Hallucinatory experience as aberrant event memory formation: Implications for the pathophysiology of schizophrenia

• Equivalence of conscious experience with event memory formation, corresponding to the equivalence of conscious imagery with event memory recall
• Role of consciousness and event memory formation in the mapping of landmarks to locations, and connectedness of the hippocampus in support of translations of locations into modes of action
• Hippocampal abnormalities in schizophrenia and their implication in the mechanism of hallucinations
• Position of hippocampal dysfunction within a network model of schizophrenia

If hallucinations are not fundamentally different from normal wakeful experiences, then the neural basis of hallucinations has to be essentially that of consciousness in general. The additional insight that consciousness reflects the formation (as opposed to consolidation) of event (episodic) memories links the pathophysiology of hallucinations to the hippocampus. Perceptions and misperceptions, insofar as they are consciously experienced, constitute contextualized and unitary phenomena (which are embedded as discrete events in the stream of consciousness); they are experiential manifestations of activity patters that recurrently emerge in the CA3 network of the hippocampus (and that are secondarily consolidated into retrievable and declarable memories). The hippocampus, forming allocentric representations of objects in their world context (event memories), is a point of convergence of neocortical sensory processing streams. Moreover, being extensively modulated by the organism's physiological state, the hippocampus embeds such representations in an emotional context and, through its output to the medial prefrontal cortex, guides decision-making and goal-selection processes. Although sensory and associative processing in the neocortex makes an important contribution to the formation of behaviourally adaptive representations in the hippocampus, it is becoming clearer that pattern formation in the hippocampus is in itself the neural correlate of consciousness and that disruptions in relational memory processing in the hippocampus can give rise to hallucinations. Neurobiological and neuroimaging findings in schizophrenia research can be integrated within the proposed conceptual framework.