Functional brain imaging of olfactory processing in monkeys

Enhanced D2 Agonism Induces Conditioned Appetitive Sexual Responses Toward Non-reproductive Conspecifics

Brain mechanisms of sexual attraction toward reproductive partners develop from a systematic interrelationship between biology (nature) and learning (nurture). However, the causes of attraction toward non-reproductive partners are poorly understood. Here, we explored the role of Pavlovian learning under dopaminergic agonism on the development of sexual preference and brain activation for young male rats. During conditioning, adult sexually naïve males received either Saline (Saline-Paired) or the D2-receptor agonist quinpirole (QNP-Paired) and cohabited in contingency, or out of contingency (QNP-Unpaired) during 24 h with an almond-scented prepubertal juvenile male (PD25). Conditioning occurred every 4 days for three trials. Social and sexual responses were assessed four days after the last conditioning trial in a drug-free test, and males chose freely between a scented young male (PD37) and a novel receptive female. Four days later, males were exposed to the conditioned odor only and brain Fos-IR and serum testosterone were analyzed. Saline-Paired and QNP-Unpaired males displayed more non-contact erections (NCEs) and genital investigations for females, whereas QNP-Paired males expressed more NCEs and genital investigations for young males. In the QNP-Paired group, exposure to the young male-paired odor evoked more Fos-IR in limbic, hypothalamic and cortical areas, but no differences in serum testosterone were observed. Cohabitation with juvenile males during enhanced D2 agonism results in atypical appetitive sexual responses and a higher pattern of brain response for the young male-paired odor, with no changes in serum testosterone. We discuss the potential implications for the development of pedophilic disorder and perhaps other paraphilias.

Functional Activities Detected in the Olfactory Bulb and Associated Olfactory Regions in the Human Brain Using T2-Prepared BOLD Functional MRI at 7T

Olfaction is a fundamental sense that plays a vital role in daily life in humans, and can be altered in neuropsychiatric and neurodegenerative diseases. Blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) using conventional echo-planar-imaging (EPI) based sequences can be challenging in brain regions important for olfactory processing, such as the olfactory bulb (OB) and orbitofrontal cortex, mainly due to the signal dropout and distortion artifacts caused by large susceptibility effects from the sinonasal cavity and temporal bone. To date, few studies have demonstrated successful fMRI in the OB in humans. T2-prepared (T2prep) BOLD fMRI is an alternative approach developed especially for performing fMRI in regions affected by large susceptibility artifacts. The purpose of this technical study is to evaluate T2prep BOLD fMRI for olfactory functional experiments in humans. Olfactory fMRI scans were performed on 7T in 14 healthy participants. T2prep BOLD showed greater sensitivity than GRE EPI BOLD in the OB, orbitofrontal cortex and the temporal pole. Functional activation was detected using T2prep BOLD in the OB and associated olfactory regions. Habituation effects and a bi-phasic pattern of fMRI signal changes during olfactory stimulation were observed in all regions. Both positively and negatively activated regions were observed during olfactory stimulation. These signal characteristics are generally consistent with literature and showed a good intra-subject reproducibility comparable to previous human BOLD fMRI studies. In conclusion, the methodology demonstrated in this study holds promise for future olfactory fMRI studies in the OB and other brain regions that suffer from large susceptibility artifacts.

The squirrel monkey model in clinical neuroscience

Clinical neuroscience research relying on animal models brought valuable translational insights into the function and pathologies of the human brain. The anatomical, physiological, and behavioural similarities between humans and mammals have prompted researchers to study cerebral mechanisms at different levels to develop and test new treatments. The vast majority of biomedical research uses rodent models, which are easily manipulable and have a broadly resembling organisation to the human nervous system but cannot satisfactorily mimic some disorders. For these disorders, macaque monkeys have been used as they have a more comparable central nervous system. Still, this research has been hampered by limitations, including high costs and reduced samples. This review argues that a squirrel monkey model might bridge the gap by complementing translational research from rodents, macaque, and humans. With the advent of promising new methods such as ultrasound imaging, tool miniaturisation, and a shift towards open science, the squirrel monkey model represents a window of opportunity that will potentially fuel new translational discoveries in the diagnosis and treatment of brain pathologies.

Novelty preferences and cocaine-associated cues influence regions associated with the salience network in juvenile female rats

Preferences for novel environments (novelty-seeking) is a risk factor for addiction, with little known about its underlying circuitry. Exposure to drug cues facilitates addiction maintenance, leading us to hypothesize that exposure to a novel environment activates a shared neural circuitry. Stimulation of the D1 receptor in the prelimbic cortex increases responsivity to drug-associated environments. Here, we use D1 receptor overexpression in the prelimbic cortex to probe brain responses to novelty-preferences (in a free-choice paradigm) and cocaine-associated odors following place conditioning. These same cocaine-conditioned odors were used to study neural circuitry with Blood Oxygen Level Dependent (BOLD) activity. D1 overexpressing females had deactivated BOLD signals related to novelty-preferences within the insula cortex and amygdala and activation in the frontal cortex and dopamine cell bodies. BOLD responses to cocaine cues were also sensitive to D1. Control females demonstrated a place preference for cocaine environments with no significant BOLD response, while D1 overexpressing females demonstrated a place aversion and weak BOLD responses to cocaine-conditioned odor cues within the insula cortex. For comparison, we provide data from an earlier study with juvenile males overexpressing D1 that show a strong preference for cocaine and elevated BOLD responses. The results support the use of a pharmacological manipulation (e.g., D1 overexpression) to probe the neural circuitry downstream from the prelimbic cortex.

Enhancement of Odor-Induced Activity in the Canine Brain by Zinc Nanoparticles: A Functional MRI Study in Fully Unrestrained Conscious Dogs

Using noninvasive in vivo functional magnetic resonance imaging (fMRI), we demonstrate that the enhancement of odorant response of olfactory receptor neurons by zinc nanoparticles leads to increase in activity in olfaction-related and higher order areas of the dog brain. To study conscious dogs, we employed behavioral training and optical motion tracking for reducing head motion artifacts. We obtained brain activation maps from dogs in both anesthetized state and fully conscious and unrestrained state. The enhancement effect of zinc nanoparticles was higher in conscious dogs with more activation in higher order areas as compared with anesthetized dogs. In conscious dogs, voxels in the olfactory bulb and hippocampus showed higher activity to odorants mixed with zinc nanoparticles as compared with pure odorants, odorants mixed with gold nanoparticles as well as zinc nanoparticles alone. These regions have been implicated in odor intensity processing in other species including humans. If the enhancement effect of zinc nanoparticles observed in vivo are confirmed by future behavioral studies, zinc nanoparticles may provide a way for enhancing the olfactory sensitivity of canines for detection of target substances such as explosives and contraband substances at very low concentrations, which would otherwise go undetected.

A direct anterior cingulate pathway to the primate primary olfactory cortex may control attention to olfaction

Behavioral and functional studies in humans suggest that attention plays a key role in activating the primary olfactory cortex through an unknown circuit mechanism. We report that a novel pathway from the anterior cingulate cortex, an area which has a key role in attention, projects directly to the primary olfactory cortex in rhesus monkeys, innervating mostly the anterior olfactory nucleus. Axons from the anterior cingulate cortex formed synapses mostly with spines of putative excitatory pyramidal neurons and with a small proportion of a neurochemical class of inhibitory neurons that are thought to have disinhibitory effect on excitatory neurons. This novel pathway from the anterior cingulate is poised to exert a powerful excitatory effect on the anterior olfactory nucleus, which is a critical hub for odorant processing via extensive bilateral connections with primary olfactory cortices and the olfactory bulb. Acting on the anterior olfactory nucleus, the anterior cingulate may activate the entire primary olfactory cortex to mediate the process of rapid attention to olfactory stimuli.

Behavioral relevance of the dynamics of the functional brain connectome

While many previous studies assumed the functional connectivity (FC) between brain regions to be stationary, recent studies have demonstrated that FC dynamically varies across time. However, two challenges have limited the interpretability of dynamic FC information. First, a principled framework for selecting the temporal extent of the window used to examine the dynamics is lacking and this has resulted in ad-hoc selections of window lengths and subsequent divergent results. Second, it is unclear whether there is any behavioral relevance to the dynamics of the functional connectome in addition to that obtained from conventional static FC (SFC). In this work, we address these challenges by first proposing a principled framework for selecting the extent of the temporal windows in a dynamic and data-driven fashion based on statistical tests of the stationarity of time series. Further, we propose a method involving three levels of clustering-across space, time, and subjects-which allow for group-level inferences of the dynamics. Next, using a large resting-state functional magnetic resonance imaging and behavioral dataset from the Human Connectome Project, we demonstrate that metrics derived from dynamic FC can explain more than twice the variance in 75 behaviors across different domains (alertness, cognition, emotion, and personality traits) as compared with SFC in healthy individuals. Further, we found that individuals with brain networks exhibiting greater dynamics performed more favorably in behavioral tasks. This indicates that the ease with which brain regions engage or disengage may provide potential biomarkers for disorders involving altered neural circuitry.

Functional MRI of the olfactory system in conscious dogs

We depend upon the olfactory abilities of dogs for critical tasks such as detecting bombs, landmines, other hazardous chemicals and illicit substances. Hence, a mechanistic understanding of the olfactory system in dogs is of great scientific interest. Previous studies explored this aspect at the cellular and behavior levels; however, the cognitive-level neural substrates linking them have never been explored. This is critical given the fact that behavior is driven by filtered sensory representations in higher order cognitive areas rather than the raw odor maps of the olfactory bulb. Since sedated dogs cannot sniff, we investigated this using functional magnetic resonance imaging of conscious dogs. We addressed the technical challenges of head motion using a two pronged strategy of behavioral training to keep dogs' head as still as possible and a single camera optical head motion tracking system to account for residual jerky movements. We built a custom computer-controlled odorant delivery system which was synchronized with image acquisition, allowing the investigation of brain regions activated by odors. The olfactory bulb and piriform lobes were commonly activated in both awake and anesthetized dogs, while the frontal cortex was activated mainly in conscious dogs. Comparison of responses to low and high odor intensity showed differences in either the strength or spatial extent of activation in the olfactory bulb, piriform lobes, cerebellum, and frontal cortex. Our results demonstrate the viability of the proposed method for functional imaging of the olfactory system in conscious dogs. This could potentially open up a new field of research in detector dog technology.

Why do capuchin monkeys urine wash? An experimental test of the sexual communication hypothesis using fMRI

Urine washing (UW) consists of depositing urine on the hands and vigorously rubbing the body. As urine contains chemical and pheromonal cues, UW may convey socially relevant information. Although ritualized UW is observed in many New World primates, including capuchin monkeys, the functional significance of UW remains unclear. In this experiment, we investigated the social signaling hypothesis of UW. Specifically, we hypothesized that UW by males conveys socially relevant signals that females can detect. We used functional magnetic resonance imaging (fMRI) to test whether adult female capuchins show differential brain activation in response to adult male and juvenile male capuchin urine. We expected to see changes in activation of structures involved in olfactory processing, including the piriform cortex, medial preoptic and anterior hypothesis, orbitofrontal cortex, hippocampus, and cerebellum. Data were acquired from four adult female capuchin monkeys. Presentations of odor stimuli (obtained from unfamiliar males) were made during fMRI acquisition using a standard ON-OFF design. All fMRI data were spatially normalized to a template and analyzed using the FMRI Expert Analysis Tool Version 5.98, part of the FMRIB's Software Library (www.fmrib.ox.ac.uk/fsl). Whole brain analyses revealed significant activations in the inferior temporal cortex, parahippocampal gyrus, precuneus, hippocampus, pulvinar, and cerebellum when females were presented with the adult male urine. Notably, significantly greater signal activation was observed in several regions associated with olfactory processing, when subjects were presented with adult male urine as compared with urine from juvenile males. Our results indicate that UW serves a social communicative function in capuchins, providing support for the sexual signaling hypothesis.

Nursing stimulation is more than tactile sensation: It is a multisensory experience

Novel sensory experiences, particularly those associated with epochal developmental events like nursing alter cortical representation, affecting memory, perception and behavior. Functional MRI was used here to test whether the sensoricortical map of the ventrum is modified during lactation. Three stimuli were used to drive cortical activation in primiparous rats: natural, artificial suckling stimulation and general mechanical rubbing of the skin of the ventrum. These stimuli significantly activated the somatosensory cortex of dams. Of the three stimuli, artificial and pup suckling robustly activated much of the cerebrum, most notably the visual, auditory and olfactory cortices. Surprisingly, activation occurred even in the absence of pups, with artificial suckling. This finding suggests that incoming information from a single modality was sufficient to drive activity of others. Enhanced sensitivity across the cortical mantle during nursing may help the dam to perceive, process, and remember stimuli critical to the care and protection of her young.

CA3 NMDA receptors are required for experience-dependent shifts in hippocampal activity

The anatomical distribution of sensory-evoked activity recorded from the hippocampal long-axis can shift depending on prior experience. In accordance with Marr's computational model of hippocampal function, CA3 NMDA receptors have been hypothesized to mediate this experience-dependent shift in hippocampal activity. Here we tested this hypothesis by investigating genetically-modified mice in which CA3 NMDA receptors are selectively knocked-out (CA3-NR1 KO). First, we were required to develop an fMRI protocol that can record sensory-evoked activity from the mouse hippocampal long-axis. This goal was achieved in part by using a dedicated mouse scanner to image odor-evoked activity, and by using non-EPI (echo planer imaging) pulse sequences. As in humans, odors were found to evoke a ventral-predominant activation pattern in the mouse hippocampus. More importantly, odor-evoked activity shifted in an experience-dependent manner. Finally, we found that the experience-dependent shift in hippocampal long-axis activity is blocked in CA3-NR1 knock-out mice. These findings establish a cellular mechanism for the plasticity imaged in the hippocampal long-axis, suggesting how experience-dependent modifications of hippocampal activity can contribute to its mnemonic function.

The influence of olfactory-induced negative emotion on verbal working memory: individual differences in neurobehavioral findings

The influence of emotion on cognition plays an important role in people's everyday life as well as in psychiatric and neurological disorders. The present study used fMRI to examine the neural correlates of cognitive-emotional interactions and its inter-individual differences. Twenty-one healthy males performed a 0-back/2-back task while negative or neutral emotion was induced by negative/neutral olfactory stimulation. Subjects revealed a differential effect of emotion on cognition; in 9 subjects, negative odor had a deteriorating influence on verbal working memory ("affected group", AG) while in 12 subjects, performance was not affected in a negative way ("unaffected group", UAG). Although no brain activation differences emerged during the working memory task, the interaction of working memory and emotion yielded significant differences between the AG and the UAG. The latter showed greater activation in the fronto-parieto-cerebellar working memory (WM) network including the precuneus while the AG demonstrated stronger activation in more "emotional" areas (mainly the temporal and medial frontal cortex) as well as compensatory activations in prefrontal regions known to be essential for the cognitive down-regulation of emotions. Hence, the UAG may have been better able to counteract the detrimental influence of negative stimulation during the 2-back task and to effectively sustain or even increase activation in the task-relevant WM network. Correlation analyses for the whole group supported this interpretation; reduced working memory performance during negative stimulation was accompanied by higher activation in the inferior frontal gyrus whereas less performance impairment was related to higher activation in the precuneus. Results confirm the importance of incorporating individual differences in emotion processing and its interaction with cognitive functions in neuroimaging.

Reciprocal connections between olfactory structures and the cortex of the rostral superior temporal sulcus in the Macaca fascicularis monkey

Convergence of sensory modalities in the nonhuman primate cerebral cortex is still poorly understood. We present an anatomical tracing study in which polysensory association cortex located at the fundus and upper bank of the rostral superior temporal sulcus presents reciprocal connections with primary olfactory structures. At the same time, projections from this polysensory area reach multiple primary olfactory centres. Retrograde (Fast Blue) and anterograde (biotinylated dextran-amine and 3H-amino acids) tracers were injected into primary olfactory structures and rostral superior temporal sulcus. Retrograde tracers restricted to the anterior olfactory nucleus resulted in labelled neurons in the rostral portion of the upper bank and fundus of superior temporal sulcus. Injections of biotinylated dextran-amine at the fundus and upper bank of the superior temporal sulcus confirmed this projection by labelling axons in the dorsal and lateral portions of the anterior olfactory nucleus, as well as piriform, periamygdaloid and entorhinal cortices. Retrograde tracer injections at the rostral superior temporal sulcus resulted in neuronal labelling in the anterior olfactory nucleus, piriform, periamygdaloid and entorhinal cortices, thus providing confirmation of the reciprocity between primary olfactory structures and the cortex at the rostral superior temporal sulcus. The reciprocal connections between the rostral part of superior temporal sulcus and primary olfactory structures represent a convergence for olfactory and other sensory modalities at the cortex of the rostral temporal lobe.

On the scent of human olfactory orbitofrontal cortex: meta-analysis and comparison to non-human primates

It is widely accepted that the orbitofrontal cortex (OFC) represents the main neocortical target of primary olfactory cortex. In non-human primates, the olfactory neocortex is situated along the basal surface of the caudal frontal lobes, encompassing agranular and dysgranular OFC medially and agranular insula laterally, where this latter structure wraps onto the posterior orbital surface. Direct afferent inputs arrive from most primary olfactory areas, including piriform cortex, amygdala, and entorhinal cortex, in the absence of an obligatory thalamic relay. While such findings are almost exclusively derived from animal data, recent cytoarchitectonic studies indicate a close anatomical correspondence between non-human primate and human OFC. Given this cross-species conservation of structure, it has generally been presumed that the olfactory projection area in human OFC occupies the same posterior portions of OFC as seen in non-human primates. This review questions this assumption by providing a critical survey of the localization of primate and human olfactory neocortex. Based on a meta-analysis of human functional neuroimaging studies, the region of human OFC showing the greatest olfactory responsivity appears substantially rostral and in a different cytoarchitectural area than the orbital olfactory regions as defined in the monkey. While this anatomical discrepancy may principally arise from methodological differences across species, these results have implications for the interpretation of prior human lesion and neuroimaging studies and suggest constraints upon functional extrapolations from animal data.

Projections from the amygdaloid complex to the piriform cortex: A PHA-L study in the rat

Projections from the amygdala to the piriform cortex are proposed to provide a pathway via which the emotional system can modulate the processing of olfactory information as well as mediate the spread of seizure activity in epilepsy. To understand the details of the distribution and topography of these projections, we injected the anterograde tracer Phaseolus vulgaris-leucoagglutinin into different nuclear divisions of the amygdaloid complex in 101 rats and analyzed the distribution and density of projections in immunohistochemically processed preparations. The heaviest projections from the amygdala to the piriform cortex originated in the medial division of the lateral nucleus, the periamygdaloid and sulcal subfields of the periamygdaloid cortex, and the posterior cortical nucleus. The heaviest terminal labeling was observed in layers Ib and III of the medial aspect of the posterior piriform cortex. Lighter projections to the posterior piriform cortex originated in the dorsolateral division of the lateral nucleus, the magnocellular and parvicellular divisions of the basal and accessory basal nuclei, and the anterior cortical nucleus. The projections to the anterior piriform cortex were light and originated in the dorsolateral and medial divisions of the lateral nucleus, the magnocellular division of the basal and accessory basal nuclei, the anterior and posterior cortical nuclei, and the periamygdaloid subfield of the periamygdaloid cortex. The results indicate that only selective amygdaloid nuclei or their subdivisions project to the piriform cortex. In addition, substantial projections from several amygdaloid nuclei converge in the medial aspect of the posterior piriform cortex. Via these projections, the amygdaloid complex can modulate the processing of olfactory information in the piriform cortex. In pathologic conditions such as epilepsy, these connections might provide pathways for the spread of seizure activity from the amygdala to extra-amygdaloid regions.