Source: Petr Novák, Wikipedia/Creative Commons

What do bigger pupils and wide-eyed amazement have in common? Wider pupil diameter increases visual sensitivity, which in turn increases the odds of having a “Wow!” moment while witnessing something (or someone) visually stunning.

According to Merriam-Webster’s website, the first known use of wide-eyed to describe the sense of “having the eyes wide open especially with wonder or astonishment” was in 1789. A century later, in 1899, Mark Twain famously wrote: “[But] the wide-eyed wonder, the reverent admiration in his face were more eloquent than any words could have been.” 

To the best of my knowledge, until recently, there’s been little neuroscience-based research into how a subconscious sense of wonder may cause the pupils in someone’s eyes to become broader in diameter. 

Recently, a pair of German researchers, Caspar Schwiedrzik and Sandrin Sudmann, discovered that when humans see something unique or extraordinary in the environment or someone’s face, our pupil diameter widens, which makes more information available for visual processing. These findings (Schwiedrzik & Sudmann, 2020) were published on May 5 in the Journal of Neuroscience

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“When light hits the retina, the pupil reflexively constricts. This determines how much light and thus how much information is available for visual processing,” the authors explain. Historically, most experts agreed that the pupillary light reflex (PLR) automatically adjusted pupil diameter strictly based on the amount of light in the environment, without any subjective influence based on conscious or unconscious intention.

The goal of this study was to identify if a brainstem reflex pathway regulates pupil diameter changes without any cognitive control or if the unconscious mind also plays a previously unrecognized role in the pupillary reflex. This research was conducted at the German Primate Center (DPZ) and the European Neuroscience Institute (ENI) in Göttingen, Germany. 

Through a series of elaborate experiments in humans and rhesus monkeys, the researchers found that pupil diameter is not only reflexively controlled by luminance variations in the environment; changes in pupil diameter are also unconsciously influenced by our mind. “Pupil control is not purely reflexive, [it] is also influenced by our unconscious thoughts,” Schwiedrzik said in a news release.

Based on these findings, it appears that the reflex pathways that regulate pupil diameter across primate species are, to some degree, influenced by cognitive control in an unconscious attempt to boost the transmission of certain visual stimuli to the brain. 

The researchers also found that pupil size moves to the rhythm of the environment. ”We find entrainment to environmental statistics in both species. This entrainment directly affects visual processing by increasing sensitivity at the environmentally relevant temporal frequency. Thus, pupil dynamics are matched to the temporal structure of the environment to optimize perception, in line with an active sensing account,” the authors conclude.