth of anesthesia in surgical patients. And, the ultimate cessation of brain activity is often made by testing pupil response. All states in the U.S. have adopted the Uniform Definition of Death Act (UDDA) which specifies cessation of all brain activity including the brain stem. One oft-stated characteristic of brain stem death is lack of pupil response.
Psychologists recognized the link between mental activity and pupil dilation long before they could measure it accurately. The earliest paper on this topic appeared in 1900. Formal studies linking pupil dilation and cognitive effort began to appear in the research literature about 50 years later.
A number of key psychologists have recognized the importance of pupil dilation in understanding human cognition. For instance, Daniel Kahneman incorporated pupil dilation as a cornerstone of his well-known theory of attention. Kahneman argued that pupil dilations were indicative of mental effort, and he carried out a number of different studies to demonstrate how pupil size changed as mental effort increased.
Similarly, Jackson Beatty studied how the pupil changed during the performance of different cognitive tasks including memory, language, reasoning, and perception tasks. Beatty and his colleagues developed the task-evoked pupillary response for measuring changes in pupil diameter, which was patterned after the evoked potential responses used in EEG measurements. This technique is still used in many laboratories today. Essentially, it requires defining a short baseline period that precedes the presentation of each stimulus. The pupil is measured during the baseline and again during the stimulus presentation and the difference is compared.
These early researchers and their associates faced many technological problems in measuring pupil size. Subjects often had to use bite bars and chin rests to restrict head movement, and cameras were primitive compared to today’s pupillometers and eye trackers. Most of the researchers recognized that factors other than mental effort could also change pupil size. Some of these factors are internal to the subject (e.g., arousal, frustration) and some are external (e.g., experimental conditions), and their impact may be large. For accurate measurement, it is important to control as many of these factors as possible. In particular, it is important to control the impact of light. Even slight variations of light (such as presenting different numbers and letters on a computer screen) may change pupil size. It became evident by the late 1990s that measurement of absolute pupil diameter itself would not be a sufficient indicator of mental effort because one could easily manipulate pupil size by changing the lighting conditions. Even the improved task-evoked pupillary response was prone to light contamination.
Under ideal conditions, measurement of absolute pupil diameter might be sufficient for estimating cognitive activity in the brain. The greater the electrical impulses received by the brainstem, the greater the number of reflex impulses sent to the pupils and the greater the pupil response. If nothing else changes, the increase in pupil diameter reflects the degree of mental activity. Unfortunately, except for very tightly controlled laboratory studies, it is impossible to hold constant all the other factors that can impact pupil size. It is especially difficult to control the impact of light in situations involving real-world activities.