Increased generalization between drug-related interoceptive stimuli with delayed testing

Although the flattening of generalization gradients over time has been widely investigated using exteroceptive stimuli, little attention has been given to generalization involving interoceptive stimuli. To investigate generalization between internal states, Sprague-Dawley rats were given either 0.835 ml/kg chloropent or 15 mg/kg sodium pentobarbital. Both drugs produced asymmetrical state-dependent retention of a passive avoidance response, that is, good retention in the "same state" conditions (i.e., the drug-drug and no drug-no drug conditions) as well as in the no drug-drug conditions but poor retention in the drug-no drug conditions, at both 1- and 7-day retention intervals. Furthermore, subjects trained in one drug state (pentobarbital or chloropent) demonstrated disrupted performance when tested 1 day later in another drug state, but good performance when tested 7 days later in the other drug state, indicating a decrement in the discriminability of the two drug states after 7 days. This outcome demonstrates that generalization gradients between drug states flatten over time. Moreover, these results suggest that memory for attributes of internal stimuli undergoes changes similar to those found for exteroceptive stimuli. Implications for contextual cues models of forgetting are considered.