Two issues in Pavlovian fear conditioning: selective fear of bright vs. dark, and CS determinants of CR form.

We examined the effects of tone, light, and dark conditioned stimuli (CSs) on barpress suppression and freezing in rats. We found more freezing to tone than to light or dark yet similar barpress suppression. In compound, the auditory and visual stimuli together evoked more suppression and freezing than did any element alone. The fact that the compounds could increase suppression speaks against a ceiling effect interpretation of the similar suppression to the elements. The data strengthen the growing evidence in fear conditioning that CSs in different modalities may evoke conditioned responses of different forms (freezing to tone vs. less freezing to light or dark) despite having similar associative values. The finding that light and dark produced similar levels of suppression and freezing argues against earlier claims (Welker & Wheatley, 1977) that rats are prepared to associate bright (but not dim) light with danger.