Responses of Single Neurons in the Human Brain During Vision and Visual Imagery

Gabriel Kreiman, Christof Koch, Itzhak Fried

Annual Meeting of the Cognitive Neuroscience Society, San Francisco, 2000

We studied the visual properties of single neurons in the temporal lobe of patients with intractable epilepsy. Based on clinical criteria, intracranial electrodes were implanted in order to localize seizure foci for surgical resection. Probes were stereotactically placed in bilateral medial temporal lobe targets including hippocampus, amygdala, entorhinal cortex and parahippocampal gyrus. We recorded the activity of individual neurons while patients viewed pictures of faces, objects, spatial layouts, abstract patterns and animals projected on a monitor and when the patients were asked to imagine these pictures with closed eyes. In each session, two pictures, A and B, were presented in random alternation, 5 times each, during 1000 ms per presentation. Immediately after this, subjects were asked to close their eyes and imagine stimulus A upon hearing a high tone and stimulus B upon hearing a low tone. Tones were presented every 3000 ms. We analyzed visual activity from 248 neurons (72 Left, 176 Right) in 10 epileptic patients. 58/248 neurons showed visual responses and 31/248 neurons showed selective visual responses. Most of the visually responsive neurons were located in the hippocampus and entorhinal cortex. We analyzed activity during visual imagery in 163 neurons (60 Left, 103 Right). 13/163 neurons showed selective changes in firing rates while subjects were asked to visually recall the same stimuli. Our results suggest that neurons in the temporal lobe may be involved in the generation or maintenance of visual images in our minds in the absence of retinal stimulation. Supported by NIH and the Center for Consciousness Studies at the University of Arizona.