Paradoxical facilitatory effect of fornix lesions on acquisition of contextual fear conditioning in mice.

The present study examined the effect of fornix lesions on freezing behavior elicited by contextual and phasic conditioned stimuli. Male mice of the C57Bl/6 strain received electrolytic lesions of the fornix. Ten days following the lesion, they were submitted to acquisition of one-trial classical fear conditioning involving the pairing of an auditory conditioned stimulus (CS) with a footshock unconditioned stimuli (US). Analysis of conditioned fear responses showed that fornix lesions enhanced the freezing response elicited by exposure to the conditioning chamber 24 h after a single tone-shock pairing. In contrast, the two groups did not differ on their fear responses during the auditory cue test. Analysis of the time-course of freezing behavior during re-exposure to the conditioning chamber suggests, however, that the observed fornix lesion-induced facilitation of freezing to the conditioning chamber is more likely due to a facilitation of the processing of a simple (unimodal) rather than polymodal (contextual) CS-US association.