Human medial temporal lobe neurons respond preferentially to personally relevant images

Abstract

People with whom one is personally acquainted tend to elicit richer and more vivid memories than people with whom one does not have a personal connection. Recent findings from neurons in the human medial temporal lobe (MTL) have shown that individual cells respond selectively and invariantly to representations of famous people [Quian Quiroga R, Reddy L, Kreiman G, Koch C, Fried I (2005) Nature 435(7045):1102-1107]. Observing these cells, we wondered whether photographs of personally relevant individuals, such as family members, might be more likely to generate such responses. To address this issue, we recorded the activity of 2,330 neurons in the human MTL while patients viewed photographs of varying personal relevance: previously unknown faces and landscapes, familiar but not necessarily personally relevant faces and landscapes, and finally, photographs of the patients themselves, their families, and the experimenters. Our findings indicate that personally relevant photographs are indeed more likely to elicit selective responses in MTL neurons than photographs of individuals with whom the patients have had no personal contact. These findings further suggest that relevant stimuli are encoded by a larger proportion of neurons than less relevant stimuli, given that familiar or personally relevant items are linked to a larger variety of experiences and memories of these experiences.

Fig. 1.

A neuron in the amygdala with a selective excitatory response to an image of one of the researchers running studies with the patient at UCLA. (A) A sampling of 10 pictures eliciting the large responses and their corresponding raster plots and peri-stimulus time histograms (PSTH). Please note that due to copyright issues, the exact images of famous people used in the study were replaced by similar photographs in this figure. Dotted lines mark stimulus onset and offset, 1 sec apart. (B) Median number of spikes (across trials) for each of the pictures presented to the patient. The horizontal line marks the threshold for defining responsiveness (see Methods). Image numbers correspond to the ones shown at the top of each picture in (A).

Fig. 2.

Proportion of images in each category generating selective responses in the MTL. Asterisks indicate significant differences between categories at P < 0.05.

Fig. 3.

Regional differences in the percentage of responses for each category of stimuli