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

Functional connectivity differences of the olfactory network in Parkinson's Disease, mild cognitive impairment and cognitively normal individuals: A resting-state fMRI study

Olfactory dysfunction is an early sign of such neurodegenerative diseases as Parkinson's (PD) and Alzheimer's (AD), and is often present in Mild Cognitive Impairment (MCI), a precursor of AD. Understanding neuro-temporal relationships, i.e., functional connectivity, between olfactory eloquent structures in such disorders, could shed light on their basic pathophysiology. To this end, we employed region-based analyses using resting-state functional magnetic resonance imaging (rs-fMRI) obtained from cognitively normal (CN), MCI, and PD patients with cognitive impairment (PD-CogImp). Using machine learning (linear and ensemble learning), we determined whether the identified functional patterns could classify abnormal function from normal function. Olfaction, as measured by objective testing, was found to be most strongly associated with diagnostic status, emphasizing the fundamental association of this primary sensory system with these conditions. Consistently lower functional connectivity was observed in the PD-CogImp cohort compared to the CN cohort among all identified brain regions. Differences were also found between PD-CogImp and MCI at the level of the orbitofrontal and cingulate cortices. MCI and CN subjects had different functional connectivity between the posterior orbitofrontal cortex and thalamus. Regardless of study group, males showed significantly higher connectivity than females in connections involving the orbitofrontal cortex. The logistic regression model trained using the top discriminatory features revealed that caudate was the most involved olfaction-related brain structure (accuracy = 0.88, Area under the Receiver operator characteristic curve of 0.90). In aggregate, our study demonstrates that resting functional connectivity among olfactory eloquent structures has potential value in better understanding the pathophysiology of several neurodegenerative diseases.

Medial to lateral frontal functional connectivity mapping reveals the organization of cingulate cortex

The functional organization of the frontal lobe is a source of debate, focusing on broad functional subdivisions, large-scale networks, or local refined specificities. Multiple neurocognitive models have tried to explain how functional interactions between cingulate and lateral frontal regions contribute to decision making and cognitive control, but their neuroanatomical bases remain unclear. We provide a detailed description of the functional connectivity between cingulate and lateral frontal regions using resting-state functional MRI in rhesus macaques. The analysis focuses on the functional connectivity of the rostral part of the cingulate sulcus with the lateral frontal cortex. Data-driven and seed-based analysis revealed three clusters within the cingulate sulcus organized along the rostro-caudal axis: the anterior, mid, and posterior clusters display increased functional connectivity with, respectively, the anterior lateral prefrontal regions, face-eye lateral frontal motor cortical areas, and hand lateral frontal motor cortex. The location of these clusters can be predicted in individual subjects based on morphological landmarks. These results suggest that the anterior cluster corresponds to the anterior cingulate cortex, whereas the posterior clusters correspond to the face-eye and hand cingulate motor areas within the anterior midcingulate cortex. These data provide a comprehensive framework to identify cingulate subregions based on functional connectivity and local organization.

Brain microstructure and connectivity in COVID-19 patients with olfactory or cognitive impairment

INTRODUCTION

The COVID-19 pandemic has affected millions worldwide, causing mortality and multi-organ morbidity. Neurological complications have been recognized. This study aimed to assess brain structural, microstructural, and connectivity alterations in patients with COVID-19-related olfactory or cognitive impairment using post-acute (time from onset: 264[208-313] days) multi-directional diffusion-weighted MRI (DW-MRI).

METHODS

The study included 16 COVID-19 patients with cognitive impairment (COVID-CM), 35 COVID-19 patients with olfactory disorder (COVID-OD), and 14 controls. A state-of-the-art processing pipeline was developed for DW-MRI pre-processing, mean diffusivity and fractional anisotropy computation, fiber density and cross-section analysis, and tractography of white-matter bundles. Brain parcellation required for probing network connectivity, region-specific microstructure and volume, and cortical thickness was based on T1-weighted scans and anatomical atlases.

RESULTS

Compared to controls, COVID-CM patients showed overall gray matter atrophy (age and sex corrected p = 0.004), and both COVID-19 patient groups showed regional atrophy and cortical thinning. Both groups presented an increase in gray matter mean diffusivity (corrected p = 0.001), decrease in white matter fiber density and cross-section (corrected p < 0.05), , and COVID-CM patients also displayed an overall increased diffusivity (p = 0.022) and decreased anisotropy (corrected p = 0.038) in white matter. Graph-based analysis revealed reduced network modularity, with an extensive pattern of connectivity increase, in conjunction with a localized reduction in a few connections, mainly located in the left hemisphere. The left cingulate, anterior cingulate, and insula were primarily involved.

CONCLUSION

Expanding upon previous findings, this study further investigated significant alterations in brain morphology, microstructure, and connectivity in COVID-19 patients with olfactory or cognitive disfunction. These findings suggest underlying neurodegeneration, neuroinflammation, and concomitant compensatory mechanisms. Future longitudinal studies are required to monitor the alterations over time and assess their transient or permanent nature.

Semantic context-dependent neural representations of odors in the human piriform cortex revealed by 7T MRI

Olfactory perception depends not only on olfactory inputs but also on semantic context. Although multi-voxel activity patterns of the piriform cortex, a part of the primary olfactory cortex, have been shown to represent odor perception, it remains unclear whether semantic contexts modulate odor representation in this region. Here, we investigated whether multi-voxel activity patterns in the piriform cortex change when semantic context modulates odor perception and, if so, whether the modulated areas communicate with brain regions involved in semantic and memory processing beyond the piriform cortex. We also explored regional differences within the piriform cortex, which are influenced by olfactory input and semantic context. We used 2 × 2 combinations of word labels and odorants that were perceived as congruent and measured piriform activity with a 1-mm isotropic resolution using 7T MRI. We found that identical odorants labeled with different words were perceived differently. This labeling effect was observed in multi-voxel activity patterns in the piriform cortex, as the searchlight decoding analysis distinguished identical odors with different labels for half of the examined stimulus pairs. Significant functional connectivity was observed between parts of the piriform cortex that were modulated by labels and regions associated with semantic and memory processing. While the piriform multi-voxel patterns evoked by different olfactory inputs were also distinguishable, the decoding accuracy was significant for only one stimulus pair, preventing definitive conclusions regarding the locational differences between areas influenced by word labels and olfactory inputs. These results suggest that multi-voxel patterns of piriform activity can be modulated by semantic context, possibly due to communication between the piriform cortex and the semantic and memory regions.

Significance of the anterior cingulate cortex in neurogenesis plasticity: Connections, functions, and disorders across postnatal and adult stages

The anterior cingulate cortex (ACC) is a complex and continually evolving brain region that remains a primary focus of research due to its multifaceted functions. Various studies and analyses have significantly advanced our understanding of how the ACC participates in a wide spectrum of memory and cognitive processes. However, despite its strong connections to brain areas associated with hippocampal and olfactory neurogenesis, the functions of the ACC in regulating postnatal and adult neurogenesis in these regions are still insufficiently explored. Investigating the intricate involvement of the ACC in neurogenesis could enhance our comprehension of essential aspects of brain plasticity. This involvement stems from its complex circuitry with other relevant brain regions, thereby exerting both direct and indirect impacts on the neurogenesis process. This review sheds light on the promising significance of the ACC in orchestrating postnatal and adult neurogenesis in conditions related to memory, cognitive behavior, and associated disorders.

The brain functional connectivity alterations in traumatic patients with olfactory disorder after low-level laser therapy demonstrated by fMRI

BACKGROUND

Low-level laser therapy (LLLT) has been clinically accepted to accelerate the nerve regeneration process after a nerve injury or transection. We aimed to investigate the neuronal basis and the influence of LLLT on brain functional networks in traumatic patients with olfactory dysfunction.

METHODS

Twenty-four Patients with traumatic anosmia/hyposmia were exposed to pleasant olfactory stimuli during a block-designed fMRI session. After a 10-week period, patients as control group and patients who had completed the sessions of LLLT were invited for follow-up testing using the same fMRI protocol. Two-sample t-tests were conducted to explore group differences in activation responding to odorants (p-FDR-corrected <0.05). Differences of functional connectivity were compared between the two groups and the topological features of the olfactory network were calculated. Correlation analysis was performed between graph parameters and TDI score.

RESULTS

Compared to controls, laser-treated patients showed increased activation in the cingulate, rectus gyrus, and some parts of the frontal gyrus. Shorter pathlength (p = 0.047) and increased local efficiency (p = 0.043) within the olfactory network, as well as decreased inter-network connectivity within the whole brain were observed in patients after laser surgery. Moreover, higher clustering and local efficiency were related to higher TDI score, as manifested in increased sensitivity to identify odors.

CONCLUSIONS

The results support that low-level laser induces neural reorganization process and make new connections in the olfactory structures. Furthermore, the connectivity parameters may serve as potential biomarkers for traumatic anosmia or hyposmia by revealing the underlying neural mechanisms of LLLT.

Cingulate Cortex: Anatomy, Structural and Functional Connectivity

SUMMARY

The cingulate cortex is a paired brain region located on the medial wall of each hemisphere. This review explores the anatomy as well as the structural and functional connectivity of the cingulate cortex underlying essential roles this region plays in emotion, autonomic, cognitive, motor control, visual-spatial processing, and memory.

Aberrant olfactory network functional connectivity in people with olfactory dysfunction following COVID-19 infection: an exploratory, observational study

BACKGROUND

Olfactory impairments and anosmia from COVID-19 infection typically resolve within 2-4 weeks, although in some cases, symptoms persist longer. COVID-19-related anosmia is associated with olfactory bulb atrophy, however, the impact on cortical structures is relatively unknown, particularly in those with long-term symptoms.

METHODS

In this exploratory, observational study, we studied individuals who experienced COVID-19-related anosmia, with or without recovered sense of smell, and compared against individuals with no prior COVID-19 infection (confirmed by antibody testing, all vaccine naïve). MRI Imaging was carried out between the 15th July and 17th November 2020 at the Queen Square House Clinical Scanning Facility, UCL, United Kingdom. Using functional magnetic resonance imaging (fMRI) and structural imaging, we assessed differences in functional connectivity (FC) between olfactory regions, whole brain grey matter (GM) cerebral blood flow (CBF) and GM density.

FINDINGS

Individuals with anosmia showed increased FC between the left orbitofrontal cortex (OFC), visual association cortex and cerebellum and FC reductions between the right OFC and dorsal anterior cingulate cortex compared to those with no prior COVID-19 infection (p < 0.05, from whole brain statistical parametric map analysis). Individuals with anosmia also showed greater CBF in the left insula, hippocampus and ventral posterior cingulate when compared to those with resolved anosmia (p < 0.05, from whole brain statistical parametric map analysis).

INTERPRETATION

This work describes, for the first time to our knowledge, functional differences within olfactory areas and regions involved in sensory processing and cognitive functioning. This work identifies key areas for further research and potential target sites for therapeutic strategies.

FUNDING

This study was funded by the National Institute for Health and Care Research and supported by the Queen Square Scanner business case.

Neural underpinning of a respiration-associated resting-state fMRI network

Respiration can induce motion and CO2 fluctuation during resting-state fMRI (rsfMRI) scans, which will lead to non-neural artifacts in the rsfMRI signal. In the meantime, as a crucial physiologic process, respiration can directly drive neural activity change in the brain, and may thereby modulate the rsfMRI signal. Nonetheless, this potential neural component in the respiration-fMRI relationship is largely unexplored. To elucidate this issue, here we simultaneously recorded the electrophysiology, rsfMRI, and respiration signals in rats. Our data show that respiration is indeed associated with neural activity changes, evidenced by a phase-locking relationship between slow respiration variations and the gamma-band power of the electrophysiological signal recorded in the anterior cingulate cortex. Intriguingly, slow respiration variations are also linked to a characteristic rsfMRI network, which is mediated by gamma-band neural activity. In addition, this respiration-related brain network disappears when brain-wide neural activity is silenced at an isoelectrical state, while the respiration is maintained, further confirming the necessary role of neural activity in this network. Taken together, this study identifies a respiration-related brain network underpinned by neural activity, which represents a novel component in the respiration-rsfMRI relationship that is distinct from respiration-related rsfMRI artifacts. It opens a new avenue for investigating the interactions between respiration, neural activity, and resting-state brain networks in both healthy and diseased conditions.

Odors Associated With Autobiographical Memory Induce Visual Imagination of Emotional Scenes as Well as Orbitofrontal-Fusiform Activation

Specific odors can induce memories of the past, especially those associated with autobiographical and episodic memory. Odors associated with autobiographical memories have been found to elicit stronger activation in the orbitofrontal cortex, hippocampus, and parahippocampus compared with odors not linked to personal memories. Here, we examined whether continuous odor stimuli associated with autobiographical memories could activate the above olfactory areas in older adults and speculated regarding whether this odor stimulation could have a protective effect against age-related cognitive decline. Specifically, we used functional magnetic resonance imaging to investigate the relationship between blood oxygen levels in olfactory regions and odor-induced subjective memory retrieval and emotions associated with autobiographical memory in older adults. In our group of healthy older adults, the tested odors induced autobiographical memories that were accompanied by increasing levels of retrieval and the feeling of being "brought back in time." The strength of the subjective feelings, including vividness of the memory and degree of comfort, impacted activation of the left fusiform gyrus and left posterior orbitofrontal cortex. Further, our path model suggested that the strength of memory retrieval and of the emotions induced by odor-evoked autobiographical memories directly influenced neural changes in the left fusiform gyrus, and impacted left posterior orbitofrontal cortex activation through the left fusiform response.

Sinonasal surgery alters brain structure and function: Neuroanatomical correlates of olfactory dysfunction

Olfactory dysfunction (OD) is more common than hearing loss, partial blindness, or blindness and can have a significant impact on the quality of life. Moreover, unexplained OD is an early biomarker in neurodegenerative diseases and increases 5-year mortality risk. Structural alterations in olfactory eloquent brain regions may represent the neuroanatomical correlates of OD. Previous studies have demonstrated reduced gray matter (GM) volume in areas of presumed olfactory relevance in patients with OD. However, being cross-sectional in nature, these studies do not provide evidence of causality, for which longitudinal work is required. At present, however, longitudinal studies addressing olfactory structural plasticity are limited, both in number and methodological approach: to our knowledge, such work has not included parallel functional imaging to confirm the relevance of structural change. We therefore performed a longitudinal multimodal neuroimaging study investigating structural and functional plasticity in 24 patients undergoing surgical treatment for chronic rhinosinusitis, compared with 17 healthy controls. We demonstrated functionally significant structural plasticity within the orbitofrontal, anterior cingulate and insular cortices, and temporal poles in patients 3 months after surgery. Of interest, GM volume decreased in these regions, in association with increased psychophysical scores and BOLD signal. To our knowledge, this is the first study to demonstrate both structural and functional plasticity of the central olfactory networks, thereby confirming these areas as neuroanatomical correlates of olfactory function/dysfunction.

Pathways for Contextual Memory: The Primate Hippocampal Pathway to Anterior Cingulate Cortex

The anterior cingulate cortex (ACC) is one of the few prefrontal areas that receives robust direct hippocampal terminations. This pathway may enable current context and past experience to influence goal-directed actions and emotional regulation by prefrontal cortices. We investigated the still ill-understood organization of the pathway from anterior hippocampus to ACC (A24a, A25, A32) to identify laminar termination patterns and their postsynaptic excitatory and inhibitory targets from system to synapse in rhesus monkeys. The densest hippocampal terminations targeted posterior A25, a region that is involved in affective and autonomic regulation. Hippocampal terminations innervated mostly excitatory neurons (~90%), suggesting strong excitatory effects. Among the smaller fraction of inhibitory targets, hippocampal terminations in A25 preferentially innervated calretinin neurons, a pattern that differs markedly from rodents. Further, hippocampal terminations innervated spines with D1 receptors, particularly in the deep layers of A25, where D1 receptors are enriched in comparison with the upper layers. The proximity of hippocampal terminations to D1 receptors may enable dopamine to enhance information transfer from the hippocampus to A25 and contribute to dopaminergic influence downstream on goal-directed action and emotional control by prefrontal cortices, in processes that may be disrupted by excessive dopamine release during uncontrollable stress.

Smell and taste dissociations in the modulation of tonic pain perception induced by a capsaicin cream application

BACKGROUND

Pain is a subjective experience characterized by sensory (intensity) and emotional (unpleasantness) aspects. Although literature reports behavioural effects on pain due to smell and taste influence, to our knowledge the relationship between tonic pain induced by a capsaicin cream and these chemosensory systems has never been explored before. The aim of this study was to investigate the modulation of olfactory and gustatory substances having different valence on tonic pain perception mediated by a capsaicin cream application.

METHODS

Sixty healthy volunteers were included in two separated experiments (N = 30 smell; N = 30 taste) and underwent different valence smell and taste stimulations, while receiving painful stimuli. Perception of pain intensity (the sensory component) and unpleasantness (the affective component) was measured with a numerical rating scale, both during the two aforementioned experiments.

RESULTS

Pain unpleasantness rating showed differences only in the smell experiment between the two odourous conditions. In particular, pleasant odour induced lower ratings of pain unpleasantness, while no significant results were found for intensity. Regarding taste, we could not observe significant effects nor for pain unpleasantness or intensity.

CONCLUSIONS

These findings highlight the potential role of pleasant odours in influencing the affective aspects of pain perception induced by this kind of tonic pain. Such evidence might provide new insight for using chemosensory substances as analgesics for modulating the cognitive aspects of neuropathic pain.

SIGNIFICANCE

This work shows the effect of smell on the emotional component of tonic pain, experimentally induced by capsaicin cream application. Previous literature investigated tonic pain in interaction with smell and/or taste stimuli, but mainly with physical methods such as temperature. Our findings add new information in this field, contributing to a deeper insight on the role of olfaction on this particular kind of tonic pain perception. This approach could open to new investigations aimed to consider odours for pain management.

Lisdexamfetamine Alters BOLD-fMRI Activations Induced by Odor Cues in Impulsive Children

INTRODUCTION

Lisdexamfetamine (LDX) is a drug used to treat ADHD/impulsive patients. Impulsivity is known to affect inhibitory, emotional and cognitive function. On the other hand, smell and odor processing are known to be affected by neurological disorders, as they are modulators of addictive and impulsive behaviors specifically. We hypothesize that, after LDX ingestion, inhibitory pathways of the brain would change, and complementary behavioral regulation mechanisms would appear to regulate decision-making and impulsivity.

METHODS

20 children were studied in an aleatory crossover study. Imaging of BOLD-fMRI activity, elicited by olfactory stimulation in impulsive children, was performed after either LDX or placebo ingestion.

RESULTS

Findings showed that all subjects who underwent odor stimulation presented activations of similar intensities in the olfactory centers of the brain. This contrasted with inhibitory regions of the brain such as the cingulate cortex and frontal lobe regions, which demonstrated changed activity patterns and intensities. While some differences between the placebo and medicated states were found in motor areas, precuneus, cuneus, calcarine, supramarginal, cerebellum and posterior cingulate cortex, the main changes were found in frontal, temporal and parietal cortices. When comparing olfactory cues separately, pleasant food smells like chocolate seemed not to present large differences between the medicated and placebo scenarios, when compared to non-food-related smells.

CONCLUSION

It was demonstrated that LDX, first, altered the inhibitory pathways of the brain, secondly it increased activity in several brain regions which were not activated by smell in drug-naïve patients, and thirdly, it facilitated a complementary behavioral regulation mechanism, run by the cerebellum, which regulated decision-making and impulsivity in motor and frontal structures.

fMRI-based Neuronal Response to New Odorants in the Newborn Brain

The sense of smell is one of the oldest and the most primitive senses mammals possess, it helps to evaluate the surrounding environment. From birth, smell is an important sensory modality, highly relevant for neonatal behavioral adaptation. Even though human newborns seem to be able to perceive and react to olfactory stimuli, there is still a lack of knowledge about the ontogeny of smell and the underlying central processing involved in odor perception in newborns. Brain networks involved in chemosensory perception of odorants are well described in adults, however in newborns there is no evidence that central olfaction is functional given the largely unmyelinated neonatal central nervous system. To examine this question, we used functional magnetic resonance imaging (fMRI) in the newborn to characterize cortical response to olfactory and trigeminal odorants. Here we show that brain response to odors can be measured and localized using functional MRI in newborns. Furthermore, we found that the developing brain, only few days after birth, processes new artificial odorants in similar cortical areas than adults, including piriform cortex, orbitofrontal cortex and insula. Our work provides evidence that human olfaction at birth relies on brain functions that involve all levels of the cortical olfactory system.

Cortical Connections Position Primate Area 25 as a Keystone for Interoception, Emotion, and Memory

The structural and functional integrity of subgenual cingulate area 25 (A25) is crucial for emotional expression and equilibrium. A25 has a key role in affective networks, and its disruption has been linked to mood disorders, but its cortical connections have yet to be systematically or fully studied. Using neural tracers in rhesus monkeys, we found that A25 was densely connected with other ventromedial and posterior orbitofrontal areas associated with emotions and homeostasis. A moderate pathway linked A25 with frontopolar area 10, an area associated with complex cognition, which may regulate emotions and dampen negative affect. Beyond the frontal lobe, A25 was connected with auditory association areas and memory-related medial temporal cortices, and with the interoceptive-related anterior insula. A25 mostly targeted the superficial cortical layers of other areas, where broadly dispersed terminations comingled with modulatory inhibitory or disinhibitory microsystems, suggesting a dominant excitatory effect. The architecture and connections suggest that A25 is the consummate feedback system in the PFC. Conversely, in the entorhinal cortex, A25 pathways terminated in the middle-deep layers amid a strong local inhibitory microenvironment, suggesting gating of hippocampal output to other cortices and memory storage. The graded cortical architecture and associated laminar patterns of connections suggest how areas, layers, and functionally distinct classes of inhibitory neurons can be recruited dynamically to meet task demands. The complement of cortical connections of A25 with areas associated with memory, emotion, and somatic homeostasis provide the circuit basis to understand its vulnerability in psychiatric and neurologic disorders.SIGNIFICANCE STATEMENT Integrity of the prefrontal subgenual cingulate cortex is crucial for healthy emotional function. Subgenual area 25 (A25) is mostly linked with other prefrontal areas associated with emotion in a dense network positioned to recruit large fields of cortex. In healthy states, A25 is associated with internal states, autonomic function, and transient negative affect. Constant hyperactivity in A25 is a biomarker for depression in humans and may trigger extensive activation in its dominant connections with areas associated with emotions and internal balance. A pathway between A25 and frontopolar area 10 may provide a critical link to regulate emotions and dampen persistent negative affect, which may be explored for therapeutic intervention in depression.

Mirror trends of plasticity and stability indicators in primate prefrontal cortex

Research on plasticity markers in the cerebral cortex has largely focused on their timing of expression and role in shaping circuits during critical and normal periods. By contrast, little attention has been focused on the spatial dimension of plasticity-stability across cortical areas. The rationale for this analysis is based on the systematic variation in cortical structure that parallels functional specialization and raises the possibility of varying levels of plasticity. Here, we investigated in adult rhesus monkeys the expression of markers related to synaptic plasticity or stability in prefrontal limbic and eulaminate areas that vary in laminar structure. Our findings revealed that limbic areas are impoverished in three markers of stability: intracortical myelin, the lectin Wisteria floribunda agglutinin, which labels perineuronal nets, and parvalbumin, which is expressed in a class of strong inhibitory neurons. By contrast, prefrontal limbic areas were enriched in the enzyme calcium/calmodulin-dependent protein kinase II (CaMKII), known to enhance plasticity. Eulaminate areas have more elaborate laminar architecture than limbic areas and showed the opposite trend: they were enriched in markers of stability and had lower expression of the plasticity-related marker CaMKII. The expression of glial fibrillary acidic protein (GFAP), a marker of activated astrocytes, was also higher in limbic areas, suggesting that cellular stress correlates with the rate of circuit reshaping. Elevated markers of plasticity may endow limbic areas with flexibility necessary for learning and memory within an affective context, but may also render them vulnerable to abnormal structural changes, as seen in neurologic and psychiatric diseases.

Anterior Cingulate Pathways May Affect Emotions Through Orbitofrontal Cortex

The anterior cingulate cortex (ACC) and posterior orbitofrontal cortex (pOFC) are associated with emotional regulation. These regions are old in phylogeny and have widespread connections with eulaminate neocortices, intricately linking areas associated with emotion and cognition. The ACC and pOFC have distinct cortical and subcortical connections and are also interlinked, but the pattern of their connections-which may be used to infer the flow of information between them-is not well understood. Here we found that pathways from ACC area 32 innervated all pOFC areas with a significant proportion of large and efficient terminals, seen at the level of the system and the synapse. The pathway from area 32 targeted overwhelmingly elements of excitatory neurons in pOFC, with few postsynaptic sites found on presumed inhibitory neurons. Moreover, pathways from area 32 originated mostly in the upper layers and innervated preferentially the middle-deep layers of the least differentiated pOFC areas, in a pattern reminiscent of feedforward communication. Pathway terminations from area 32 overlapped in the deep layers of pOFC with output pathways that project to the thalamus and the amygdala, and may have cascading downstream effects on emotional and cognitive processes and their disruption in psychiatric disorders.

Posterior Orbitofrontal and Anterior Cingulate Pathways to the Amygdala Target Inhibitory and Excitatory Systems with Opposite Functions

The bidirectional dialogue of the primate posterior orbitofrontal cortex (pOFC) with the amygdala is essential in cognitive-emotional functions. The pOFC also sends a uniquely one-way excitatory pathway to the amygdalar inhibitory intercalated masses (IM), which inhibit the medial part of the central amygdalar nucleus (CeM). Inhibition of IM has the opposite effect, allowing amygdalar activation of autonomic structures and emotional arousal. Using multiple labeling approaches to identify pathways and their postsynaptic sites in the amygdala in rhesus monkeys, we found that the anterior cingulate cortex innervated mostly the basolateral and CeM amygdalar nuclei, poised to activate CeM for autonomic arousal. By contrast, a pathway from pOFC to IM exceeded all other pathways to the amygdala by density and size and proportion of large and efficient terminals. Moreover, whereas pOFC terminals in IM innervated each of the three distinct classes of inhibitory neurons, most targeted neurons expressing dopamine- and cAMP-regulated phosphoprotein (DARPP-32+), known to be modulated by dopamine. The predominant pOFC innervation of DARPP-32+ neurons suggests activation of IM and inhibition of CeM, resulting in modulated autonomic function. By contrast, inhibition of DARPP-32 neurons in IM by high dopamine levels disinhibits CeM and triggers autonomic arousal. The findings provide a mechanism to help explain how a strong pOFC pathway, which is poised to moderate activity of CeM, through IM, can be undermined by the high level of dopamine during stress, resulting in collapse of potent inhibitory mechanisms in the amygdala and heightened autonomic drive, as seen in chronic anxiety disorders.SIGNIFICANCE STATEMENT The dialogue between prefrontal cortex and amygdala allows thoughts and emotions to influence actions. The posterior orbitofrontal cortex sends a powerful pathway that targets a special class of amygdalar intercalated mass (IM) inhibitory neurons, whose wiring may help modulate autonomic function. By contrast, the anterior cingulate cortex innervates other amygdalar parts, activating circuits to help avoid danger. Most IM neurons in primates label for the protein DARPP-32, known to be activated or inhibited based on the level of dopamine. Stress markedly increases dopamine release and inhibits IM neurons, compromises prefrontal control of the amygdala, and sets off a general alarm system as seen in affective disorders, such as chronic anxiety and post-traumatic stress disorder.

Chemosensory processing in children with attention-deficit/hyperactivity disorder

BACKGROUND

In attention-deficit/hyperactivity disorder (ADHD) not only deficits in dopamine-related cognitive functioning have been found but also a lower dopamine-sensitive olfactory threshold. The aim of the present study was to proof that only olfactory but not trigeminal sensitivity is increased in ADHD. Structural magnetic resonance imaging (MRI) was used to show increased olfactory bulb (OB) volume- a structure which is strongly shaped by olfactory performance through the mechanism of neuroplasticity (e.g. synaptogenesis). To elucidate whether cortical mechanisms are involved in altered olfaction in ADHD, functional MRI (fMRI) was introduced.

METHODS

A total of 18 boys with ADHD and 17 healthy controls (aged 7-12) were included in the study. Olfactory as well as trigeminal detection thresholds were examined. OB sizes were measured by means of structural MRI and an analysis of effective functional (fMRI) coupling of primary olfactory cortex was conducted. The frontal piriform cortex (fPIR) was chosen as seed region because of its importance in processing both trigeminal and olfactory stimuli as well as having profound influence on inner OB-signaling.

RESULTS

Increased olfactory sensitivity as well as an increase in OB volume was found in ADHD. There were no group differences in sensitivity towards a trigeminal stimulus. Compared to healthy controls, the fPIR in ADHD was more positively coupled with structures belonging to the salience network during olfactory and, to a lesser extent, during trigeminal stimulation.

CONCLUSIONS

Olfactory functioning is superior in subjects with ADHD. The observed increase in OB volume may relate to higher olfactory sensitivity in terms of neuroplasticity. During the processing of chemosensory stimuli, the primary olfactory cortex in ADHD is differently coupled to higher cortical structures which might indicate an altered top-down influence on OB structure and function.

Area 4 has layer IV in adult primates

There are opposing views about the status of layer IV in the primary motor cortex (area 4). Cajal described a layer IV in area 4 of adult humans. In contrast, Brodmann found layer IV in developmental but not in adult primates and called area 4 'agranular'. We addressed this issue in rhesus monkeys using the neural marker SMI-32, which labels neurons in lower layer III and upper layer V, but not in layer IV. SMI-32 delineated a central unlabeled cortical stripe in area 4 that corresponds to layer IV, which was populated with small interneurons also found in layer IV in 'granular' areas (such as area 46). We distinguished layer IV interneurons from projection neurons in the layers above and below using cellular criteria. The commonly used term 'agranular' for area 4 is also used for the phylogenetically ancient limbic cortices, confusing areas that differ markedly in laminar structure. This issue pertains to the systematic variation in the architecture across cortices, traced from limbic cortices through areas with increasingly more elaborate laminar structure. The principle of systematic variation can be used to predict laminar patterns of connections across cortical systems. This principle places area 4 and agranular anterior cingulate cortices at opposite poles of the graded laminar differentiation of motor cortices. The status of layer IV in area 4 thus pertains to core organisational features of the cortex, its connections and evolution.