ABSTRACT
Although a substantial amount of behavioral evidence supports the existence of two different stimulus processing systems posses sing the hypothesized characteristics of the reinforcer and habitua tive systems, additional evidence is required from neurological studies before we can have confidence in these constructs. There are many possible constructs that can be developed to explain the same behavioral phenomena; however, only one has biological exis tence. Therefore, there has to be a conjunction of evidence from both behavioral and neurological research, all pointing to the same hypothesized construct, to be sure that it is not only plausible but also actually in operation in the brain.
