Give me a break! Unavoidable fatigue effects in cognitive pupillometry

Abstract

Pupillometry has a rich history in the study of perception and cognition. One perennial challenge is that the magnitude of the task-evoked pupil response diminishes over the course of an experiment, a phenomenon we refer to as a fatigue effect. Reducing fatigue effects may improve sensitivity to task effects and reduce the likelihood of confounds due to systematic physiological changes over time. In this paper, we investigated the degree to which fatigue effects could be ameliorated by experimenter intervention. In Experiment 1, we assigned participants to one of three groups-no breaks, kinetic breaks (playing with toys, but no social interaction), or chatting with a research assistant-and compared the pupil response across conditions. In Experiment 2, we additionally tested the effect of researcher observation. Only breaks including social interaction significantly reduced the fatigue of the pupil response across trials. However, in all conditions we found robust evidence for fatigue effects: that is, regardless of protocol, the task-evoked pupil response was substantially diminished (at least 60%) over the duration of the experiment. We account for the variance of fatigue effects in our pupillometry data using multiple common statistical modeling approaches (e.g., linear mixed-effects models of peak, mean, and baseline pupil diameters, as well as growth curve models of time-course data). We conclude that pupil attenuation is a predictable phenomenon that should be accommodated in our experimental designs and statistical models.

Keywords: experiment design; fatigue; growth curve analysis; pupillometry.

FIGURE 1

Picture of the four toys offered to participants in the Kinetic Breaks condition.

FIGURE 2

Timings for the pupillometry task. All data collection occurred during sections with red fixations crosses.

FIGURE 3

(a) Fatigue of the pupil response in Experiment 1 is visualized by summarizing predicted GCA fits by quartile (i.e., collapsing trials 1–20, 21–40, 41–60, and 61–80). Quartiles of trials (“1st,” “2nd,” “3rd,” and “4th” lines) are for visualization only and were not used for analyses. Size of the pupil is shown on the y-axis, and time within the trial (where zero is the start of the stimulus) is shown on the x-axis. Dashed vertical lines show the average offset of the stimuli. (b) Changes in peak pupil diameter (left), mean pupil diameter (middle), and baseline pupil diameter (right) across trials are shown for each random assignment group of Experiment 1. Lines represent model fits and points represent mean values of the raw data summarized across all participants for each trial. Note that the peak and mean pupil diameter measures reflect baselined values, whereas baseline pupil diameter is a summary of the raw arbitrary units used for the baselining process.

FIGURE 4

Visualization of the importance of modeling trial (i.e., time across the experiment) is shown using data from the control condition in Experiment 1. Here, raw data (gray dots) and predicted GCA fit lines are summarized by quartile (i.e., collapsing trials 1–20, 21–40, 41–60, and 61–80). Quartiles of trials are for visualization only and were not used for modeling. Size of the pupil is shown on the y-axis, and time within the trial (where zero is the start of the stimulus) is shown on the x-axis.

FIGURE 5

(a) Fatigue of the pupil response in Experiment 2 is visualized by summarizing predicted GCA fits by quartile (i.e., collapsing trials 1–20, 21–40, 41–60, and 61–80). Quartiles of trials are for visualization only and were not used for analyses. Size of the pupil is shown on the y-axis, and time within the trial (where zero is the start of the stimuli) is shown on the x-axis. Dashed vertical lines show the average offset of the stimuli. (b) Changes in peak pupil diameter (left), mean pupil diameter (middle), and baseline pupil diameter (right) across trials are shown for each random assignment group of Experiment 2. Lines represent model fits and points represent mean values of the raw data summarized across all participants for each trial. Note that the peak and mean pupil diameter measures reflect baselined values, whereas baseline pupil diameter is a summary of the raw arbitrary units used for the baselining process.