The ability to conjure pictures in the mind’s eye enables us to remember the past and imagine the future. It also allows us to plan, navigate and create works of art. In a study published April 9 in Science, researchers report that imagining an object reactivates some of the same neurons involved in seeing it in the first place, providing new insight into how mental imagery is produced in the brain.
Previous research had hinted that the neurons involved in perceiving and imagining images overlapped. These studies used various methods, such as asking participants to view and then imagine pictures while lying in a functional MRI scanner, to show that the same brain regions were involved in these processes. But whether the same individual neurons were involved remained an open question, says Ueli Rutishauser, a neuroscientist at Cedars-Sinai Medical Center in Los Angeles.
Because measuring neuronal activity requires electrodes in the brain, Rutishauser and colleagues studied 16 adults with epilepsy who had already had electrodes temporarily implanted into their brains to identify the origin of their seizures. Participants viewed hundreds of images from five categories — faces, text, plants, animals and everyday objects — while researchers recorded activity from over 700 neurons in the ventral temporal cortex, a region involved in representing visual objects. Of those, about 450 selectively responded to individual categories. Machine learning then revealed that 80 percent of those category-responsive neurons were selective to specific visual features within the images.
The researchers then examined what happened when six of the participants conjured mental images of some of the objects they had previously viewed. About 40 percent of the neurons active during perception responded similarly when objects were visualized in the mind’s eye. To confirm their findings, the researchers used data from the neural recordings to reconstruct the pictures the participants were asked to recall.
These observations support the idea that the human brain implements what some call a generative model, where the reactivation of the neural code used to perceive objects enables us to create mental images, says study coauthor Varun Wadia, a neuroscientist at Cedars-Sinai. In addition, because disruptions to mental imagery may contribute to certain psychiatric conditions, such as schizophrenia and PTSD, better understanding the neuronal underpinning of mental imagery could pave the way for new therapies, Wadia adds.
Scientists had built hypotheses about mental imagery and other cognitive processes on the assumption that the same neurons are active during perception and imagination, but this had not been proven until now, says Nadine Dijkstra, a neuroscientist at University College London who was not involved in this work. “This was a study that the field was waiting for.”
Whether similar mechanisms are at play in more complex forms of mental imagery, such as the ability to dream up a novel piece of visual art, remains to be seen, Rutishauser says. “But this presents a clear hypothesis for how things like this could work.”
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