| Previous research has demonstrated that patterned electrical stimulation of the olfactory bulb (Mouly et al., Behavioral Brain Research 17: 45--58, 1985) or of its output pathway, the lateral olfactory tract (LOT) (Roman et al., Brain Research, 418: 221--226, 1987), can be used as a discriminative cue in a learning task (so-called "electric odors"). Because olfactory bulb stimulation activates association fibers from piriform cortex as well as the LOT, the present study compared the effectiveness of stimulation of these two fiber systems as a discriminative cue. Male Long-Evans rats with chronically implanted electrodes were water deprived and trained on a Go/No-Go task in which they initiated a trial by breaking a photobeam in a nosepoke operandum. Animals received a water reward for performing a second nosepoke on "go" trials in which the first nosepoke triggered patterned electrical stimulation (a single train of four pulses at 40 Hz), and for withholding a response on "no-go" trials in which no stimulation was given. All animals attained high levels of performance during initial training, which used stimulation that activated both the LOT and cortical association fibers (coactivation). Animals were then tested for their ability to respond to stimulation that selectively activated each fiber system on different trials. Stimulation of cortical association fibers was found to be significantly more effective as a discriminative cue than was LOT stimulation. In a separate test, stimulation of cortical association fibers was found to be effective as a discriminative cue even if the number of pulses in an "electric odor" was reduced from four to one. Analysis of potentials evoked by the training stimulation during Phase I revealed that potentiation of a late component evoked by association fiber activation occurred during training, while the population EPSP evoked by LOT activation was unchanged. Taken together, these results indicate that activation of cortical association fibers plays a critical role in the effectiveness of "electric odors," and emphasize the importance of this fiber system in olfactory learning. |
