Enrichment to odors improves olfactory discrimination in adult rats

The authors tested how prior odor enrichment affects the spontaneous discrimination of both preexposed and novel odors. Experimental rats were exposed to single odors or to pairs of similar or dissimilar odors for 1-hr periods twice daily over 20 days. Spontaneous discriminations between pairs of similar odors were tested before and after the odor exposure period using an olfactory habituation task. The authors found that (a) experimental rats did not spontaneously discriminate similar odor pairs before the exposure period, whereas they spontaneously discriminated them after the enrichment period, and (b) the improvement of performance was not selective for the odors used during enrichment. These results show that odor experience changes perception in the manner predicted based on other groups' electrophysiological experiments.     

Detection and discrimination of carvone enantiomers in rats with olfactory bulb lesions

Rats with lesions of dorsal and dorsolateral bulbar sites known to be differentially responsive to carvone enantiomers were tested for their ability to detect (+)-carvone, to discriminate between (+)-carvone from (-)-carvone, and to discriminate (+)-carvone from mixtures of both enantiomers after they had been pre-trained or not pre-trained on these tasks prior to surgery. In postoperative tests, rats pre-trained on the enantiomer discrimination problems made somewhat fewer errors than those not pre-trained, but experimental rats performed as well as controls (those that had one intact olfactory bulb) within both conditions and on each task. These results indicate that removal of most bulbar sites known to be differentially responsive to carvone enantiomers and the consequent disruption of normal patterns of bulbar input produced in response to carvones are largely without effect on the ability of rats to discriminate between these odors.     

Effect of bilateral olfactory bulbectomy on discrimination avoidance conditioning in rats

Studies were made with the two-way shuttle box method on the acquisition of discrimination avoidance learning by olfactory bulbectomized rats in relation to changes in emotional behavior. Bulbectomized rats showed a marked increase in locomotor activity, with accompanying augmentation of the reactivity and the appearance of muricidal behavior. Initially, the bulbectomized rats showed elevated conditioned avoidance responses to both the CS+ and the CS-. In later stages, there was a continued slow increase in responses to the CS+ accompanied by a decrease in responses to the CS-, until responses to both stimuli were only slightly elevated above the levels shown by control rats. This result suggests that olfactory bulbectomy does not affect discrimination ability itself, but the impairment of discrimination during the initial stages is resulted from hyperemotionality induced by olfactory bulbectomy.     

The olfactory control of meal pattern in rats

The role of the olfactory input in the control system regulating the free food intake of rats has been investigated. In a first experiment it was shown that the mean daily intake of 20 adult male and female rats, made anosmic by the bilateral ablation of olfactory bulbs, did not change significantly after surgery. But, as a result of the bulbar removal, the day to day fluctuation of this daily intake was significantly increased. In a second experiment the pre and postoperative recording of the daily feeding pattern (meal size and intervals) revealed that the olfactory bulbs ablation induced a more or less persistant and typical nibbling pattern. The stereotaxic electrolytic lesion of the cortico-medial amygdaloid olfactory projection, interrupting one of the olfacto-hypothalamic pathways, induced the same disrupted feeding pattern. The results are discussed in relation to control systems regulating meal sizes and meal to meal intervals in normal rats. It is suggested that the orosensory and particularly the olfactory control is responsible for determining the normal feeding pattern made of large and consistant meals followed by long intervals of nonfeeding.     

Sparing of an olfactory discrimination habit following extensive neocortical removals in rats

Adult male albino rats, blinded by enucleation, learned a tap water-acetone discrimination and subsequently sustained bilateral destruction of the occipito-temporal, parietal, lateral fronto-parietal, dorsal fronto-parietal or cingulate areas. The retention test disclosed that the various brain-damaged groups were not significantly inferior to the control group in savings scores. These data, which confirm the findings of Swann [13], suggest that certain olfactory experiences are not stored within the neocortex.     

Acquisition and retention of separate elements of a conditioned olfactory discrimination in preweanling rats

Acquisition and retention of separate elements of an olfactory discrimination were tested in 15- and 20-day-old preweanling rats. Four or 8 conditioning trials were given in Experiment 1. Each rat was presented one odor always followed by footshock (CS+) and another never paired with footshock (CS-). Conditioning to both stimuli was assessed through 3 types of olfactory preference tests involving comparison between CS+ and a novel odor, CS- and a novel odor, or CS+ and CS-. The results indicated that for 15- and 20-day-olds, both stimuli become excitatory early in training; further conditioning trials diminished the excitation previously accrued to the CS-, and the olfactory discrimination became apparent. When levels of conditioning were equated, retention was tested after intervals of 4 min, 3 days, or 8 days (Exp. 2). Rate of forgetting was more rapid for the 15-day-olds, but both ages of subjects showed similar patterns of forgetting, which included a progressive decrease in the aversion to the CS+ but an increase in aversion to the CS-.     

A comparison of discrimination and reversal learning for olfactory and visual stimuli in aged rats

The present study investigated age-related differences in discrimination and reversal learning for olfactory and visual stimuli in 6-month and 24-month-old rats. Rats were trained to discriminate between two pseudo-randomly selected odors or objects. Once each animal reached a criterion on discrimination trials, the reward contingencies were reversed. Young and aged rats acquired the olfactory and visual discrimination tasks at similar rates. However, on reversal trials, aged rats required significantly more trials to reach the learning criterion on both the olfactory and visual reversal tasks than young rats. The deficit in reversal learning was comparable for odors and objects. Furthermore, the results showed that rats acquired the olfactory task more readily than the visual task. The present study represents the first examination of age-related differences in reversal learning using the same paradigm for odors and objects to facilitate cross-modal comparisons. The results may have important implications for the selection of memory paradigms for future research studies on aging.     

Gabaergic control of olfactory learning in young rats.

Olfactory learning in young rats correlates with neural plasticity in the olfactory bulb, and involves noradrenergic modulation of reciprocal dendrodendritic synapses between mitral cells and GABAergic granule cells. The purpose of this study was to examine, in vivo, the consequences of manipulating bulbar GABA transmission during training. In the first experiment, postnatal day 11 rat pups were trained in an olfactory associative learning task with citral odor and foot shock as the conditioned and unconditioned stimuli, respectively. The pups received continuous infusion of saline or the GABA(A) receptor agonist muscimol into the olfactory bulbs throughout a 30-min training session. The pups were then tested on postnatal day 12 for a preference for or an aversion to citral odor. Saline-infused control pups developed an aversion to citral odor. The GABA(A) receptor agonist muscimol impaired this aversive learning in a dose-dependent manner. In the second experiment, pups were exposed to the odor for 30 min while receiving continuous intrabulbar infusion of a low or high dose of the GABA(A) receptor antagonist bicuculline, without any other reinforcer. Depending on whether a low (0.2 nmol/bulb) or high (1.0 nmol/bulb) dose of bicuculline was infused, the pups showed a preference or an aversion for citral odor after infusion of low and high doses, respectively. These results indicate that disinhibition of mitral cells in the olfactory bulb is critical for olfactory learning in young rats, and suggest that the degree of disinhibition is an important determinant in acquiring either preference or aversion for the conditioned odor.     

Rapid olfactory processing implicates subcortical control of an olfactomotor system

Sniffs are modulated in response to odor content. Higher concentrations of odor induce lesser-volume sniffs. This phenomenon implicates a neural feedback mechanism that measures sensory input (odor concentration) and modulates motor output (sniffing) accordingly. Here we used air-dilution olfactometry to probe the time course of this olfactomotor mechanism. A stainless-steel computer-controlled olfactometer, equipped with mass flow controllers, temperature and humidity control, and on-line photo-ionization detection, was coupled to a highly sensitive pneumatotachograph that measured nasal flow. The olfactometer was used to generate four ascending concentrations of the odorants propionic acid and phenethyl alcohol. Sniff volume was inversely related to odor concentration (P > 0.0001). Sniffs were uniform and concentration independent for the initial 150 ms but acquired a concentration-dependent flowrate as early as 160 ms following sniff onset for propionic acid (P > 0.05) and 260 ms for phenethyl alcohol (P > 0.05). Considering that odorant transduction takes around 150 ms and odorant-induced cortical evoked potentials have latencies of around 300 ms, the rapid motor adjustments measured here suggest that olfactomotor sniff feedback control is subcortical and may rely on neural mechanisms similar to those that modulate eye movements to accommodate vision and ear movements to accommodate audition.     

Genetic control of sniffing and marking responses in mice

Observations on adult male mice from two inbred strains (SRH and SRL), their intercrosses, and their backcrosses provided evidence that a genetic relationship exists between two behavioral acts directed at food pellets in a novel environment: sniffing and marking. This supports the idea that these responses serve a communicative, possibly territorial, function.     

Faster acquisition of an olfactory discrimination following septal lesions in male albino rats.

Normal rats and rats with septal lesions were maintained on a 23.5-hr water deprivation schedule and trained to bar press for water reinforcement, which was available during the presentation of one odor (SD) but not another (Sdelta). Vanilla and vinegar were the olfactants. Both groups showed evidence of discrimination within the first 2-hr of training and reached asymptotic discrimination ratios greater than 90 percent, but the rats with septal lesions reached successively higher levels of discrimination faster than the controls. The results suggest a septal inhibitory influence on the olfactory bulbs.     

Individual and synergistic effects of sniffing frequency and flow rate on olfactory bulb activity

Is faster or stronger sniffing important for the olfactory system? Odorant molecules are captured by sniffing. The features of sniffing constrain both the temporality and intensity of the input to the olfactory structures. In this context, it is clear that variations in both the sniff frequency and flow rate have a major impact on the activation of olfactory structures. However, the question of how frequency and flow rate individually or synergistically impact bulbar output has not been answered. We have addressed this question using multiple experimental approaches. In double-tracheotomized, anesthetized rats, we recorded both the bulbar local field potential (LFP) and mitral/tufted cells' activities when the sampling flow rate and frequency were controlled independently. We found that a tradeoff between the sampling frequency and the flow rate could maintain olfactory bulb sampling-related rhythmicity and that only an increase in flow rate could induce a faster, odor-evoked response. LFP and sniffing were recorded in awake rats. We found that sampling-related rhythmicity was maintained during high-frequency sniffing. Furthermore, we observed that the covariation between the frequency and flow rate, which was necessary for the tradeoff seen in the anesthetized preparations, also occurred in awake animals. Our study shows that the sampling frequency and flow rate can act either independently or synergistically on bulbar output to shape the neuronal message. The system likely takes advantage of this flexibility to adapt sniffing strategies to animal behavior. Our study provides additional support for the idea that sniffing and olfaction function in an integrated manner.     

Speed and accuracy of olfactory discrimination in the rat

The sense of smell is typically thought of as a 'slow' sense, but the true temporal constraints on the accuracy of olfactory perception are not known. It has been proposed that animals make finer odor discriminations at the expense of additional processing time. To test this idea, we measured the relationship between the speed and accuracy of olfactory discrimination in rats. We found that speed of discrimination was independent of odor similarity, as measured by overlap of glomerular activity patterns. Even when pushed to psychophysical limits using mixtures of two odors, rats needed to take only one sniff (<200 ms at theta frequency) to make a decision of maximum accuracy. These results show that, for the purpose of odor quality discrimination, a fully refined olfactory sensory representation can emerge within a single sensorimotor or theta cycle, suggesting that each sniff can be considered a snapshot of the olfactory world.     

Sensory responses in motor cortex neurons during precise motor control

Precentral neurons whose activity changed with precisely controlled small supination-pronation movements almost invariably showed sensory responses to small perturbations which pronated or supinated the forearm in conscious monkeys. Conversely, most units failing to show changes with small movements but being recruited with large ballistic movements were unaffected by these kinesthetic inputs. Moreover, perturbations which yielded intense short-latency unit responses when delivered during postural stability usually failed to modify unit activity when delivered during a ballistic movement. It is suggested that sensory feedback continuously modulates motor cortex neuron discharge during accurate positioning and precise fine movement, whereas during ballistic movements such modulation is greatly attenuated.     

An olfactory biconditional discrimination in the mouse

Male CD-1 mice were given a biconditional discrimination task with four odors; A, B, C, and D. Mice were presented with odor compounds AC+, BD+, BC-, AD- for thirteen days. Pieces of odorized filter paper were placed in the bottom of odor pots and covered with bedding. On reinforced AC and BD trials, sugar was buried in the bedding, and on nonreinforced AD and BC trials no sugar was present. Following training, simultaneous nonreinforced tests were given between AD and AC, and between BC and BD. The mice spent more time digging in the previously reinforced odor compounds than in the previously nonreinforced compounds. In a second experiment, mice were conditioned to dig in AC+ and not BD-. In a subsequent test with the separate elements they dug more in A and C than in B and D, indicating that the biconditional discrimination had not been solved on the basis of complete perceptual blending. The data demonstrate that mice are capable of olfactory configural learning when solving a biconditional discrimination.     

Sleep enhances inhibitory behavioral control in discrimination learning in rats

Sleep supports the consolidation of memory, and it has been proposed that this enhancing effect of sleep pertains in particular to memories which are encoded under control of prefrontal–hippocampal circuitry into an episodic memory system. Furthermore, repeated reactivation and transformation of such memories during sleep are thought to promote the de-contextualization of these memories. Here, we aimed to establish a behavioral model for the study of such sleep-dependent system consolidation in rats, using a go/nogo conditional discrimination learning task known to essentially depend on prefrontal–hippocampal function. Different groups of rats were trained to criterion on this task and, then, subjected to 80-min retention intervals filled with spontaneous morning sleep, sleep deprivation, or spontaneous evening wakefulness. In a subsequent test phase, the speed of relearning of the discrimination task was examined as indicator of memory, whereby rats were either tested in the same context as during training or in a different context. Sleep promoted relearning of the conditional discrimination task, and this effect was similar for testing memory in the same or different context (p < 0.001). Independent of sleep and wakefulness during the retention interval, animals showed faster relearning when tested in the same context as during learning, compared with testing in a different context (p < 0.001). The benefitting effect of sleep on discrimination learning was primarily due to an enhancing effect on response suppression during the nogo stimulus. We infer from these results that sleep enhances memory for inhibitory behavioral control in a generalized context-independent manner and thereby might eventually also contribute to the abstraction of schema-like representations.     

Reduced olfactory discrimination in patients on chronic hemodialysis

Eleven patients on maintenance hemodialysis, aged 19–31, and 16 healthy controls, aged 19–25, rated the similarity in odor for all 91 combinations of pairs of 14 commercial food flavors. A multidimensional scaling procedure, INDSCAL, was applied to the similarity measurements, yielding a common two-dimensional space in which flavors rated similar in odor were arranged close to one another in the space; flavors judged dissimilar were located distant from one another. INDSCAL is an individual difference model which provides weights for each subject on each of the dimensions of the common multidimensional space. The weights as well as the arrangements of flavors in multidimensional spaces for individual subjects indicated that the renal patients had sharply reduced ability to judge qualitative odor differences between food flavors. Hedonic ratings were obtained for all subjects on the 14 flavors as well. The direction of preference was similar for both renal patients and controls, with the odor of fruits preferred over meats. However, the median hedonic ratings for the two groups, renals and controls, revealed that renal patients rated thirteen of the fourteen odors more unpleasant than the controls.     

Odor discrimination in single turtle olfactory receptor neuron

To explore the ability of odor discrimination of olfactory receptor neurons, current responses to odorant cocktails were recorded from an isolated olfactory neuron of the turtle. Twenty-five percent of the neurons tested responded to both cAMP-dependent and the IP₃-dependent odorant cocktails. Application of the cAMP-dependent (or the IP₃-dependent) odorant cocktail to the neuron after an inward current induced by the IP₃-dependent (or the cAMP-dependent) odorant cocktail was adapted induced a large inward current in the neuron. The results suggest that at least two different receptors exist in a single olfactory neuron.     

Prefrontal and tegmental electrical activity during olfactory stimulation in virgin and lactating rats

This study was designed to analyze if the electroencephalographic (EEG) activity of the medial prefrontal cortex (mPFC) and the ventral tegmental area (VTA) was modified throughout various reproductive stages of female rats exposed to pup-associated or non-associated odors. Simultaneous EEG recordings were obtained from right and left mPFC and VTA in adult female rats during the smelling of nest bedding and female bedding under three reproductive states: proestrus-estrus (P-E), diestrus (D) and lactation (L). Absolute (AP) and relative (RP) powers of three EEG band frequencies, and interhemispheric correlation (r) of EEG activity were calculated and compared among conditions. During the awake-quiet condition, RP of the 12-21 Hz band was significantly higher in the right and left mPFC as well as in the right VTA of lactating rats as compared to P-E rats. During the smelling of nest bedding, the RP of the 8-11 Hz in the mPFC became increased while that of the 6-7 and 12-21 Hz decreased in the three reproductive stages. In the VTA, this phenomenon was mainly observed in lactating rats. Only the RP of the lower frequencies (6-7 and 8-11 Hz bands) was higher in the right mPFC and in the left and right VTA, respectively, of the lactating rats with respect to P-E rats, while that of the 12-21 Hz band was lower in lactating as compared to P-E rats. Moreover, the interprefrontal correlation of the lower-frequency bands was higher in relation to smelling of nest bedding in diestrus and lactating rats, whereas during the smelling of female bedding the correlation of the 6-7 Hz band was increased only in the diestrus rats as compared to P-E rats. These data indicate that EEG activity recording is a sensitive tool to study the functionality of the mPFC and VTA during different reproductive states and suggest the possible participation of these structures in the processing of olfactory stimuli associated to pups to modulate the motivational and performance processes, crucial for the expression of maternal behavior.