Sloan/Swartz Centers for Theoretical Neurobiology
Annual Summer Meeting 2004

Single-unit activity in the human hippocampus-amygdala complex
during visual recognition memory

Ueli Rutishauser, CalTech

Summary of the talk (Adobe Acrobat format):
http://www.urut.ch/pdfs/CSHL_sloanMeeting04_Short.pdf

Abstract

The hippocampus has long been indicated in memory encoding; it's role in
retrieval and recognition, however, is unclear. Neurons coding for
familiarity and novelty are thought to be an important part of memory
retrieval. We investigated the properties of human medial temporal lobe
(MTL) neurons using in-vivo recordings from intracranial depth electrodes
implanted in epilepsy surgery patients. Specifically, we investigated i)
whether MTL neurons respond to visual stimuli, ii) whether neuronal
activity predicts novelty/familiarity and iii) whether neuronal activity
is selective for visual categories. To accomplish this, we developed a
technique for recording and separating large numbers of single cells from
the extracellular signal that we record while we ask patients to perform
visual psychophysical experiments. We are able to cleanly separate up to
40 simultanously recorded single cells recorded from 24 channels. We find
that a significant number of neurons (30-50% of all neurons) respond to
visual stimuli, that is, they predict whether the subject is currently
viewing a visual stimulus or not. About half of these neurons' activity
was modulated by whether the visualized stimulus has been seen before or
not e.g.novelty). We find neurons that encode novelty with a significant
firing increase as well as neurons that encode novelty with a significant
firing decrease.

We furthermore find neurons that specifically respond to visual categories
(animals versus non-animals). These findings suggest that the human MTL is
involved in the processing of visual stimuli. Contrary to previous imaging
studies, this findings also suggest that the human hippocampus is part of
the visual recognition memory system.