Pupillometry

The pupil does not change size only because of light. It dilates a few tenths of a millimetre when the task tightens and returns when the demand passes, an effect described by KahnemanDaniel KahnemanHalf the work this catalogue cites passes through him: anchoring, availability, the endowment effect, peak–end, the two systems.5 references in this work:2011Thinking, fast and slow1966Pupil diameter and load on memory1993When more pain is preferred to less: adding a better end1990Experimental tests of the endowment effect and the Coase theorem1991Anomalies: the endowment effect, loss aversion, and status quo biasSee on Wikipedia ↗See in the bibliography → and BeattyBeattyShowed that the pupil dilates in proportion to mental effort, the measure that makes cognitive load observable from outside.One reference in this work:1982Task-evoked pupillary responses, processing load, and the structure of processing resourcesSee in the bibliography → (1966) in an experiment in which dilation tracked, digit by digit, the number of figures the person had to hold in memory. Beatty (1982) consolidated the finding as an index of load: the response grows with demand and separates levels of difficulty within a single task. For design, it is the instrument that measures the cost of a screen. It measures how much the screen cost, not whether the person liked it. There are three set-up precautions. The first is luminosity, because any variation in brightness contaminates the measure, and the versions you compare need equivalent brightness. The second is that the pupil rises with surprise and with interest the same way it rises with effort, which is why it only separates difficulty within a single task. The third is age, which shrinks the pupil more than load dilates it, and that makes comparing a group of twenty-year-olds with one of sixty-year-olds unworkable without correcting the baseline.