Sparse representation of visual information by single neurons in the human medial temporal lobe

Gabriel Kreiman, Christof Koch, Itzhak Fried

Annual Meeting, Cognitive Neuroscience Society, 2002

We studied the responses of over 1000 single neurons in the medial temporal lobe during presentation of complex visual stimuli, expanding on our previous study (Kreiman et al. 2000, Nat. Neurosci. 3:946:953). Subjects were patients with pharmacologically intractable epilepsy who were implanted with depth electrodes in order to localize the seizure focus for potential surgical resection. Recording sites were chosen based on clinical criteria and included the hippocampus, amygdala, entorhinal cortex and parahippocampal gyrus. Stimuli were presented for 1000 ms and included emotional faces, household objects, spatial layouts, animals, cars, photographs and drawings of famous people, foodstuffs and abstract patterns. We observed that 11% of the units responded selectively, changing their firing rate upon presentation of different pictures within a category of stimuli. In contrast, 6% of the neurons displayed more finely tuned responses by increasing their firing rate to only one or a few individual stimuli. The latter group showed shorter latencies and better signal-to-noise ratio as determined by the probability of misclassification in an ROC analysis. Most of the recorded neurons were outside the clinically determined seizure focus area. Our results suggest a sparse representation of complex visual information in the human medial temporal lobe. Furthermore, we observed neurons with different tuning properties, some presumably representing features beyond those characterized by the physical aspects of the pictures.