The temporal relationship between sniffing and the limbic 19 rhythm was studied in rats during odor discrimination reversal learning. The I3 rhythm was monitored as rhythmic slow wave activity (RSA) in the dorsal hippocampal formation, and cyclic nasal airflow (sniffing) was monitored with a thermocouple in the nasal cavity. The training procedures required animals to perform a sequence of whole body locomotion toward one wall of an arena, followed by investigatory sniffing of stimuli through a port while otherwise standing still. Hippocampal RSA was present reliably during the periods of investigatory sniffing. Analyses based on the fast Fourier transform (FFT) demonstrated that this RSA tended to be lower in frequency and amplitude than RSA which occurred during locomotory approach. Other analyses based on the FFT were developed to characterize the nature and parameters of the temporal relationship between rhythmic sniffing and hippocampal RSA as a function of the dominant sniffing frequency during the periods of stimulus sampling. The phase difference between sniffing and RSA tended to vary linearly with frequency so as to maintain a preferred latency relationship between the onset of each sniff cycle and a particular phase of the hippocampal RSA. The phase of RSA to which sniffing was related differed across animals and was correlated with electrode position relative to the phase reversal layers within the hippocampal formation. These results therefore are consistent with the interpretation that, during the periods of stimulus sampling, the sniffs were being timed to maintain a preferred latency relationship with the pacemaker activity which drives the 8 rhythm and the recorded RSA.The consistency with which the animals exhibited the preferred latency relationship varied during the course of training. Across animals, this correlation between sniffing and 8 activity was consistently high during the trials which immediately preceded the achievement of criterion level performance, and the correlation was reduced during the criterion run and/or subsequent trials of overtraining. Thus, the tendency of the animals to exhibit this relationship was not associated specifically with correct performance. Rather, the correlation tended to be highest when the animals were most likely to be evaluating the behavioral relevance of stimuli and were in the process of modifying their responses to those stimuli. The timing of investigatory sniffs as a function of 6' cycle phase may be important for the neural processing of sensory and/or motor information of relevance for response modification.

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Single olfactory bulb units were studied in two macrosmatic species of rodents under conditions intended to preserve the cyclical stimulation which normally accompanies nasal breathing. Patterns of unit activity related to the inhalation cycle were observed in all animals, often in the absence of specific stimuli, and could not be explained in simple mechanical terms. Distinctive changes in these patterns occurred in response to certain odors, and were generally independent of changes in the overall firing frequency. These findings indicate that a change in the overall firing frequency of unit discharges is neither a necessary nor sufficient measure of responsiveness to odors in the rodent olfactory bulb, and that stimulus-specific temporal distributions of unit firing may be involved in olfacto-endocrine activities.

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The centrifugal projections from the various subdivisions of the anterior olfactory nucleus (AON) can be categorized into four groups based on the organization of terminal fields in the main olfactory bulb (MOB). Pars lateralis and dorsalis have bilaterally asymmetric laminar projections to the MOB. The ipsilateral projections terminate primarily in the superficial half of the granule cell layer and in the deep third of the glomerular layer, whereas the contralateral projections terminate primarily in the superficial half of the granule cell layer and do not extend into the glomerular layer. Pars ventralis and posterior have bilaterally symmetric laminar projections with heavy terminations both in the superficial half of the granule cell layer and in the deep third of the glomerular layer. Pars medialis sends predominantly ipsilateral projections to the deep half of the granule cell layer. Pars externa has predominantly contralateral projections with a very narrow terminal field immediately deep to the internal plexiform layer. The projections to the MOB from the ventral hippocampal rudiment (HR) and the piriform cortex (PC) are exclusively ipsilateral. The projections from the ventral HR terminate primarily in the deep half of the granule cell layer. The projections from the PC also terminate predominantly in the granule cell layer, but there is a progressive shifting of terminal fields from the superficial half of this layer toward deeper regions for centrifugal axons arising from progressively more caudal levels of the PC. The laminar termination patterns of cortical afferents to the ipsilateral MOB thus are correlated with the mediolateral axis of the olfactory peduncle and the rostrocaudal axis of the piriform cortex. The centrifugal axons from these various sources enter directly into the granule cell layer of the caudal MOB or pass through the internal plexiform layer of the accessory olfactory bulb to reach the middle and anterior part of the MOB. We have termed these two routes the final common bulb pathway. The centrifugal axons from the laterally situated sources join the anterior and bulbar limbs of the anterior commissure before entering the final common bulbar pathway. In contrast, the centrifugal axons from pars medialis and the ventral HR travel diffusely in the cellular layer of the ipsilateral olfactory peduncle. A small component of the centrifugal projections from the PC travels in association with the lateral olfactory tract.