Table 1.

Experimental Design

Conditioning					Test
CS Type		Drug		Trials
Light	X	Saline	X	8	Light?
Compound		Naltrexone
No CS

Note. Animals were conditioned to fear a light CS, compound light-noise CS or trained in the absence of an explicit CS (No CS group). Eight-trial conditioning proceeded under the influence of the opioid antagonist naltrexone or the saline vehicle. The CS type and drug condition were manipulated using a 3 × 2 factorial design. The following day, animals were tested to one presentation of the light CS.