Existing pupillometers indiscriminately illuminate the whole pupil with LED light. Using modern photonic technology, we have developed a pupillometer that has surgical precision in illumination. It uses a laser beam, the cross section of which consists of discrete pixels that can be either dark or bright. Hence we obtain spatial selectivity in the illumination of the pupil, and hence spatial selectivity in the stimulation of the retina. With computer control, we can obtain any illumination arrangement in a 5×5 grid of 25 pixels. Moreover, we can precisely computer-control the photon number per unit time per illuminated pixel. Finally, using infrared light exactly superimposed on the green stimulus light we obtain precise information on the illumination geometry on the pupil through the reflection of the infrared light.

We show that by illuminating different single pixels we obtain qualitatively different pupil responses, conveying very rich yet until now inaccessible information about the relevant neural circuits. We can even induce oscillations in the pupil diameter by illuminating the boundary of the pupil (figure e on the left). When illuminated the pupil closes, but then the brain thinks “there’s no light”, prompting the pupil to reopen and so forth. Finally, by lighting up all available pixels (now 25) we obtain an average kind of response (figure f on the left).