Activity of human hippocampal formation and amygdala neurons during memory testing

Single and multiple unit recordings were made from fine wires stereotaxically implanted in the hippocampus (HC), hippocampal gyrus (HCG), and amygdala (Am) of psychomotor epileptics. During a series of memory and control tests presented on slides, 21 of 155 HCG units, 15 of 59 HC units, and 2 of 54 Am units showed what appeared to be simple phasic or tonic visual responses. Twenty-seven other units, found only in the HCG, changed firing only during slides requiring a choice ('choice units'). A given choice unit responded during choices indicated verbally or manually, and during tasks requiring recall of Recent Memory, various visual discriminations, and expressions of preference. Choice units were not affected by sensory stimulation or motor activity in contexts not requiring choice. Phasically inhibited choice units had higher firing rates and lower signal-to-noise ratios than tonically excited units. Whether an electrode recorded a choice unit was unrelated to if it recorded a response to hyperventilation, or was in an area of epileptic pathology. Recordings were also made during an interview lasting several hours and eliciting a wide range of behaviors. Five of the 131 HCG units fired in repeated extended bursts, at least 50 times background during recall of word pairs or of the patient's hospital room. The unit response did not occur during numerous control tasks possessing similar overt sensory, motor, and social concomitants, but not requiring Recent Memory.