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What goes on in our minds is usually a private affair, but sometimes, our eyes can betray our thoughts. When someone is thinking of a bright image, their pupils will noticeably shrink, researchers have found, even if there’s nothing to look at and they weren’t anticipating light.

The finding builds on previous research that indicates how our pupil response is connected to working visual memory. When we try and recall something, the visual memory can be traced all the way down from higher-order areas to the earliest synapses in our visual systems.

“The thought-provoking corollary to our findings,” the authors write, “is that the pupils provide a reliable measure of what is in the focus of the mind.”

When we humans turn our visual attention to something, our senses often respond in vigorous and selective ways. When that something is a bright light, our pupils contract; when it’s dark, our pupils dilate.

Recent research has shown that this ‘pupillary’ response is regulated by higher brain functions, and most probably by visual sensory areas. Other studies suggest that pupil dilation coincides with an increase in mental activity, and a review last year found that the more tasks we keep track of, the greater their size.

So what happens to our pupils when we focus on a memory that’s triggered by a sound, rather than an image? This latest study used auditory cues to find out.

The team used three experiments, totalling hundreds of trials, in which 22 healthy men and women were shown either a dark or light image in association with a specific sound. In the first experiment, the auditory cues were linked to brightness, whereas in the second experiment, they were linked to location.

The last experiment built on the second by including learned spatial and timing expectations so the participants could anticipate the right answer.

Across all three studies, the dilation of the pupil was linked more to dark grating than to bright grating. And this effect was found even when the brightness of the image was not an acknowledged part of the participant’s memory trial.

“The results provide surprising and consistent evidence that pupil responses are under top-down control by cognitive factors, even when there is no direct adaptive gain for such modulation, since no visual stimuli were presented or anticipated,” the authors write.

“The results also strengthen the view of sensory recruitment during working memory, suggesting even activation of sensory receptors.”

A study released on a pre-print server last year also found something similar. When participants turned their mind’s eye on a bright stimulus they had previously memorised, their pupils contracted.

Unlike these previous findings, however, the authors of the new study say their results are unambiguous. The last study did not separate attention in working memory from the anticipation of a bright stimulus.

Just before you go to turn on a light, for example, your pupils will constrict to prepare you for the sudden onslaught; if that’s what the participants were doing, then it’s not so much about memory retrieval at all.

Extending previous studies of attention in working memory, the authors claim the pupillary response can be flexibly regulated according to the attention and selection of a memorised object and its brightness.

The authors propose that pupil regulation in working memory is not necessarily advantageous, but rather a vestige of higher order cognitive areas using basic sensory systems to do their own thing.

To date, however, it’s still not clear if a pupil’s response to brightness plays an active role in retaining memories, or whether it’s simply connected to those memories because it helped make them.

Whatever the relationship, it’s worth exploring more.

“These findings,” the authors conclude, “highlight the repurposing of sensory mechanisms to guide adaptive behavior based on attention functions and memory traces, all of the way down to the pupils.”

The study was published in PNAS.

Source: Simply Imagining a Bright Light Can Be Enough to Change Your Pupil Size, Study Finds