Search strategies on a holeboard are impaired in rats with ventral tegmental damage: animal model for tests of thought disorder
Introduction: Attention-related mechanisms distinguish relevance from irrelevance. A disturbance of this capability underlies thought-disorder in schizophrenia (e.g. see Oades, Attention and Schizophrenia. Pitman Press, 1982). Stimulus choice strategies depend, inter alia, on such selective mechanisms and are anomalous in some patients with schizophrenia, a disorder in which ventral tegmental area (VTA) functions have been postulated to be impaired. Here, the effects of VTA damage on making the relevance/irrelevance distinction and the formation of problem-solving strategies has been studied in rats. Methods: Food-deprived animals searched for food pellets placed consistently in 4 holes of a 16-hole-board (figure 2). They were presented with 9 sessions of 10 trials/session. VTA damage resulted from coagulation with a stylet inserted down a stereotaxically implanted cannula, sham operations consisted of cannula placement alone (figure 1). Results: 1/ a) Across sessions the control group reduced the number of empty hole-visits (errors) more rapidly than the lesioned animals: b) the proportion of repeated visits to relevant holes (had contained food, working memory errors) to irrelevant holes (had never contained food, reference memory errors) increased for intact, but not for lesioned animals. 2/ Intact animals developed a preferred sequence of hole-visits (a strategy) across sessions: this habit was not learned by the lesioned animals. 3/ The animals with VTA damage developed a preference across trials within a session and maintained a preference for the first-hole visited across sessions (i.e. were capable of simple learning), but switched their preferred overall strategy (hole-visit-sequence) between sessions. Conclusions: .The results are discussed in terms of a) the overall behaviour of the animals, and b) the interaction between selective attention and the establishment of a short-term working memory - both for the efficiency of search (errors) and the strategy that facilitates search success (hole-visit-sequence). It is proposed that VTA function contributes to the succesful deployment of attention-related strategies in rodents, and that these strategies model those impaired when patients with schizophrenia have to interpret words with multiple meanings, categories in card-sorting tasks - to assess the contingencies or context that normally control the making of a choice.
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