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Title: Mechanisms of reward representation and memory formation
Author: Barron, H. C.
ISNI:       0000 0004 8503 7311
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2016
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Prior experience can be used to guide decision-making. It allows potential outcomes to be explicitly represented before choices are made and facilitates prospective evaluation. Here, I show that when a decision outcome constitutes a familiar food reward, a rich representation of outcome identity can be measured in human orbitofrontal cortex. However, in the absence of prior experience, the consequences of a novel choice can still be imagined. In the human brain I show that this is achieved by invoking multiple independent memories in hippocampus and medial prefrontal cortex. Relying on past experience to represent a novel outcome does, however, only persist for a short time period. To prevent cognitive interference the brain must employ mechanisms to ensure that neural representations are sufficiently decorrelated despite receiving similar inputs. One such mechanism involves reducing the representational overlap of existing and novel memories by attenuating excitatory connections that lie between these representations. I show that such attenuation can be achieved by repeatedly rehearsing existing memories. Once independent, novel memories can provide a basis from which to construct future novel experiences. An alternative mechanism employed to ensure neural representations are sufficiently decorrelated involves the formation of inhibitory replicas of newly-formed excitatory connections. This second mechanism is thought to be critical in cortex where a precise balance between excitation and inhibition (EI) is maintained to prevent run-away excitation. Here, by combining a range of techniques, I show evidence for such selective inhibitory rebalancing in humans. Following learning, I show that the expression of newly-formed cortical associations reduces. However, by modulating EI balance via a local reduction in the concentration of GABA, cortical associations can be re-expressed. This suggests that in the human brain, newly formed cortical excitatory synapses are subsequently rebalanced by opposing and proportional changes at inhibitory synapses.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available