Figure 1. Behavioral methods.

A. Spontaneous detection task. Rats were presented with odorants in their homecage. A weighing dish with a scented filter paper was put on top of the cage and the time rats spent investigating the odor was recorded. B. Rats were presented with mineral oil (MO) during three trials separated by 5 min ITIs. During the 4^th trial an odor diluted to 10⁻², 10⁻⁴ or 10⁻⁵ Pa was presented. If rats investigated the odorant significantly more than the MO during the 3^rd presentation, it was assumed that they were able to detect the odorant. C. Reward motivated odor detection. Rats were first shaped to retrieve a sucrose pellet from a dish filled with bedding. D. During detection testing, they were presented with a choice of a rewarded dish scented with an odorant at 10⁻² Pa and an unrewarded, unscented dish, for five trials. During the remaining 15 trials, the scented odor was randomly presented at 10⁻², 10⁻⁴ and 10⁻⁵ Pa. The dish in which the rat looked for the reward was recorded. E. CCh control experiment. Rats were habituated in a setup similar to A. They were first presented with a an unscented filter paper, then during three trials with a straight chain aliphatic odorant diluted to 1 Pa. During trials five and six, odorants of the same functional group differing by one or two carbons from the habituated odor were presented in random order. If the rats investigated the novel test odors significantly more than the habituated odor it was assumed that they discriminated between the two odorants.