Thinking about "Thinking Harder"

At its best, cognitive psychology can seem like magic. We can use techniques like pulsation threshold masking, stroop tasks, or phonological suppression to infer the kinds of hidden mental processes that modulate behavior. Here's one of the least used instruments in the cognitive psychologist's toolbox: pupillometrics.

Pupil dilation is consistently sensitive to mental effort (aka "capacity utilization" or some function of both absolute task demand and individual differences in ability) in mental arithmetic, sentence comprehension, letter matching, stroop, human-computer interaction (drag-and-drop, search), problem solving, imagery, rehearsal and retrieval from STM, and delayed tone discrimination tasks. Confounding variables include ambient lighting, baseline pupil diameter, spontaneous variations in pupil diameter (which seem to be suppressed during cognitive load), and direction of eye gaze, all of which can be controlled with various procedures. Diameter changes occur within 1200 ms of cognitive demand onset, and constriction occurs with similar speed, but these speeds are dependent on ambient lighting level. The absolute magnitude of diameter change differs between individuals but trends are consistent, as typically measured (average pupil diameter, or percent change in pupil diameter from baseline, with various sample rates). The mechanism relating cognitive load to pupil diameter is unknown, but is hypothesized to be a consequence of cortical modulation of the reticular formation, which is itself thought to modulate the pupillary control system.

Pupil dilation can measure what it means to "think harder" because it is a function of capacity utilization; it reflects not only task demands, and not only individual ability, but some combination of the two. In fact, one can even track moment-by-moment cognitive effort in digit span tasks: progressive pupil dilation occurs as each digit is presented, and progressive constriction occurs as each digit is recalled. Maximal pupil dilation corresponds both with the reported time period of maximal effort, and with cognitive components of task structure as determined through GOMS analysis. For all the non-believers, you can even see practice effects, such that tasks will show less absolute dilation after extended practice, despite "task demand" remaining ostensibly constant.

Pupil dilation is certainly a valuable addition to other neuroindices (EEG, fMRI) and appears to be relatively stable across development. Pupillary responses can even be seen in infants less than 4 months old, in response to various social stimuli (such as pictures of their mothers).