There is compelling evidence that memory is supported by multiple, functionally independent subsystems that distinguish declarative from non-declarative memories (Tulving, 1972). The declarative subsystems, episodic and semantic memory, have been studied intensively, largely in isolation from each other. Relatively little attention has been paid to the interplay been episodic and semantic memory. This thesis constitutes a series of behavioural, neuroimaging, and computational investigations aimed at elucidating the factors and mechanisms that mediate interactions between episodic and semantic memory. Event-Related Potentials (ERPs) are used to isolate processes implicated in episodic and semantic memory interactions on the basis of known ERP effects. Experimental investigations vary factors that target semantic memory either directly or indirectly. Direct manipulations alter the semantic content of word pairs by modulating their lexicality (words vs. non-words) or coherence (categorical vs. non-categorical). Indirect manipulations focus episodic encoding towards semantic or non-semantic aspects of the to-be-encoded word pairs. This thesis investigates whether such manipulations influence episodic memory and if so, in what form. The behavioural and ERP data provide clear evidence for distinct episodic and semantic interactions at the level of semantic organisation and lexical representation. Episodic retrieval, which is supported by recollection and familiarity according to dual process theories (Yonelinas, 2002), reveals enhanced familiarity for semantically organised stimuli. This effect is dependent on semantically deep encoding strategies. By contrast differences in the lexicality of stimuli modulated both familiarity and recollection. To provide an account for why different types of interactions are obtained a computational memory model is proposed. This model uses a single network to simulate a dual process model of episodic retrieval and gives insight into processes that may support interactions between episodic and semantic memory. Thus, this thesis provides novel evidence for different types of episodic and semantic memory interactions dependent on the kind of semantic manipulation and specifies the mediating mechanisms leading to such interactions.

The proper functioning of nervous systems requires electrical activity to be tightly regulated. Perturbations in the intrinsic properties of neurons, and in excitatory input, are imposed throughout nervous system development as cell morphology and network activity evolve. In mature nervous systems these changes continue as a result of synaptic plasticity and external stimuli. It is therefore likely that homeostatic mechanisms exist to regulate membrane conductances that determine the excitability of individual neurons, and several mechanisms have been characterised to date. This thesis characterises a novel in vitro model for homeostatic control of intrinsic excitability. The principal finding is that cultured hippocampal neurons respond to chronic depolarisation over a period of days by attenuating their response to injected current. This effect was found to depend on the level of depolarisation and the length of treatment, and is accompanied by changes in both active and passive membrane conductances. In addition, the effect is reversible and dependent on L-type calcium channel activity. Using experimental data to parameterise a conductance-based computer model suggests that the changes in conductance properties account for the observed differences in excitability.