The basolateral amygdala is critical for learning about neutral stimuli in the presence of danger, and the perirhinal cortex is critical in the absence of danger

Abstract

The perirhinal cortex (PRh) and basolateral amygdala (BLA) appear to mediate distinct aspects of learning and memory. Here, we used rats to investigate the involvement of the PRh and BLA in acquisition and extinction of associations between two different environmental stimuli (e.g., a tone and a light) in higher-order conditioning. When both stimuli were neutral, infusion of the GABAA, muscimol, or the NMDA receptor (NMDAR) antagonist ifenprodil into the PRh impaired associative formation. However, when one stimulus was neutral and the other was a learned danger signal, acquisition and extinction of the association between them was unaffected by manipulations targeting the PRh. Temporary inactivation of the BLA had the opposite effect: formation and extinction of an association between two stimuli was spared when both stimuli were neutral, but impaired when one stimulus was a learned danger signal. Subsequent experiments showed that the experience of fear per se shifts processing of an association between neutral stimuli from the PRh to the BLA. When training was conducted in a dangerous environment, formation and extinction of an association between neutral stimuli was impaired by BLA inactivation or NMDAR blockade in this region, but was unaffected by PRh inactivation. These double dissociations in the roles of the PRh and BLA in learning under different stimulus and environmental conditions imply that fear-induced activation of the amygdala changes how the brain processes sensory stimuli. Harmless stimuli are treated as potentially harmful, resulting in a shift from cortical to subcortical processing in the BLA.

Figure 1.

Sensory preconditioning of an association between S2 and a neutral S1. A, Behavioral demonstration of sensory preconditioning (Experiment 1A: Group PP, n = 7; Group PU, n = 8; and Group UP, n = 8). Left, Freezing to S2 during the test for sensory preconditioned fear. Right, Freezing to S1 during the test for first-order conditioned fear. B, The roles of the PRh and BLA in the acquisition of a sensory preconditioned association (Experiment 1B: Group PRh-VEH, n = 10; Group PRh-MUS, n = 7; Group BLA-VEH, n = 8; Group BLA-MUS, n = 8). Left, Freezing to S2 during the test for sensory preconditioned fear. Right, Freezing to S1 during the test for first-order conditioned fear. C, The involvement of PRh NMDAR in acquisition of a sensory preconditioned association (Experiment 1C: Group VEH, n = 14; Group IFEN, n = 14). Left, Freezing to S2 during the test for sensory preconditioned fear. Right, Freezing to S1 during the test for retention of first-order conditioned fear. Data shown are means ± SEM.

Figure 2.

Microinfusion cannula placements as verified on Nissl-stained coronal sections for the PRh (left) and BLA (right). Sections are based on the atlas of Paxinos and Watson (1997).

Figure 3.

Pre-extinction of a sensory preconditioned association requires activation of the PRh and activation of NMDAR containing the NR2B subunit (Experiment 2: PE-MUS, n = 9; PE-IFEN, n = 9; PE-VEH, n = 9; NO-PE, n = 9). Left, Freezing to S2 during the test for pre-extinction of a sensory preconditioned association. Right, Freezing to S1 during the test for retention of first-order conditioned fear. Data shown are means ± SEM.

Figure 4.

Second-order conditioning of an association between S2 and a dangerous S1. A, Behavioral demonstration of second-order conditioned fear (Experiment 3A: Group PP, n = 8; Group PU, n = 8; Group UP, n = 8). Left, Freezing to S2 during the test for second-order fear. Right, Freezing to S1 during the test for first-order conditioned fear. B, The roles of the PRh and BLA in acquisition of second-order conditioned fear (Experiment 3B: Group PRh-VEH, n = 8; Group PRh-MUS, n = 9; Group BLA-VEH, n = 8; Group BLA-MUS, n = 8). Left, Freezing to S2 during the test for second-order fear. Right, Freezing to S1 during the test for first-order conditioned fear. Data shown are means ± SEM.

Figure 5.

The roles of the PRh and BLA in extinction of sensory preconditioned fear. A, Extinction of sensory preconditioned fear does not require activation of the PRh or NMDAR in the PRh (Experiment 4A: EXT-MUS, n = 8; EXT-IFEN, n = 8; EXT-VEH, n = 8; NO-EXT, n = 7). Left, Freezing to S2 during the test for extinction of sensory preconditioned fear. Right, Freezing to S1 during the test for retention of first-order conditioned fear. B, Extinction of sensory preconditioned fear requires activation of the BLA and NMDAR in the BLA (Experiment 4B: EXT-MUS, n = 8; EXT-IFEN, n = 8; EXT-VEH, n = 9; NO-EXT, n = 9). Left, Freezing to S2 during the test for extinction of sensory preconditioned fear. Right, Freezing to S1 during the test for retention of first-order conditioned fear. Data shown are means ± SEM.

Figure 6.

Sensory preconditioning in a dangerous context. A, Behavioral demonstration of acquisition of a sensory preconditioned association in a dangerous context (Experiment 5A: Group PP, n = 9; Group PU, n = 8; Group UP, n = 8). Left, Freezing to S2 during the test for sensory preconditioning. Right, Freezing to S1 during the test for first-order conditioned fear. Data shown are means ± SEM. B, Acquisition of a sensory preconditioned association in a dangerous context requires activation of the BLA but not the PRh (Experiment 5B: BLA-MUS, n = 8; VEH, n = 10; PRh-MUS, n = 7). Left, Freezing to S2 during the test for sensory preconditioning. Right, Freezing to S1 during the test for first-order conditioned fear. C, Acquisition of a sensory preconditioned association in a dangerous context requires activation of NMDAR in the BLA (Experiment 5C: Group VEH, n = 8, Group IFEN, n = 7). Left, Freezing to S2 during the test for sensory preconditioning. Right, Freezing to S1 during the test for first-order conditioned fear. Data shown are means ± SEM.

Figure 7.

Pre-extinction of a sensory preconditioned association in a dangerous context requires activation of the BLA but not the PRh, and activation of NMDAR in the BLA (Experiment 6: VEH, n = 12; PRh-MUS, n = 8; BLA-MUS, n = 8; BLA-IFEN, n = 7). Left, Freezing to S2 during the test for sensory preconditioning. Right, Freezing to S1 during the test for first-order conditioned fear. Data shown are means ± SEM.