The horror movies often use enlarged pupils in the eyes to depict characters encountering extreme fear or surprising events. This has become a classic technique in cinematic expression. So, why do people’s pupils dilate when they are surprised?
According to the renowned medical institution Cleveland Clinic in the United States, the pupil is the black circular hole located in the center of the iris, with the iris being the colored part of the eye. The muscles inside the iris are responsible for regulating the size of the pupil, and this regulation typically depends on the intensity of light, making the pupil a crucial part of the visual system.
When exposed to bright light, the pupil contracts and becomes smaller; whereas in dim light, the pupil dilates and enlarges. This response is known as the pupillary light reflex.
The pupil is protected by the cornea and conjunctiva. The cornea is a transparent dome-shaped tissue covering the surface of the eyeball above the pupil, providing protection. Another layer of tissue called the conjunctiva also protects the pupil and the entire eyeball.
Under normal circumstances, the pupil only dilates or constricts in response to external factors like light. However, a previous study revealed that a man in Germany could directly control the size of his pupils through thought, which was previously considered impossible.
Brown University stated in a press release on August 5th that when people encounter unexpected information, their pupils often dilate. A study led by researchers at the university indicated that this physiological response signifies changes occurring within the brain, assisting individuals in adapting to new situations.
The author of the study report and associate professor of neuroscience and cognitive and psychological sciences at the university, Matt Nassar, mentioned that pupil dilation signifies a heightened state of alertness, which can be beneficial in navigating the constantly shifting scenarios in this world.
The research suggested that this increased alertness marks the brain transitioning into a new mode, instantly altering how individuals perceive their surroundings and learn from them.
Nassar explained that unexpected events trigger activity in the locus coeruleus region of the brain. The locus coeruleus is a major source of norepinephrine, a chemical messenger crucial in driving the body’s fight-or-flight response. This activity is correlated with changes in pupil diameter and specific brain wave patterns recorded by electroencephalography (EEG).
In this study, researchers designed an experiment involving colored squares appearing on a screen, requiring 63 participants to predict the upcoming images repeatedly.
These participants then viewed a different set of squares, reported their observations, and made new predictions for the following images. Throughout this process, researchers collected physiological data from participants, including changes in pupil diameter and EEG signals.
The researchers found that unexpected colors (colors diverging from expectations) triggered pupil dilation and enhanced specific brain wave activities – indicating that the brain was allocating considerable cognitive resources to process this “unexpected” new information.
They also discovered that when the images seemed to match participants’ initial expectations, they tended to rely on their existing predictions rather than careful observation – the brain’s learning process adjusted to a “lazier mode.”
The pupil responses and measurements of brain waves indicated that participants could learn from unexpected outcomes and adjust their subsequent predictions accordingly.
The researchers concluded that encountering unexpected situations led to a surge in norepinephrine, acting as a “refresh” mechanism – signifying the brain’s adaptation to new information altering initial expectations.
Nassar elaborated, stating, “The brain retains a certain mental context, and once it recognizes that you are in a new context, it replaces it with the new one. The changes in pupil diameter and specific EEG readings serve as external markers showing the brain is loading a new context.”
The findings of the study were published on August 5th in the journal “Nature Human Behaviour.”