Electronic noses combine an array of cross-selective gas sensors with a pattern recognition engine to identify odors. Pattern recognition of multivariate gas sensor response is usually performed using existing statistical and chemometric techniques. An alternative solution involves developing novel algorithms inspired by information processing in the biological olfactory system. The objective of this dissertation is to develop a neuromorphic architecture for pattern recognition for a chemosensor array inspired by key signal processing mechanisms in the olfactory system. Our approach can be summarized as follows. First, a high-dimensional odor signal is generated from a chemical sensor array. Three approaches have been proposed to generate this combinatorial and high dimensional odor signal: temperature-modulation of a metal-oxide chemoresistor, a large population of optical microbead sensors, and infrared spectroscopy. The resulting high-dimensional odor signals are subject to dimensionality reduction using a self-organizing model of chemotopic convergence. This convergence transforms the initial combinatorial high-dimensional code into an organized spatial pattern (i.e., an odor image), which decouples odor identity from intensity. Two lateral inhibitory circuits subsequently process the highly overlapping odor images obtained after convergence. The first shunting lateral inhibition circuits perform gain control enabling identification of the odorant across a wide range of concentration. This shunting lateral inhibition is followed by an additive lateral inhibition circuit with center-surround connections. These circuits improve contrast between odor images leading to more sparse and orthogonal patterns than the one available at the input. The sharpened odor image is stored in a neurodynamic model of a cortex. Finally, anti-Hebbian/ Hebbian inhibitory feedback from the cortical circuits to the contrast enhancement circuits performs mixture segmentation and weaker odor/background suppression, respectively. We validate the models using experimental datasets and show our results are consistent with recent neurobiological findings.

In this thesis lateralization of olfactory functions was investigated by both behavioral and electrophysiological assessment, the latter with the olfactory event-related potential (OERP) technique. The olfactory sense is primarily ipsilateral in that a stimulus that is presented to one nostril is initially processed in the same hemisphere. This makes it possible to observe differences between stimulated nostrils as an indication of hemispheric difference. Study I explored differences in olfactory cognitive functions with respect to side of rhinal stimulation and demonstrated that familiarity ratings are higher at right- compared to left-nostril stimulation. No differences were found in episodic recognition memory or free identification, possibly reflecting inter-hemispheric interactions in higher cognitive functions. Effects of repetition priming were present in odor identification and tended to be more pronounced when tested via left nostril. Study II further investigated the effect of previous exposure in odor identification by a different experimental set-up, and demonstrated effects of repetition priming when tested via left- but not right-nostril stimulation. This finding indicates the importance of reconsidering possible sequential effects in olfactory research. Study III examined methodological aspects of an OERP protocol with respect to stimulus duration, which was used in Study IV. No differences in amplitudes or latencies where found between the stimulus durations of 150, 200 and 250 ms, suggesting the commonly used duration of 200 ms in a standard protocol. Study IV investigated laterality effects in OERPs with respect to side of stimulation and electrode site. The results showed consistent amplitudes and latencies regardless of rhinal side of stimulation. Larger amplitudes were demonstrated on left hemisphere and midline compared to right hemisphere, possibly explained by smaller N1/P2 amplitudes at the right-hemisphere sites at left-nostril stimulation. Apart from a proposed OERP protocol, the findings support the notions of a right-hemisphere predominance in processes related to olfactory perception and indicate, in accordance with other findings, a left-side advantage in conceptual repetition priming.

Olfactory function is impaired in aging and dementia. The general aim of this thesis was to identify variables that predict olfactory function and dysfunction (assessed with an odor identification test) in middle-aged and elderly adults. The thesis investigated whether odor identification performance was associated with demographic variables, cognitive function, the ApoE gene, dementia, and other health-related variables. The ApoE-ε4 allele is associated with cognitive impairment and Alzheimer’s disease, the most common form of dementia. The studies included in this thesis used data from the Betula study, a large-scale, population-based prospective study on aging, memory, and health. Study 1 investigated demographic and cognitive predictors of odor identification ability in non-demented participants. The results showed that younger age, female sex, and high education contributed to better odor identification ability. Cognitive speed and vocabulary had a small additional influence. Study 2 included information about ApoE genotypes, dementia and other health-related variables. The results indicated that the ApoE-ε4 allele was associated with odor identification impairment among the elderly, but not middle-aged adults. Participants who were demented at the time of testing or became demented within five years after testing exhibited olfactory impairments. Interestingly, the age-related olfactory impairment in ε4-carriers was independent of clinical dementia within five years. In Study 3, decline in global cognitive status over a five-year test-retest interval was predicted in a sample of elderly participants. The major result was a three-way interaction reflecting that odor identification impairment, old age, in combination with the ε4 allele predicted a larger cognitive decline. However, odor identification impairment did not predict cognitive change in elderly who were non-carriers of the ε4 allele. Overall, the results indicate that odor identification impairment in elderly is related to ApoE-ε4, cognitive decline, and clinical and pre-clinical stages of dementia. Theoretical and practical implications of the results are discussed. Furthermore, it is proposed that in order to effectively predict clinical dementia or cognitive decline from olfactory assessment in the elderly, variables that mediate (e.g. neuropathology) or moderate (e.g. age) the associations between olfactory function, the ε4 allele, and dementia need to be further evaluated, preferably in studies using longitudinal assessment.

In this study I assessed the effect of objects and odors as environmental enrichment for two captive snow leopards (Uncia uncia) at Kolmården Wildlife Park. Five odors (lavender, lemon balm, cumin, cinnamon and catnip) were used to impregnate four different types of enrichment objects (boomer balls®, tennis balls, ropes, logs). During test sessions, one odorized and one non-odorized enrichment object of the same type were introduced in the snow leopards’ outdoor enclosure. The behavior, activity and location of the snow leopards were recorded and compared to their behavior during baseline sessions. During the test sessions I recorded the number and types of interactions with the enrichment objects. Both snow leopards interacted more often with odorized than with non-odorized enrichment objects. The number of interactions differed markedly between the types of enrichment objects and between the different odors. Both snow leopards interacted most often with boomer balls® and least often with logs. They interacted most often with cinnamon and least often with lemon balm (Brahma) and catnip (Binu). The results suggest that both the type of object and the odor play a role in capturing the interest of the snow leopards. The snow leopards behavioral diversity increased during the study and they could increase their performance of species-specific behavior. I did not see any indications of habituation during the testing period. The results indicate that enrichment objects impregnated with odors can be an effective environmental enrichment for captive snow leopards.

Odor discrimination ability was tested in four female South African fur seals (Arctocephalus pusillus pusillus) using a food-rewarded two-choice instrumental conditioning paradigm. The seals’ ability to distinguish between members of homologous series of aliphatic ketones (2-butanone to 2-heptanone) and alcohols (1-butanol to 1-heptanol) was assessed. The results showed that three out of four seals successfully discriminated between all of their stimulus combinations in both classes of odorants. One seal succeeded to reach the discrimination criterion with all 2-ketones but failed with all 1-alcohols. No significant correlation between odor discrimination performance and structural similarity of the odorants in terms of differences in carbon chain length was found in either of the two chemical classes. Furthermore, it was found that the 2-ketones were significantly better discriminated than the 1-alcohols. The fact that both classes of odorants are known to be present in the natural environment of seals provides a possible explanation as to why most of the seals were able to successfully discriminate between them. The results of the present study support the notion that the sense of smell may play an important role in behavioral contexts such as social communication, foraging and reproductive behavior of fur seals.

Les techniques modernes d'imagerie fonctionnelle du cerveau utilisent le métabolisme cérébral comme marqueur d'activité neuronale. En effet le cerveau dépend intimement des apports sanguins en métabolites pour son fonctionnement. Cependant les mécanismes de régulation du métabolisme sont encore mal connus. Dans cette étude nous avons utilisé le modèle du bulbe olfactif chez le rat anesthésié pour caractériser la consommation d'oxygène en réponse à une stimulation physiologique. La quantification précise de la vascularisation du bulbe olfactif a pu mettre en évidence que la couche glomérulaire, très dense en synapses, est l'une des zones les plus vascularisées du cerveau. Cette couche est aussi le lieu d'une intense consommation d'oxygène lors du traitement de l'information olfactive. Par contraste, la couche du nerf, complètement dénuée d'interactions synaptiques et très peu vascularisée, consomme peu d'oxygène. L'étude pharmacologique de ces réponses métaboliques nous a permis de montrer que le compartiment post-synaptique du glomérule est le siège de cette intense activité métabolique. Cette dernière est aussi dépendante du traitement de l'information olfactive qui est effectué à la fois dans le bulbe olfactif et à la périphérie, dans la cavité nasale. Ceci nous a permis de caractériser l'effet de l'adaptation périphérique sur la consommation d'oxygène et le traitement local de l'information olfactive. Enfin, nous avons décrit en détail l'importance des phénomènes de diffusion au niveau du réseau microvasculaire dans le rééquilibrage transitoire du taux d'oxygène local.

Crayfish (Procambarus clarkii) form dominance hierarchies, which are patterns of repeated fights with expected outcomes of winner and loser. Establishment of hierarchies allows dominants the first access to limited resources over subordinates, and leads to behavioral and cellular changes corresponding to the social status. Here, the animals¡¯ responses to an unexpected unilateral touch, a non-social stimulus, were examined with respect to their social status and to their social context. Isolates oriented to the stimulus source with raised claws and elevated posture. Dominants also oriented to the stimulus both when tested alone and in the presence of a subordinate. Subordinates oriented to the stimulus while separated from their familiar dominant partner; however, they avoided it when tested while paired with the dominant. In subsequent tests first while semi-separated from the dominant and later while fully separated, the same subordinates displayed more orienting responses as the duration of post-fight separation increased. These results suggest that the lingering effects of recent social experience influence the behavior of subordinate animals. During fights, crayfish release urine toward each other, providing critical chemosensory cues for establishing hierarchies. Throughout the lifespan, new neuronal precursors are added into clusters of olfactory local and projection interneurons (clusters 9 and 10). Here, the effect of pair-wise social experience on neurogenesis in these brain regions was examined using the proliferation marker bromodeoxyuridine. Groups of proliferating cells in clusters 9 and 10 formed distinctive comma shapes. The BrdU-positive nuclei in the head part of the comma were smaller and more circular than those in the tail part of the comma. Subordinates had fewer new neuronal precursors surviving in cluster 9 after 14 days than did dominants. Mitotic activity was not influenced by social status. The effect of social experience on neurogenesis remained when the effect of body growth rate on neurogenesis was removed. In conclusion, social domination enhances cell survival compared to social subordination. Although the function of these surviving newborn neuronal precursors is unknown they may enhance the learning ability of dominant crayfish.

Odor signals mediate a variety of behaviors in animals across a diversity of taxa. Despite dramatic morphological differences between animals from different taxa, several important features of olfactory system organization and processing are similar across animals. Because of this similarity, a number of different organisms including mammals, insects, and decapod crustaceans serve as valuable model systems for understanding general principles of olfactory processing. As in other organisms, including both vertebrates and insects, the chemosensory system of decapod crustaceans is organized into multiple anatomically distinct neuronal pathways. The two main pathways (the aesthetasc/ olfactory lobe pathway and non-aesthetasc/ lateral antennular neuropil pathway) originate in different populations of antennular sensilla and project to different neuropils in the brain. The functional significance of this parallel organization is not well understood in crustaceans or in many other species. Although in some insect species the functions of parallel pathways are clearly delineated by the types of odors processed by each, functional differences between parallel pathways in other organisms are much less distinct. A critical step towards understanding the functional significance of the multiple chemosensory pathways is to identify the specific behaviors that are driven by each pathway. Using spiny lobsters and crayfish as model organisms, the importance of each pathway was examined in three different behavioral contexts: (1) orientation to a distant food odor, (2) shelter selection in response to conspecific chemical signals, and (3) determination of conspecific social status. In each study, selective ablations of specific populations of antennular sensilla were performed, and the behavior of ablated animals was compared to that of intact controls. Results show that either the aesthetasc or non-aesthetasc pathway is capable of driving orientation to food odors, suggesting functional redundancy between the pathways in this behavior. In contrast social odors are processed preferentially by the aesthetasc pathway rather than the non-aesthetasc pathway, suggesting a unique role for the aesthetasc pathway in this context. As in other organisms possessing multiple chemosensory pathways, the dual antennular pathways in crustaceans display both unique and overlapping functions depending on the chemicals examined, and the behavioral context in which the signal is presented.

Adaptive reproductive behavior requires the ability to recognize and approach possible mating partners in the environment. Syrian hamsters (Mesocricetus auratus) provide a useful animal model by which to study the neural processing of sexual signals, as mate recognition in this species relies almost exclusively on the perception of social odors. In the laboratory, male hamsters prefer to investigate female odors compared to male odors, and this opposite-sex odor preference provides a sensitive measure of the underlying neural processing of sexual stimuli. In addition to chemosensory cues, reproductive behavior in hamsters also requires sufficient levels of circulating gonadal steroid hormones, which reflect the reproductive state of the animal. These chemosensory and hormone signals are processed within an interconnected network of ventral forebrain nuclei, and within this network, the posteromedial cortical amygdala (PMCo) and medial amygdala (MA) are the only nuclei that both receive substantial chemosensory input and are also highly sensitive to steroid hormones. Although a large body of evidence suggests that the MA is critical for generating attraction to sexual odors, the specific role of the PMCo in regulating odor-guided aspects of male reproductive behavior has never been directly tested. Furthermore, detailed analyses of the MA suggest that separate, but interconnected sub-regions within this nucleus process odors differently. Specifically, the anterior MA (MeA) receives the majority of chemosensory input and responds to a variety of social odors, whereas the posterodorsal MA (MePD) receives less chemosensory input but contains the vast majority of steroid receptors. In order to further elucidate how the PMCo and/or MA process sexual odors, this dissertation addressed the following research questions: (1) Is the PMCo required for the expression of either opposite-sex odor preferences or male copulatory behavior? (2) Are functional interactions between MeA and MePD required for the expression of opposite-sex odor preferences? (3) How do MeA and MePD regulate odor responses within the MePD and MeA, respectively? (4) Are odor and/or hormone cues conveyed directly between MeA and MePD? Together, these experiments provide a comprehensive analysis of the functional and neuroanatomical substrates by which the brain processes sexual odors and generates appropriate behavioral responses to these stimuli.

The honeybee is one of the model species in insect olfaction and its sense of smell is well studied. However, knowledge about the spectrum of odorants detectable to honeybees is limited. One class of odorants that has never been tested so far are the amino acids, which are important constituents of floral nectar. The experiments reported here were conducted in order to (1) determine if the odor of amino acids is detectable to honeybees (Apis mellifera), and (2) determine olfactory detection thresholds in honeybees for detectable amino acid odors. To this end, the proboscis extension reflex, a classical conditioning paradigm that takes advantage of the honeybee’s ability to build a robust association between an odor stimulus and a nectar reward, was used. The results demonstrate that five out of 20 amino acids presented at 100 mM were detectable. The honeybees’ median olfactory detection thresholds for these five amino acids are 12 mM for L-tyrosine and L-cysteine, 50 mM for L-asparagine and L-tryptophan, and 100 mM for L-proline. These threshold values are high in comparison to naturally occurring concentrations in floral nectar, and compared to threshold values obtained in vertebrate species. One possible explanation for these findings is that the size of the olfactory receptor repertoire of honeybees limits their olfactory capabilities in terms of detectability and sensitivity for the odor of amino acids