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Trimethyltin (TMT) is a neurotoxic organometal which produces a variety of learning and memory impairments in laboratory animals and humans, including impairments of avoidance learning, maze learning, and problem solving. Two studies investigated the effects of TMT exposure on serial-pattern learning in rats. Rats in both experiments were intubated once with either 0 or 7.0 mg/kg TMT 1 week prior to the pattern-learning procedure. Rats learned serial patterns composed of various quantities of brain-stimulation reward (BSR) pulses; they received BSR quantities in a predetermined order for leverpresses in a discrete-trial operant task. In Experiment 1, all rats received two serial patterns (20-10-0 vs. 1-29-0 pulses of BSR) that alternated within each daily session of 100 patterns. In Experiment 2, all rats received two serial patterns (18-10-6-3-1-0 vs. 18-1-3-6-10-0 pulses of BSR) that alternated within each daily session. In Experiment 1, TMT-exposed rats learned both their patterns more slowly than did controls. In Experiment 2, TMT-exposed rats learned the formally simple 18-10-6-3-1-0 pattern of BSR quantities faster than did controls, but were significantly slower than controls in learning the formally more complex 18-1-3-6-10-0 pattern. In both experiments, however, TMT exposure did not affect either the reinforcing properties of BSR or rats’ asymptotic performance. These results support the notion that TMT exposure impaired some aspects of the rote processes involved in serial-pattern learning in rats, yet spared the rats’ ability to encode some representation of the formal rule-based structure of the pattern. The results thus suggest that the processes involved in learning simple versus complex serial patterns may be mediated by different systems in the brain. |
