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.

1. Neural activity was recorded from the orbitofrontal cortex (OF) of rats performing an eight-odor discrimination task that included predictable associations between particular odor pairs. A modified linear discriminant analysis was employed to characterize the population response in each trial of the task as a point in an N-dimensional activity space with the firing rate of each cell in the population represented on one of the N dimensions. The ability of the ensemble to discriminate among conditions of a variable was reflected in the tendency of population responses to cluster together in this activity space for repetitions of a given condition. We assessed coding of several variables describing the period of odor sampling, focusing on aspects of current, past, and future events reflected in single-neuron firing patterns, in ensembles composed of 22-138 cells active during the period when the rats sampled the discriminative stimulus in each trial. 2. OF ensembles performed well at discriminating variables with relevance to task demands represented in single-neuron firing patterns, specifically the physical attributes and assigned reward contingency of the current odor as well as the expectation of reward in the following trial that could be inferred from the predictable associations between particular pairs of odors. OF ensembles were able to correctly identify the identity and assigned reward contingency of the current odor in up to 52% (chance = 12.5%) and 99% (chance = 50%) of all trials, respectively, such that the observed behavioral performance required a population of 5,364 odor-responsive cells in the case of odor identity and only 40 cells in the case of valence. Expectations regarding upcoming rewards based on both assigned response contingency and associations between particular pairs of odors were correctly classified in up to 67% (chance = 20%) of all trials such that the observed level of behavioral performance required a population of 3,169 cells. 3. Other information represented in the single-neuron firing patterns, such as the identity and reward contingency of the preceding odor and specific odor-odor associations, was poorly encoded by OF ensembles. Thus neural ensembles in OF may represent only some of the information reflected in single-neuron activity. Stable coding of only the most useful and relevant information by the ensemble might emerge from the tuning properties of single neurons under the influence of the task at hand, producing in the well-trained animal the observed pattern of broad and diverse coding by single neurons and selective, task-relevant coding by neural ensembles in OF.

The ''core'' subnucleus of the mediodorsal thalamic nucleus (MD) receives direct input from olfactory cortex in the rat. This part of MD projects to the frontal neocortex of the rhinal sulcus (RS), while other parts of the MD project to the anterior medial wall of the neocortex (MW). In this study, rats were tested on odor threshold, detection of specific odors, and three odor discriminations both before and after lesions of MD, MW, RS, or sham lesions. Olfactory threshold and detection ability remained unaffected by any of these lesions; however, subjects with MD and RS lesions were markedly impaired on odor discriminations. Furthermore, subjects were more profoundly impaired when the stimuli to be discriminated were novel or difficult to associate. Thus, MD and its ''olfactory'' projection target, RS, seem to mediate cognitive, rather than sensory aspects of odor-guided behavior in the rat.

The role of the thalamocortical ''olfactory'' pathway in odor preferences and sexual behavior was examined in the hamster, a species which depends crucially upon olfaction for mating. Before and after ablation of primary or secondary olfactory structures, male hamsters were tested for threshold, interest, and preference for the odor of an estrus female hamster and other odors. Sexual competency was also observed in daily mating sessions. Disruption of the primary olfactory pathway resulted in an absence of interest in odors and mating. Lesions of mediodorsal thalamic nucleus or frontal neocortex of the rhinal sulcus did not result in anosmia, but did eliminate or alter odor preferences and resulted in inappropriate, inefficient, precopulatory and copulatory behavior. It appears that substructures in the thalamofrontal pathway play a role in discriminative or cognitive aspects of processing adaptively significant stimuli.