Abstract

Neural circuit level descriptions of cognitive processes are difficult to measure in humans, but can provide important insight into brain function. Here I will present two sets of studies that illustrate how non-invasive measurements of the human brain can be used to index neural circuit mechanisms responsible for memory and decision-making. In the first set of studies, I will ask how memories are stored in cortex given that learning disrupts the precise balance between excitation and inhibition. To address this question I combine 7T fMRI and MRS with transcranial direct current stimulation,and show that recently established memories lie dormant in cortex but can be unmasked when the concentration of cortical GABA is down-regulated. This data suggests that during memory formation, modifications at excitatory synapses are complimented by proportional changes at inhibitory synapses. In the second set of studies, I will ask how memories can be used to represent novel experiences, to facilitate evaluation and choice of novel items. Using representational fMRI, I will show that novel experiences are represented by combining multiple memories in hippocampus and prefrontal cortex. This suggests that in the absence of direct experience, existing memories can be compiled to allow for imagination of future choices.