Case Report: Direct Voluntary Control of Pupil Size

Article: Man Can Change His Pupil Size on Demand
Source: ScienceAlert, via Live Science
Published: September 1, 2021

Pupil contriction and dilation are controlled by the autonomic nervous system, a process that was once thought to be automatic in response to stimuli, such as light/darkness, arousing emotions, or mental effort. While indirect voluntary control of pupil size in response to imaginary light has been documented in the literature—for example, imagining a "sunny sky" or a "dark room" resulted in pupillary constriction and dilation, respectively—directly controlling the iris sphincter and dilator musculature was thought to be impossible. That is, until a psychology student in Germany approached his professor, one of the researchers, about his unusual ability to "tremble" his eyeballs. Known by his initials D.W., the student reports, "Constricting the pupil feels like gripping, tensing something; making it larger feels like fully releasing, relaxing the eye." Interestingly, D.W. described having initially practiced the ability by focusing in front of or behind an object, not unlike the miosis and mydriasis that occurs as a result of convergence and divergence of the eyes, respectively. According to the authors, "[I]t seems plausible that [D.W.] could have learned to gain control over the pupillary response by decoupling pupil size changes from accommodation and vergence in the near triad." After a while, D.W. states that all he needed to do to change his pupil size was to concentrate, noting that he doesn't have to imagine bright or dark environments.

D.W.'s eyes at different stages of dilation

The researchers performed a variety of tests, such as measuring the voltage on the skin as a proxy for mental effort, to rule out indirect ways of controlling pupil size. They then measured D.W.'s dilation of pupil diameter to be around 0.8 mm and constriction of pupil diameter to be around 2.4 mm. Moreover, even closer than his near point of accommodation or NPA (that is, the distance at which accommodation is maximal, at which a small object held in front of the eye, such as the tip of a pen, cannot be clearly resolved), D.W. could voluntarily constrict his pupils further. The authors report, "Even at maximal accommodation, [D.W.] voluntarily constricted his pupil without changing vergence and could improve visual acuity by >6 diopters." Task-based functional magnetic resonance imaging (fMRI) showed increased activity in brain areas responsible for volitional impulses, in this case, the dorsolateral prefrontal cortex, adjacent premotor areas, and supplementary motor area. And although the researchers cannot elucidate any connections between these cortical areas and the sympathetic or parasympathetic pathways that ultimately control autonomic pupillary reactions, nor can they definitively rule out that D.W. was using indirect strategies to change his pupil size, they found no evidence of such from the tests they performed. Accordingly, they conclude that this is the first reported case of direct voluntary control of pupil size.

My rating of this study: ⭐⭐⭐🌸

Eberhardt LV, Grön G, Ulrich M, et al. "Direct voluntary control of pupil constriction and dilation: Exploratory evidence from pupillometry, optometry, skin conductance, perception, and functional MRI." International Journal of Psychophysiology.  168:33-42. October 2021. https://doi.org/10.1016/j.ijpsycho.2021.08.001