The measurement of pupil diameter in psychology (in short, "pupillometry") has just celebrated 50 years. The method established itself after the appearance of three seminal studies (Hess & Polt, 1960, 1964; Kahneman & Beatty, 1966). Since then, the method has continued to play a significant role within the field, and pupillary responses have been successfully used to provide an estimate of the "intensity" of mental activity and of changes in mental states, particularly changes in the allocation of attention and the consolidation of perception. Remarkably, pupillary responses provide a continuous measure regardless of whether the participant is aware of such changes. More recently, research in neuroscience has revealed a tight correlation between the activity of the locus coeruleus (i.e., the "hub" of the noradrenergic system) and pupillary dilation. As we discuss in this short review, these neurophysiological findings provide new important insights to the meaning of pupillary responses for mental activity. Finally, given that pupillary responses can be easily measured in a noninvasive manner, occur from birth, and can occur in the absence of voluntary, conscious processes, they constitute a very promising tool for the study of preverbal (e.g., infants) or nonverbal participants (e.g., animals, neurological patients).

The authors have declared no conflicts of interest for this article.

The violation-of-expectation (VOE) paradigm and related methods are the main tools used to study high-level cognition in preverbal infants. Infants' differential looking to conceptually implausible/impossible events has been used as an index of early cognitive competence in many areas, including object knowledge, physics, language, and number. However, an event's plausibility is commonly confounded with its perceptual novelty or familiarity, leading to a variety of interpretations for looking time data (Bogartz, Shinskey & Speaker, 1997). This illustrative study demonstrates the value of factorial designs, in which perceptual (novelty-familiarity) and conceptual (possible-impossible) variables are independently and jointly explored. It also introduces pupil dilation as a viable and complementary dependent measure to study infant cognition. We show that pupil data can assist in the interpretation of otherwise equivocal looking time data. The discussion focuses on methodological considerations in infancy research.

To be published in Behavioral and Brain Sciences (in press) Cambridge University Press 2007 Below is the unedited précis of a book that is being accorded BBS multiple book review. This preprint has been prepared for potential commentators who wish to nominate themselves for formal commentary invitation. Please do not write a commentary unless you receive a formal invitation. Invited commentators will receive full instructions. Commentary must be based on the book.*

Pupil dilation is a useful, noninvasive technique for measuring the change in cognitive load. Since it is implicit and nonverbal, it is particularly useful with preverbal or nonverbal participants. In cognitive psychology, pupil dilation is most often measured by corneal reflection eyetracking devices. The present study investigates the effect of gaze position on pupil size estimation by three common eyetracking systems. The task consisted of a simple object pursuit situation, as a sphere rotated around the display screen. Systematic errors of pupil size estimation were found with all three systems. Implications for task-elicited pupillometry, especially for gaze-contingent studies such as object tracking or reading, are discussed.