It’s often called the mind’s eye. “I can look at an object in the world around me, but I can also close my eyes and imagine the object,” says Varun Wadia, a brain scientist at Cedars-Sinai Medical Center and Caltech. Visual imagination helps people conjure faces, navigate with mental maps and picture unseen parts of objects.
Wadia and colleagues reported in Science that imagined and perceived objects activate the same neurons and use the same neural code. “This has not been demonstrated before at the neural level,” says Kalanit Grill-Spector of Stanford, who was not involved in the study. The finding moves researchers closer to building computer models of vision and vision disorders and could aid development of prosthetic devices to restore sight. Thomas Naselaris of the University of Minnesota adds that the result helps explain how imagination fills in unseen parts of three-dimensional objects or assembles familiar pieces into novel combinations, like a unicorn.
The study recorded individual neurons in 16 epilepsy patients who already had implanted electrodes. The team monitored more than 700 neurons per participant while subjects viewed hundreds of images from categories such as faces, animals, plants, words and small objects. Researchers focused on neurons in the ventral temporal cortex, a region involved in object recognition, and recorded both which neurons fired and how often, allowing them to decipher the neural code for each image.
In a second phase, participants closed their eyes and imagined objects they had previously seen. About 40% of the neurons that responded during perception reactivated during imagination, Wadia says, and they did so with roughly equal strength. The overlap was large enough that the researchers could identify which object a participant was imagining and recover details about it—its size, angle and whether it was typically found indoors or outdoors, says Ueli Rutishauser of Cedars-Sinai and Caltech.
The result corroborates earlier brain-imaging studies suggesting shared circuits for seeing and imagining, but it goes further by showing this at the level of individual neurons—something fMRI cannot do. It also builds on work by Doris Tsao and others on how visual systems encode faces and objects.
The study doesn’t explain aphantasia, a condition in which people report an inability to voluntarily form mental images. Rutishauser describes meeting a scientist who said, “I don’t know what you’re talking about. I don’t see anything when I close my eyes.” Scientists suspect people with aphantasia rely on words or concepts rather than visual imagery, but understanding their neural mechanisms will require direct neuronal studies.