Fig. 5.

Stimulation of the posterior intralaminar nucleus (PIN), but not other medial geniculate nuclei, serves as an effective unconditioned stimulus (US) in auditory fear conditioning. (A) Behavioral conditioned bradycardia (“Condit”, n = 25) or lack thereof in a sensitization control group (“Sens.”, n = 9) to tone and shock. Guinea pigs received 10 trials of the CS and US unpaired followed by 30 trials of CS-US paired in the Conditioning group, while there was no change in protocol for the Sensitization control group. Note elicitation of novelty-induced bradycardia during the first five trials, which rapidly habituated by the end of the second block of unpaired presentation (block 2) for both groups. However, when pairing was initiated, the Conditioning group rapidly developed CS-elicited bradycardia during the first block of five trials, which continued to increase in magnitude during training (Edeline and Weinberger, 1992). (B) Substitution of stimulation of the PIN for a shock US produces conditioned bradycardia. However, stimulation of the ventral division (MGv), medial division (MGm) and dorsal/suprageniculate nuclei (MGd/SG, data combined) was ineffective and did not differ significantly from the Sensitization control group that received tone and shock unpaired (see [A] above). The ordinate is the magnitude of change in heart rate during the CS relative to pre-trial baseline. (Cruikshank et al., 1992)