Vivid visual images can be voluntarily generated in our minds in the absence of simultaneous visual input. While trying to count the number of flowers in Van Gogh's Sunflowers, understanding a description or recalling a path, subjects report forming an image in their “mind's eye”1. Whether this process is accomplished by the same neuronal mechanisms as visual perception has long been a matter of debate1,2,3. Evidence from functional imaging1,4,5,6,7,8, psychophysics1,9, neurological studies2 and monkey electrophysiology10,11,12 suggests a common process, yet there are patients with deficits in one but not the other3,13. Here we directly investigated the neuronal substrates of visual recall by recording from single neurons in the human medial temporal lobe14,15 while the subjects were asked to imagine previously viewed images. We found single neurons in the hippocampus, amygdala, entorhinal cortex and parahippocampal gyrus that selectively altered their firing rates depending on the stimulus the subjects were imagining. Of the neurons that fired selectively during both vision and imagery, the majority (88%) had identical selectivity. Our study reveals single neuron correlates of volitional visual imagery in humans and suggests a common substrate for the processing of incoming visual information and visual recall.
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This work was supported by grants from NIH, the Centre for Consciousness Studies at the University of Arizona and the Keck Foundation. We thank M. Zirlinger for discussions, T. Fields, C. Wilson, E. Isham and E. Behnke for help with the recordings, F. Crick for comments and I. Wainwright for editorial assistance. We also thank all the patients who participated in these studies.
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