Pupil size and pupil change correlate with distinct cortical neurochemical systems across the adult lifespan

The human pupil does far more than simply adjust to the brightness of a room. While it shrinks in harsh light and widens in the dark to regulate vision, it also reacts to the mind's internal state. When a person is surprised, focused, or mentally engaged, their pupils often dilate, or widen, even if the light remains constant. For decades, scientists have suspected that these spontaneous changes are a window into the brain's "locus coeruleus," a tiny, deep-seated cluster of nerve cells that acts as the brain's primary source of norepinephrine, a chemical messenger crucial for alertness and attention. Because this brain region is difficult to image directly, researchers have long hoped that measuring the pupil could serve as a reliable, non-invasive proxy for its activity. However, as people age, the biology of the brain changes, and the reliability of this connection has remained unclear. If the link between the pupil and the brain's alertness system weakens or shifts with age, then using pupil measurements to study the aging brain could lead to misleading conclusions.

To investigate this, a team of researchers at Cornell University brought together 89 adults ranging in age from 19 to 78 to perform a visual task while inside a magnetic resonance imaging scanner. The participants watched a screen where circles of different sizes and brightness appeared at random intervals. Most circles were standard, but occasionally a larger circle, known as an "oddball," would appear, prompting the participant to press a button. The researchers recorded the participants' pupil size continuously at a very high speed and simultaneously scanned their brains to measure blood flow, which serves as a proxy for neural activity. They were particularly interested in whether the pupil's size and the speed of its change were linked to activity in the brain's noradrenergic system, and whether these links held true across the entire adult lifespan.

The study revealed that the pupil's behavior is not a single, unified signal but rather carries two distinct types of information that operate on different timelines and connect to different parts of the brain. The researchers found that the sheer size of the pupil at any given moment correlates with brain activity in a specific pattern, while the rapid change in pupil size—how quickly it expands or contracts—correlates with a completely different pattern of brain activity. These two signals do not just happen at the same time; they are tied to different chemical systems within the brain. The rapid changes in pupil size were strongly linked to areas of the brain rich in the norepinephrine transporter, the protein that manages the brain's alertness chemical. This confirms that when the pupil suddenly widens or narrows, it is indeed tracking the activity of the brain's alertness system. In contrast, the steady size of the pupil was linked to brain areas dense with opioid and cannabinoid receptors, suggesting that the baseline size of the pupil reflects a different, slower-acting chemical state, possibly related to pain regulation or mood, rather than immediate alertness.

Crucially, the study examined how these connections hold up as people get older. The researchers found that the relationship between rapid pupil changes and the brain's alertness system remains significant throughout life, but it does weaken slightly in older adults. Specifically, the number of brain areas that showed a strong link to pupil changes decreased in the oldest participants, and the strength of the connection to the norepinephrine system was lower in older adults compared to younger ones. However, the link between the steady pupil size and the opioid-related brain areas did not change with age at all; it remained just as strong in a 70-year-old as in a 20-year-old. This distinction is vital because it suggests that while the brain's alertness system may become slightly less synchronized with pupil movements as we age, other aspects of brain chemistry reflected in the pupil remain stable.

The researchers also looked directly at the brain region responsible for alertness, the locus coeruleus, but found that the signal from this tiny structure was too faint to measure reliably in individual participants using standard scanning techniques. Instead of giving up, they mapped the pupil's influence across the entire brain. They discovered that the rapid changes in pupil size were most strongly associated with a vast network of brain regions involved in attention and sensory processing, spanning from the back of the brain to the brainstem. This widespread pattern, rather than a single pinpoint signal, is what the pupil seems to track. The study also ruled out the idea that these brain-pupil links were simply caused by changes in heart rate or breathing, as the researchers carefully removed those factors from their data and the patterns remained.

Ultimately, the work clarifies that the pupil is a complex instrument that tells two different stories. One story is about the speed of change, which tracks the brain's alertness system and shows a modest decline in its coordination with age. The other story is about the steady state, which tracks a different chemical system and remains consistent throughout life. This means that if scientists want to use pupil measurements to understand the aging brain, they must be careful to distinguish between the size of the pupil and how fast it is changing. By separating these two signals, researchers can better understand which parts of the brain are aging and which are staying the same, offering a more precise way to monitor brain health without invasive procedures. The findings suggest that while the brain's alertness machinery may slow its coordination with the eye as we age, the fundamental chemical landscape of the brain remains accessible through these subtle, measurable shifts in the pupil.