The precision of a memory can be seen in the eyes

"The eyes are the window to your soul," according to Shakespeare. They're also an extremely accurate guide to the precision of a memory, according to a study in the Journal of Experimental Psychology: Learning, Memory, and Cognition. Though psychologists have known for a long time that there are links between pupil size and memory, this new finding is striking, and could have some interesting real-world applications.

Pupil size can indicate the individual's degree of brain arousal. Previous work has shown that our pupils dilate in response to cognitive load, and also that changes in pupil size are linked not only to memory processing, but also to attention and decision-making. According to a study published last year, people whose pupils dilate more while performing a memory task also tend to have better working memory.

Ádám Albi at Budapest University and Péter Pajkossy at the University of Szegad set out to explore whether there are differences in the amount of pupil dilation when a person recognises something as being familiar, or has a really precise memory of it. In everyday life, that could be the difference between bumping into someone in the street and being fairly sure that you've seen their face somewhere before — or not only remembering their face, but also their name, exactly where you met them, and what they were wearing at the time.

Albi and Pajkossy first created a set of 160 relatively uncommon Hungarian words that contained two or three syllables. Then, a total of 28 participants were shown half of these words, one by one, in two experimental blocks. Each word was presented on a screen, somewhere on the circumference of an invisible circle.

During each of two test phases, the participants were shown 40 of the words that had appeared during the preceding learning phase, mixed in with 40 words from the set that they hadn't seen before. These test words also appeared one by one, but in the centre of the screen.

Each time, the participants were asked whether they had seen the word during the learning block. Then, if it was among the words that were presented, they were asked to use a slider to indicate exactly where on the screen it had appeared.

During the test phases, the size of one pupil was monitored. When the researchers analysed their results they found, as expected from previous work, that when the participants correctly recognised a word, their pupil was bigger than it was when they correctly indicated that a test word was new.

Crucially, however, Albi and Pajkossy also found that the more accurate the participants were at positioning a test word on the screen, the bigger their pupil. The relationship between the two measures was completely linear, they write.

This work suggests that the extent of pupil dilation during retrieval of a memory is driven by two things. Firstly, the mere recognition of the target (in this case a word), but secondly, by neural processes that underlie the precise recollection of contextual details (in this case, the location of each word on the screen).

While these findings are no doubt exciting for researchers looking for another window into memory and cognitive processes, they also open up the possibility for a number of real-world applications. For example, it could theoretically be possible to one day use pupil size as an indicator of the precision of a specific memory in a range of situations, including education and police investigations. More research, however, is needed before any such applications.

Read the paper in full:
Albi, Á., & Pajkossy, P. (2025). Pupil dilation accompanying successful recognition is linearly related to memory precision. Journal of Experimental Psychology: Learning, Memory, and Cognition. Advance online publication. https://doi.org/10.1037/xlm0001467

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