Consciousness, Memory, and Hallucinations

Philosophical insights into the nature of conscious experience allow us to relate neurobiological findings to a pathophysiological model of hallucinations. The world around us is a dreamlike and subjective experience that in the state of normal wakefulness, but not in states of dreaming or hallucinations, is constrained, through sensory input, by external physical reality. In patients diagnosed with schizophrenia and in some organic conditions, perception can be underconstrained by sensory data emanating from the physical world as a result of peripheral sensory impairment or increased random neural activity in specific thalamic nuclei. Sensory processing streams converge via the entorhinal cortex on the hippocampus, where self-organizing activity patterns in cornu ammonis region 3 (CA3) situate landmarks and objects within an allocentric representation. Conscious experience, which is primarily externalized and hence allocentric, may be an emergent property of self-organizing states adopted, at θ rhythm, by the CA3 autoassociation network. The ability of CA3 to bind various feature units into a coherent whole, the view-dependent and allocentric nature of information encoded by CA3, and the connectedness of CA3 with neocortical sensory processing streams – they all support the implication of CA3 in normal and abnormal consciousness. Excessive pyramidal cell activity in CA3, due to deficient inhibition by inhibitory basket interneurons, leading to event memory formation that is poorly modulated by entorhinal input, may be another mechanism for the generation of hallucinations in patients diagnosed with schizophrenia.