Odor Pairs Research Articles

The Enduring Impact of Perceptual Memories

unts ow the perceptual/sensory memories of infants and young children endure long into adulthood has been the subject of much clinical speculation and artistic expresion from Proust’s evocative madeleine to Truman Capote’s hristmas memories of making and delivering fruitcakes with his eloved Sook. What makes each of these literary accounts otable is the relation between early stress and loss in the hildren’s/authors’ lives and their enduring memories strongly ooted in olfactory and other sensory cues. In this issue, evelinges et al. (pages 1070–1079) bring basic science to these escriptions. Their report on the long-term effects of infant emories on adult learning and stress response adds an imporant new line of work to a growing literature on the enduring ffect of early stressors on adult functioning. This report also has mportant implications for clinicians and clinical investigators. Sevelinges et al. present a series of experiments based on onditioning neonatal rats 8 to 12 days old with a paired odor peppermint) and shock, an unpaired odor only, or no condiioning. In adulthood, these three infant training groups were ivided into three adult groups, a paired, unpaired, or odor-only roup, using the same conditioning odor used in infancy or a ovel odor with or without the conditioning odor from infancy as contextual cue. Their central findings are exposure to predictble stress (paired odor-shock) in infancy is associated with ttenuated fear conditioning in the same animals grown to dulthood and exposed to the same odor stimulant when ompared with the nonexposed infant/adult paired conditioning roup. When adult animals exposed to predictable stress (paired onditioning) as infants were conditioned to a different odor ith the original odor from infancy used as a contextual cue, heir fear-conditioned learning to a novel stimulus was impaired. vidence for learning from infancy was supported by the obseration that adult conditioning to a novel odor without the riginal odor as a contextual cue was not impaired at the ehavioral or functional level. These behavioral differences in adults exposed to the original dor were also associated with attenuated amygdala and enanced olfactory activity as adults assessed with 2-deoxyglucose 2-DG) autoradiography techniques. Because 2-DG autoradiogaphy does not indicate whether the attenuation in amygdala ctivity is in inhibitory or excitatory pathways, the authors use aired-pulse stimulation of the olfactory bulb to assess the unctional link between olfactory and amygdala activity. Norally, enhanced olfactory bulb activity is associated with an ncreased inhibitory signal to the amygdala as was seen in the ulse-pairing assessments in adults not exposed to shock as nfants. But in adults exposed as infants to paired odor-shock or redictable stress, olfactory inhibitory input to the amygdala was ttenuated and became facilitative at a longer interpulse. These hanges were not dependent on the presence of the odor xperienced in infancy and suggest that basolateral amygdalar

Forward and backward second-order Pavlovian conditioning in honeybees

Second-order conditioning (SOC) is the association of a neutral stimulus with another stimulus that had previously been combined with an unconditioned stimulus (US). We used classical conditioning of the proboscis extension response (PER) in honeybees (Apis mellifera) with odors (CS) and sugar (US). Previous SOC experiments in bees were inconclusive, and, therefore, we attempted to demonstrate SOC in the following three experiments: (Experiment 1) After differential conditioning (pairing odor A with US and presenting odor B without US), the bees experienced two pairs of partially overlapping odors, either a new odor C followed by a previously reinforced odor A (C-A) or a new odor C followed by a previously nonreinforced odor B (C-B). (Experiment 2) After differential conditioning, bees were presented with C-A or A-C. (Experiment 3) Bees were first presented with C-A or A-C before differential conditioning and were tested with odor C. We observed: (Experiment 1) 40% of the bees showed PER to the C-A presentation, but only 20% showed PER to the C-B presentation. (Experiment 2) 40% of the bees showed PER to the C-A presentation, while only 20% showed PER to the reversed sequence A-C. Experiments 1 and 2 showed that a previously reinforced odor can be a secondary reinforcer for excitatory SOC only with forward-pairing. (Experiment 3) PER toward C was lower (15%) in bees presented with A-C than with C-A (25%). This showed that backward SOC is not as effective as forward SOC. These results help to delineate different conditions that are critical for the phenomenon of SOC.

Enduring Effects of Infant Memories: Infant Odor-Shock Conditioning Attenuates Amygdala Activity and Adult Fear Conditioning

Early life adverse experience alters adult emotional and cognitive development. Here we assess early life learning about adverse experience and its consequences on adult fear conditioning and amygdala activity. Neonatal rats were conditioned daily from 8-12 days-old with paired odor (conditioned stimulus, CS) .5mA shock, unpaired, odor-only, or naive (no infant conditioning). In adulthood, each infant training group was divided into three adult training groups: paired, unpaired or odor-only, using either the same infant CS odor, or a novel adult CS odor without or with the infant CS present as context. Adults were cue tested for freezing (odor in novel environment), with amygdala (14)C 2-DG autoradiography and electrophysiology assessment. Infant paired odor-shock conditioning attenuated adult fear conditioning, but only if the same infant CS odor was used. The (14)C 2-DG activity correlated with infant paired odor-shock conditioning produced attenuated amygdala but heightened olfactory bulb activity. Electrophysiological amygdala assessment further suggests early experience causes changes in amygdala processing as revealed by increased paired-pulse facilitation in adulthood. This suggests some enduring effects of early life adversity (shock) are under CS control and dependent upon learning for their impact on later adult fear learning.

Olfactory discrimination ability of CD-1 mice for aliphatic aldehydes as a function of stimulus concentration

Using an operant conditioning paradigm, we tested the ability of CD-1 mice to discriminate between members of a homologous series of aliphatic aldehydes presented at four different concentrations. We found that the mice were clearly capable of discriminating between all odorant pairs when stimuli were presented at concentrations of 1, 0.01, and 0.001 ppm (corresponding to four, two, and one log unit above the highest individual detection threshold) with no significant difference in performance between these concentrations. In contrast, the animals generally failed to discriminate above chance level when stimuli were presented at 0.0001 ppm (corresponding to the highest individual detection threshold) although stimuli were clearly detectable. Further, we found a significant negative correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length. These findings suggest that an increase in stimulus concentration of only one log unit above detection threshold appears to be sufficient for recruitment of additional subpopulations of odorant receptors to allow for qualitative recognition of aliphatic aldehydes.

Opposing effects on muscarinic acetylcholine receptors in the piriform cortex of odor-trained rats

We combined pharmacological studies and electrophysiological recordings to investigate modifications in muscarinic acetylcholine (ACh) receptors (mAChR) in the rat olfactory (piriform) cortex, following odor-discrimination rule learning. Rats were trained to discriminate between positive and negative cues in pairs of odors, until they reached a phase of high capability to learn unfamiliar odors, using the same paradigm ("rule learning"). It has been reported that at 1-3 d after the acquisition of odor-discrimination rule learning, pyramidal neurons in the rat piriform cortex show enhanced excitability, due to a reduction in the spike-activated potassium current I(AHP), which is modulated by ACh. Further, ACh and its analog, carbachol (CCh), lost the ability to reduce the I(AHP) in neurons from trained rats. Here we show that the reduced sensitivity to CCh in the piriform cortex results from a decrease in the number of mAChRs, as well as a reduction in the affinity of the receptors to CCh. Also, it has been reported that 3-8 d after the acquisition of odor-discrimination rule learning, synaptic transmission in the piriform cortex is enhanced, and paired-pulse facilitation (PPF) in response to twin stimulations is reduced. Here, intracellular recordings from pyramidal neurons show that CCh increases PPF in the piriform cortex from odor-trained rats more than in control rats, suggesting enhanced effect of ACh in inhibiting presynaptic glutamate release after odor training.

Drosophila α/β Mushroom Body Neurons Form a Branch-Specific, Long-Term Cellular Memory Trace after Spaced Olfactory Conditioning

Functional optical imaging showed that odor or electric shock stimuli presented to the fly causes transient calcium influx into the two major axon branches of alpha/beta mushroom body (MB) neurons. One pairing of odor and electric shock stimuli or multiple, massed pairings did not alter odor-evoked calcium influx. In contrast, animals that received multiple, spaced pairings exhibited a robust increase in calcium influx into the MB axons when tested at 9 or 24 hr after training, but not at 3 hr. This modification occurred only in the alpha branch of the neurons and was blocked by mutation of the amnesiac gene, inhibition of protein synthesis, or the expression of a protein blocker of the transcription factor Creb. Thus, behavioral long-term olfactory memory appears to be encoded as a branch-specific modification of calcium influx into the alpha/beta MB neurons that occurs after spaced training in a protein synthesis-, Creb-, and amnesiac-dependent way.

Involvement of the left anterior insula and frontopolar gyrus in odor discrimination

Discriminating between successively presented odors requires brief storage of the first odor's perceptual trace, which then needs to be subsequently compared to the second odor in the pair. This study explores the cortical areas involved in odor discrimination and compares them with findings from studies of working-memory, traditionally investigated with n-back paradigms. Sixteen right-handed subjects underwent H(2) (15)O positron emission tomography during counterbalanced conditions of odorless sniffing, repeated single odor detection, multiple odor detection, and conscious successive discrimination between odor pairs. Eight odorants were delivered using a computer-controlled olfactometer through a birhinal nasal cannula. Conscious successive odor discrimination evoked significantly greater activity in the left anterior insula and frontopolar gyrus when compared to reported sensory detection of the identical odors. Additional activation was found in the left lateral orbital/inferior frontal and middle frontal gyri when discrimination was compared to the odorless condition. The left anterior insula is likely involved in the evaluation of odor properties. Consistent with other studies, frontopolar and middle frontal gyrus activation is more likely related to working memory during odor discrimination.

Olfactory discrimination ability of CD-1 mice for a large array of enantiomers

With use of a conditioning paradigm, the ability of eight CD-1 mice to distinguish between 15 enantiomeric odor pairs was investigated. The results demonstrate a) that CD-1 mice are capable of discriminating between all odor pairs tested, b) that the enantiomeric odor pairs clearly differed in their degree of discriminability and thus in their perceptual similarity, and c) that pre-training with the rewarded stimuli led to improved initial but not terminal or overall performance. A comparison between the proportion of discriminated enantiomeric odor pairs of the CD-1 mice and those of other species tested in earlier studies on the same discrimination tasks (or on subsets thereof) shows a significant positive correlation between discrimination performance and the number of functional olfactory receptor genes. These findings provide the first evidence of a highly developed ability of CD-1 mice to discriminate between an array of non-pheromonal chiral odorants. Further, they suggest that a species’ olfactory discrimination capabilities for these odorants may be correlated with its number of functional olfactory receptor genes. The data presented here may provide useful information for the interpretation of findings from electrophysiological or imaging studies in the mouse and the elucidation of odor structure-activity relationships.

Olfactory memory capacity of the cricketGryllus bimaculatus

Olfactory learning in insects is a useful model for studying neural mechanisms underlying learning and memory, but memory storage capacity for olfactory learning in insects has not been studied. We investigate whether crickets are capable of simultaneously memorizing seven odour pairs. Fourteen odours were grouped into seven A/B pairs, and crickets in one group were trained to associate A odours with water reward and B odours with saline punishment for all the seven pairs. Crickets in another group were trained with the opposite stimulus arrangement. Crickets in all the groups exhibited significantly greater preference for the odours associated with water reward for all the seven odour pairs. We conclude that crickets are capable of memorizing seven odour pairs at the same time.

Individual olfactory learning in Camponotus ants

We studied olfactory learning in two ant species, Camponotus mus from Argentina and Camponotus fellah from Israel. To this end, we established an experimental laboratory protocol in which individual ants were trained to associate odours with gustatory reinforcers. Ants were trained individually to forage in a Y-maze in which two odours had to be discriminated. One odour was positively reinforced with sucrose solution and the other was negatively reinforced with quinine solution. After a training session of 24 trials, ants of both species learned to differentiate the two odour pairs, the structurally dissimilar limonene and octanal, and the structurally similar heptanal and 2-heptanone. In nonreinforced tests, ants consistently chose the odour previously reinforced with sucrose solution and spent more time searching in the arm of the maze presenting this odour. Learning performances were more robust in the case of limonene versus heptanal. These results thus show for the first time that individual ants perceive and learn odours in controlled laboratory conditions.

Olfactory enrichment improves the recognition of individual components in mixtures

Olfactory enrichment improves the recognition of individual components in mixtures. PHYSIOL BEHAV 000-000, 2006.Odor mixtures can exhibit synthetic (the mixture is qualitatively different from its components) or elemental properties (the components are recognizable). We tested how prior olfactory enrichment affects the recognition of individual components in binary mixtures. Experimental rats were exposed to pairs of similar odors for one-hour periods twice daily over 20 days. Spontaneous discrimination between the binary mixture and the individual components of similar pairs of odors was tested before and after the odor enrichment. We found that, after the enrichment period, rats could discriminate components in binary mixtures that had not been discriminated prior to the enrichment period, and that this increase in discrimination capability was not always specific to the odorants used during the enrichment period.

Chiral behavior of TGS gas sensors: Discrimination of the enantiomers by the electronic nose

The electronic nose measurement system composed of TGS a gas sensor array, cooperating with the support vector machine (SVM) to discriminate between enantiomeric odor pairs: (+) and (−) of α-pinene, carvone and limonene forms is proposed in the paper. The array of the semiconductor TGS gas sensors of an electronic nose responds with a signal pattern characteristic of each enantiomer type. The SVM classifier is responsible for the recognition of this pattern. This study demonstrates that the ability of recognition of these enantiomers can be explained as the chiral behavior of the TGS gas sensors. Chirality of these sensors can be joined with the chirally selective adsorption on the metal oxide crystal surfaces. The ability of some inorganic crystalline surfaces to adsorb selectively chiral molecules is widely spread in the nature.

Dissociable Codes of Odor Quality and Odorant Structure in Human Piriform Cortex

The relationship between odorant structure and odor quality has been a focus of olfactory research for 100 years, although no systematic correlations are yet apparent. Animal studies suggest that topographical representations of odorant structure in olfactory bulb form the perceptual basis of odor quality. Whether central olfactory regions are similarly organized is unclear. Using an olfactory version of fMRI cross-adaptation, we measured neural responses in primary olfactory (piriform) cortex as subjects smelled pairs of odorants systematically differing in quality and molecular functional group (as one critical attribute of odorant structure). Our results indicate a double dissociation in piriform cortex, whereby posterior regions encode quality (but not structure) and anterior regions encode structure (but not quality). The presence of structure-based codes suggests fidelity of sensory information arising from olfactory bulb. In turn, quality-based codes are independent of any simple structural configuration, implying that synthetic mechanisms may underlie our experience of smell.

Olfactory learning in the one-day old rat: Reinforcing effects of isoproterenol

Within 24 h of their birth-induced norepinephrine surge, rat pups were tested for effects of a β-receptor agonist, isoproterenol, on olfactory learning. Experiment 1 found no effect of isoproterenol on conditioning by pairing an odor (CS) with intraoral saccharin infusions. There was, however, unexpectedly strong responding in the unpaired control condition, which had the same contingency between the CS and isoproterenol as the paired condition. Experiment 2 found that pairings of odor and isoproterenol alone were sufficient for enhancing responding to the odor. Experiment 3 determined that isoproterenol had acted independently as a US for associative conditioning rather than facilitating nonassociative learning by mere exposure to the odor. These effects of isoproterenol as a US are consistent with the results of previous studies with older rats.

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.

Reinforcing properties of ethanol in neonatal rats: Involvement of the opioid system.

Toward understanding why infant rats ingest high levels of ethanol without initiation procedures, the authors tested effects of mu and kappa receptor antagonists on ethanol reinforcement in neonatal rats. After an intracisternal injection of CTOP (micro antagonist), nor-Binaltorphimine (kappa antagonist), or saline, newborn (3-hr-old) rats were given conditioning pairings of an odor with intraorally infused ethanol or a surrogate nipple with ethanol administered intraperitoneally (to minimize ethanol's gustatory attributes). In each case, these opioid antagonists reduced or eliminated ethanol's reinforcement effect. The same effects occurred with saccharin as the reinforcer in olfactory conditioning. The results imply that activation of mu and kappa receptors, apparently acting jointly, is necessary for reinforcement or that antagonists of this activity impair basic conditioning.

An automated olfactory discrimination procedure with rats.

Although rats are commonly used as subjects in psychological research, few researchers have utilized their olfactory capabilities. An automated chamber capable of presenting multiple odor stimuli to 5 locations was used to train rats on 3 simple dis crimination and reversal tasks. Unique odor pairs were used for each and reversal task (Sets A, B, & C). All rats readily learned each task, taking longer to learn the reversal than the initial discrimination. Stimulus location did not influence accuracy, with only two exceptions. Moreover, rats showed no position biases for any of the 5 locations. These data demonstrate the feasibility of testing rats using olfactory cues in an automated apparatus with as many as 5 stimulus locations. This apparatus and procedure have potential applications to many areas of psychological investigation, including extra-dimensional shifts, compound stimulus training, and conditional discriminations. Key words: olfactory, stimulus location, nose poke, rats ********** Rats are commonly used subjects in psychological research that are frequently tested on tasks that require bar pressing or maze running. Moreover, these tasks are typically used to examine associative, or learning in rats. Few researchers, however, have supplanted these methodologies reliant on pressing and running behavior with those that take advantage of rats' superior sense of smell. For example, a search of the abstracts on PsychINFO returned over 23,000 references using the keyword discrimination, but only 627 when the keywords rat and discrimination were combined. Utilizing odor cues in tests involving rats is advantageous because this species possesses a biological preparedness for learning about odors. For example, Nigrosh, Slotnick, and Nevin (1975) demonstrated that when responding was brought under control of redundant odor and tone cues, control of behavior was more effective with the odor cue (but see Neill & Harrison, 1987, for a discussion on the variables influencing auditory learning in rats). Moreover, developing reliable behavioral tests that take advantage of rodents' olfactory sense has the potential to increase the utility of animal models of conditions, such as mental retardation, that are partially defined in terms of their behavioral effects (e.g, McIlvane & Cataldo, 1996). Nonetheless, the absence of utilizing olfactory stimuli in studies of learning remains notable. One possible reason is the inherent difficulty of working with them. Despite this, some researchers have attempted the challenge. Bunsey and Eichenbaum (1996) trained rats on matching to sample conditional discriminations using olfactory stimuli. Everyday household spices were mixed into sand and placed in small cups. Rats were required to dig into the cups to obtain reinforcement (a piece of chocolate). This procedure was effective in establishing reliable matching behavior. Moreover, the basic procedure has been replicated (Dudchenko, Wood, & Eichenbaum, 2000; Wood, Dudchenko, & Eichenbaum, 1999) and has been successfully modified for simple training with both rats and mice (Dusek & Eichenbaum, 1997; Mihalick, Langlois, Krienke, & Dube, 2000, respectively). Despite the success of this procedure, there are some disadvantages: a large number of scented cups need to be mixed prior to each session, the researchers have to manually present each stimulus during the session, and data have to be recorded by hand. Pena, Pitts and Galizio (2006) have taken this innovative idea further by conducting sessions in a standard operant chamber and streamlining the sand-cup delivery process. Baited sand cups are presented to the rats by sliding a tray that fits underneath the floor in and out of the chamber. This procedure involves less contact with the subjects during the session but still requires manual data recording. …

CROSS‐MODAL ADDITIVITY OF TASTE AND SMELL

ABSTRACT Subjects were simultaneously given subthreshold levels of taste and odor stimuli, delivered orally, for both a commonly paired and an uncommonly paired taste–odor combination. Results indicate cross‐modal summation of subthreshold concentrations of both taste–odor pairs when the olfactory stimulus is delivered orally. Results of control studies suggest that the summation was indeed across modalities, and not due to the taste of the odor compound or the smell of the taste compounds. Furthermore, results indicate that regardless of taste–odor pair commonness, taste and smell can combine in a completely additive fashion (i.e., at threshold detectability when both stimuli are presented simultaneously at 50% threshold level) if the taste–odor pair is presented orally. In several instances, but not all, measured probabilities exceeded those predicted by probability summation, indicating that hyperadditive mixing often occurs, but there do seem to be individual differences. Cross‐modal summation, regardless of taste–odor pair commonness, has broader implications for the development of foods, beverages and pharmaceuticals, especially in masking undesirable tastes and smells.

DIFFERENTIAL EFFECTS OF TWO BLOCKERS OF SMALL CONDUCTANCECa2+-ACTIVATEDK+CHANNELS, APAMIN AND LEI-DAB7, ON LEARNING AND MEMORY IN RATS

SK channels are responsible for long-lasting hyperpolarization following action potential and contribute to the neuronal integration signal. This study evaluates the involvement of SK channels on learning and memory in rats, by comparing the effects of two SK channel blockers, i.e., apamin which recognizes SK2 and SK3 channels, and lei-Dab7 which binds SK2 channels only. lei-Dab7 totally competes and contests apamin binding on whole brain sections (IC(50): 11.4 nM). Using an olfactory associative task, intracerebroventricular blocker injections were tested on reference memory. Once the task was mastered with one odor pair, it was then tested with a new odor pair. Apamin (0.3 ng), injected before or after the acquisition session, improved new odor pair learning in a retention session 24 hours later, whereas lei-Dab7 (3 ng) did not significantly affect the mnesic processes. These results indicated that the blockage of SK channels by apamin facilitates consolidation on new odor associations; lei-Dab7, containing only SK2 subunits, remains without effect suggesting an involvement of SK3 channels in the modulation of the mnesic processes.

Functional Significance of Olfactory-induced Oscillations in the Human Amygdala

We recorded directly from the amygdalar nucleus of nine epileptic patients performing a delayed odor-matching recognition memory task. Time-frequency analysis of the responses to the odorants revealed that the stimulations elicited induced oscillatory responses, as well as already described olfactory evoked potentials. These oscillatory responses were composed of two frequency components--one in the beta band (15-25 Hz) and a faster one, in the low gamma band (25-35 Hz)--both of which lasted during the full duration of the inspiration. In pairs of identical odorants, the power of gamma oscillations was weaker for the second odorant (the target) than for the first one (the sample). We observed no such difference when the first and second odorants of a pair were different. Thus, gamma oscillations in the amygdala are weaker for repeated stimuli, a mechanism known as repetition suppression. This is consistent with an involvement of the human amygdala in the encoding and retrieval of olfactory information independently of its hedonic properties, at least in epileptic patients. Altogether, our results corroborate in humans evidence found in animals that oscillations serve as a common coding process of olfactory information.