Sensitization as a mechanism for multiple chemical sensitivity: relationship to evolutionary theory

Multiple chemical sensitivity (MCS) is a disorder in humans attributed to prior chemical exposure. Sensitization is an amplification of neuronal responsiveness that elicits increased behavioral responding to stimuli, and occurs in a recently developed rat model of MCS. Rats were exposed to repeated formaldehyde (Form) and their response in three behavioral tests, including locomotor activity after a cocaine challenge, conditioned fear, and behavioral avoidance of Form, was assessed. In all three tests, rats demonstrated sensitized behaviors, implicating amplified responding within specific limbic brain regions. Evolutionary theory in the context of MCS specifies how the behavioral strategies of those with MCS are consistent with the notion that their self-perceived sense of survival and reproductive fitness may be threatened by chemical exposures. This behavior may be mediated by the same limbic brain regions that become sensitized after repeated chemical exposure in animals.     

Effects of a delay-reinforcement procedure on performance under IRT>t schedules

Water-deprived rats were studied under a compound schedule that prescribed that responses terminating interresponse times (IRTs) greater than a fixed value t₁ (IRT > t₁ component schedule) initiated a delay of reinforcement interval t₂, at the end of which water was presented if the subject did not respond ( > t₂ component schedule). If the subject responded before the t₂ interval elapsed, the IRT > t₁ component schedule was re-initiated and water was not presented. The IRT > t₁ and > t₂ component schedules were not differentially correlated with distinctive stimuli. Rate of responding during the IRT > t₁ component decreased as a function of the value of t₂. The magnitude of the decreases in response rate appeared to be proportional to the subject's rate under the IRT > t schedule with no delay of reinforcement (t₂ = 0 sec). The effects were independent of the parameter value of the IRT > t₁ component schedule and of the rate of reinforcement. The results suggested that “efficiency” of performance under IRT > t schedules can be increased by appropriately arranging brief delays of reinforcement.     

The central tendency effect in stimulus generalization: Effects of establishing a “preexperimental” frame of reference

An experiment was performed to test the hypothesis that in a voluntary generalization task subjects encode the training stimulus (TS) in relation to the adaptation-level (AL) prevailing at the time of TS presentation. If so, then during a subsequent generalization test the point of maximal generalized responding should bear this same relationship to the AL of the generalization test series. To test this hypothesis, three major groups of subjects (n = 80) were given preexposures with dim lights (low preexperimental AL group), bright lights (high preexperimental AL group), or no preexposure (control group). Next, half of each group was exposed to a TS and a subsequent generalization (recognition) test including this brightness and four others. The other half of each group was exposed to the same TS and subsequent generalization stimuli, but their task was to rate the stimuli on a 7-point bright-dim scale. This provided AL measures which could be correlated with generalization performance in the comparable generalization groups. The preexposure condition was successful in producing the expected low or high preexperimental ALs, as reflected in different ratings of the TS when it was presented. Furthermore, these effects persisted throughout testing, as evidenced in ALs (i.e., rating scale midpoints) which reflected the preexposure condition. Finally, the difference between the point of maximal generalized responding and the AL of the generalization test series was highly and significantly correlated with the difference between the original rating of the TS and scale midpoint. This supports the maj or experimental hypothesis of the study.