Methodological Aspects of Pupillometry

In this chapter, we focus on methodological aspects of pupillometry. We first survey existing types of equipment that allow the measurement of pupil size. We then discuss how a pupillometry experiment is first designed and subsequently set up in the controlled environment of a laboratory. We discuss pupillometry as a quantitative method that provides measurements on an interval scale (millimeters) that directly describes a real-world physical dimension (the size of the eye opening). Hence, statistical tests on quantitative data are typically used to compare results in each of the conditions of the experiment and to infer relationships between experimental conditions and behavior or physiology. Since there is no minimal pupil-size change that counts as a valid or meaningful pupillary signal (e.g., of increased mental effort or arousal), all pupil-size changes must be compared through inferential statistical methods to compare changes relative to a baseline or control condition while considering individual differences. We also discuss control and removal of potential artifacts when measuring the pupil in specific experimental situations. As we shall see, one needs to consider a variety of things and potential problems when doing a pupillometry experiment. However, many aspects are no different from hypothesis testing in any other field of empirical science.