Ecole Normale Supérieure Paris
Wednesday 3 December 2008
Seminar Room B10 (Basement)
Alexandra House, 17 Queen Square, London, WC1N 3AR
Functional but not anatomical separation of "what" and "when"
in prefrontal cortex
During short-term memory maintenance, different neurons in prefrontal cortex (PFC), recorded under identical conditions, will generally show a wide variety of temporal dynamics and response properties. A specific example is given by a working memory task in which a monkey must memorize a stimulus f1 for a couple of seconds, and then compare it with a subsequent stimulus f2. We investigated the activity of a large population of neurons during the short-term memory phase of this task and show that (1) on the population level, the neural activity can be decomposed into a set of mutually orthogonal representations, (2) this decomposition leads to a natural separation of stimulus information ("what") and information about the passage of time during short-term memory ("when"),
(3) this separation of stimulus and time-information finds no reflection on the single-cell level, (4) a simple (linear) network model can reproduce the neural activities and suggests that time and f1 information may be controlled by different mechanisms. In conclusion, we find a functional separation of "what" and "when" information, no direct anatomical correlate, but a possible correlate of this separation on the level of network mechanisms. [Joint work with Ranulfo Romo and Carlos Brody.]