Table 1.

Behavioral demonstration of sensory preconditioned fear

	SPC	First-order	S2 test	S1 test
Group PP	S2 − S1	S1 − US	S2−	S1−
Group PU	S2 − S1	S1/US	S2−	S1−
Group UP	S2/S1	S1 − US	S2−	S1−

S1 and S2 were a tone and flashing light stimulus (counterbalanced), and the US was electric footshock. A minus sign (−) between two events denotes that the events were paired; a virgule (/) denotes that the two events were explicitly unpaired; A minus sign (−) following one event denotes that the event was presented alone (in the absence of shock).