A hippocampal theory of schizophrenia

Hippocampal dysfunction in schizophrenia

Although not necessarily primary to the disease, hippocampal dysfunction in schizophrenia is suggested by morphological changes in the hippocampal formation reported in schizophrenic patients. This notion receives additional support from studies showing that 1) similar behavioral deficits are exhibited by both schizophrenics and animals with hippocampal lesions, and 2) some of these behavioral deficits are reversed by neuroleptics in both schizophrenics and lesioned animals. A brain-mapped neural network model is used to explain how some impairments in attention can be caused by hippocampal dysfunction and ameliorated by dopaminergic blockers.

Associative deficit accounts of disrupted latent inhibition and blocking in schizophrenia

Latent inhibition (attenuated responding to a signal due to signal-alone presentations preceding the signal-outcome pairings) and blocking (attenuated responding to Signal B due to Signal A being paired with the outcome prior to pairings of an AB compound with the outcome) are reportedly absent in acute schizophrenics. The common assumption that these phenomena reflect the normal functioning of attention and the observation that rats administered low doses of amphetamine show a similar disruption has resulted in the development of an animal model of attentional dysfunction in acute schizophrenia. Here, we selectively review the experimental and clinical literature concerning latent inhibition and blocking, their disruption in acute schizophrenia, and the current status of this model. We conclude that the construct validity of the model is compromised if latent inhibition and blocking are viewed in attentional terms because experimental data indicate both phenomena can be better understood in associative terms. We favor a framework in which disruption of latent inhibition and blocking in acute schizophrenics is viewed as an inability to compare and express stored representations (i.e. associative performance deficit). This change of perspective does not undermine the potential value of the model, but rather suggests that the nature of its validity should be reconsidered.

Nucleus accumbens, entorhinal cortex and latent inhibition: a neural network model

A neural network model of classical conditioning (Schmajuk, Lam, and Gray, J. Exp. Psychol.: Anim. Behav. Process, 22, 1996, 321-349) is applied to the description of the neural substrates of latent inhibition. Experimental data suggest that latent inhibition might be controlled by a circuit that involves the hippocampus, the entorhinal cortex, the nucleus accumbens, and the mesolimbic dopaminergic projection from the ventral tegmental area to the accumbens. By mapping different nodes and connections in the model onto this brain circuit, computer simulations demonstrate that, in most cases, the model provides a good quantitative description of: (1) the impairment of latent inhibition by lesions of the shell of the nucleus accumbens; (2) the restoration of latent inhibition by haloperidol following lesions of the shell; (3) the preservation of latent inhibition by lesions of the core of the nucleus accumbens; (4) the facilitation of latent inhibition by combined shell core lesions and by core lesions with extended conditioning; (5) the impairment of latent inhibition following lesions of the entorhinal cortex or the hippocampus; and (6) the restoration of latent inhibition by haloperidol following lesions of the entorhinal cortex and ventral subiculum. In addition, the model is able to describe neural activity in the nucleus accumbens.

Schizophrenia. A cognitive model and its implications for psychological intervention

It is argued in this article that information-processing models enable us to link psychotic phenomena to their neural bases. The core abnormality is viewed as a disturbance in the integration of sensory input with stored material. The performance of schizophrenic subjects on tasks derived from both animal learning theory and human experimental psychology is consistent with the model. The way in which such a disturbance relates to schizophrenic symptoms is outlined. It may result from an abnormality at one or more points in the neural circuit responsible for generating predictions of subsequent sensory input; in particular the hippocampus and related brain structures are implicated. The potential relevance of the model for psychological intervention is discussed.

A simple (or simplistic?) cognitive model for schizophrenia

An approach which views schizophrenia as a disturbance of information processing appears promising as a way of linking biological and clinical aspects of the disorder. A review of research in this area led to the suggestion that the basic disturbance in schizophrenia is "a weakening of the influences of stored memories of regularities of previous input on current perception". This formulation leads to the prediction that in certain circumstances, schizophrenics may perform better than normal subjects. Recent studies employing tasks derived from human experimental psychology provide evidence in support of the model. It is argued that the link between information processing disturbances and biological abnormalities may be facilitated by the use of paradigms derived from animal learning theory (latent inhibition and Kamin's blocking effect). On both tasks the pattern of performance of acute schizophrenics is consistent with the cognitive model. The ways in which such an information processing disturbance may lead to schizophrenic symptomatology are outlined, with particular reference to the formation and maintenance of delusional beliefs. The core cognitive abnormality may result from a disturbance in any of the brain structures involved in the prediction of subsequent sensory input. The proposed circuit, which draws heavily on Gray's model, implicates in particular the hippocampus and related areas and is consistent with studies of brain pathology in schizophrenia.