In recent experiments conducted in Hungary, researchers uncovered a subtle yet revealing connection between the eyes and memory. By closely monitoring pupil size while 28 participants recalled previously seen words, scientists observed that the degree of pupil dilation correlated with the clarity and precision of the memory.
In the moments when someone recognized a word they had seen earlier, their pupils subtly widened. And when they could pinpoint exactly where it had appeared on the screen, their pupils grew even more. It was as if their eyes were whispering, “I remember.”
This curious finding is at the heart of a new study published in the Journal of Experimental Psychology: Learning, Memory, and Cognition. Led by researchers Ádám Albi of the Budapest University of Technology and Economics and Péter Pajkossy of the University of Szeged, the study uncovers a powerful connection between memory and eye behavior. It builds on decades of research linking cognitive effort to pupil dilation—but takes it one critical step further.
A Window Into Memory
Psychologists have known since the 1960s that our pupils dilate when we concentrate. This happens when solving math problems, trying to recall a phone number, or focusing intently on a task. But a lesser-known discovery from the 1970s hinted that our pupils also respond to recognition: they widen when we see something familiar. It’s called the “pupil old/new effect.”
What Albi and Pajkossy set out to discover was whether our pupils could also reflect the quality of a memory—not just whether something feels familiar, but how clearly and precisely we recall it.
To test this, they asked participants to study a list of rare Hungarian words. Researchers briefly flashed each word on the screen along the edge of an invisible circle. Later, the participants were shown a mix of old and new words centered on the screen. For each one they recognized, they had to indicate—using a slider—exactly where they’d seen it the first time. All the while, an infrared camera watched a single eye.
The results were remarkable. Pupil size reliably indicated memory strength. A new word caused little change. A familiar word led to modest dilation. But the pupils dilated the most when someone precisely recalled where on the screen a word had first appeared. The correlation was so consistent it was linear.
In other words, your pupils don’t just reveal if you remember. They reveal how well you remember.
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From the Lab to the Courtroom
The implications of this work are wide-ranging. Imagine you’re on a jury, trying to decide whether an eyewitness’s testimony can be trusted. Or you’re a teacher wondering how well a student understands what they just studied. Could a glance at the eyes give you the answer?
“Pupil dilation could serve as a non-invasive marker of memory quality in settings such as education, clinical assessment, or legal testimony,” Mohamad El Haj, a neuropsychologist at the University of Nantes, who was not involved in the study told Popular Science.
Because measuring pupil size is inexpensive and non-invasive—unlike MRI scans or EEG readings—it could one day become a common tool. Albi himself notes that pupil measurement is “methodologically simpler” and could be deployed in real-world scenarios.
Still, the science behind it is far from settled.
“To date, there is no consensus on the precise cognitive and neurobiological mechanisms that drive pupil responses during different forms of memory retrieval,” Albi noted.
One leading theory involves the locus coeruleus, a tiny brainstem region responsible for regulating arousal and attention. It activates during salient moments—those that demand focus—and also triggers pupil dilation. That might explain why a vivid, detailed memory elicits a stronger response than a vague sense of familiarity.
Night Shifts in the Memory Factory
Interestingly, the idea that pupil behavior reflects memory precision isn’t just limited to conscious recall. A study published earlier this year in Nature offers another layer to the story—one that plays out while we sleep.
Researchers at Cornell University tracked the brains and eyes of mice as they moved through daily learning tasks and then rested. During sleep (mice can sleep with their eyes open), the mice’s pupils changed size in synchrony with different types of memory replay. When their pupils constricted, the brain revisited fresh memories. When they dilated, older memories resurfaced.
“It’s like new learning, old knowledge, new learning, old knowledge,” neuroscientist Azahara Oliva, a co-author of the study told Science Alert. “And that is fluctuating slowly throughout the sleep.”
The team found that if they disrupted certain brain wave patterns—called sharp-wave ripples—during the pupil-contracted phase, the mice later struggled to remember the newly learned information. This suggests that eye behavior is tied not just to conscious remembering, but to the nightly process of solidifying memories.
Though these findings come from mice, the implications for humans are tantalizing. Our eyes might not just reflect what we remember while awake—but also how our memories are being quietly sorted and stored while we sleep.
Together, these studies add fuel to a growing idea: that the eyes may serve as a nonverbal, continuous report card of memory.
In artificial intelligence, engineers wrestle with a problem called “catastrophic forgetting”—where new data overwrites old information in neural networks. But biology, it seems, has already solved this problem. Memory systems in the brain may separate and manage information across time, with pupil size acting as a readout of what’s currently on the mental stage.