Authors: |
*J. M. MOSCARELLO1, J. LEDOUX2,3;
1Ctr. for Neural Sci., 2New York Univ., New York, NY; 3Nathan Kline Inst., Orangeburg, NY |
Abstract: |
The nervous system is designed to acquire elaborate information about the ways in which an individual’s actions can alter the state of their world. Instrumental learning, in which the frequency of an action changes as a result of the outcome it produces, is an important means by which humans and animals learn how to influence their environments. Using a signaled active avoidance paradigm for rodents, in which a response performed during a tone CS leads to the omission of a foot shock US, we have demonstrated that instrumental control over the delivery of an aversive US profoundly alters the subject's threat assessment of the associated CS. Indeed, as training proceeds, the acquisition of the avoidance response corresponds with marked decreases in CS-evoked freezing, an important defensive reaction to predicted threats. However, in a contingency degradation experiment, which obstructs instrumental learning through the addition of noncontingent reinforcement (i.e. omission of the US in the absence of any instrumental action), the avoidance response was not acquired and freezing remained high even though the overall level of shock was held constant between contingency degraded and full contingency groups. Thus, we conclude that there is a strong relationship between learned control over the US and reduced defensive reactions to the CS. Intriguingly, CS-evoked freezing remains suppressed even in when the cue is presented in contexts that do no allow for the avoidance response. A subsequent inactivation study reveals that this effect is dependent on the dorsal hippocampus - muscimol infused into that structure prior to CS presentation in a non-training environment strongly increases conditioned freezing. These data suggest a model in which dorsal hippocampus generates an inhibitory memory as a result of instrumental control over aversive outcomes, and this memory is applied to fundamentally remap the animal's assessment of conditioned threat, even in ambiguous circumstances. |