Leveraging the pupillary light reflex for cognitive pupillometry: An initial characterization of the PLR in two data sets

Cognitive pupillometry in the “wild” (outside laboratory control) faces the difficulty of low signal-to-noise due to relatively larger scale pupil size fluctuations induced by luminance variation. A proposed method to improve inference of cognitive processes in natural environments is to amplify the cognitive signal by methodologically controlling for, or statistically removing, the influence of the pupillary light reflex (PLR). Here we suggest an alternative method by purposefully inducing PLR’s and measuring variation in the shape of the PLR waveform to infer cognitive states. To explore this possibility, we induced the PLR in two different experiments while subjects performed a wide range of cognitive tasks. In one experiment participants navigated a stressful naturalistic virtual environment while searching for targets. During the experiment the screen flashed white to mimic a flashbang to induce a stress response and PLR. In the second experiment participants viewed simple flashing stimuli to induce the PLR before and after completing a battery of cognitive tests. First, our results show that aspects of the PLR vary over time and when concomitant with experimental manipulations of cognitive processes. Second, we demonstrate statistically significant and interesting relationships between aspects of the PLR (I.e., latency of the PLR, velocity and magnitude of the constriction) and behavioral performance. Together these results suggest that the PLR variation has the potential to be used reliably as a measure of cognitive processing and may be especially useful in naturalistic experimentation.