Odorants elicit evoked potentials in the human amygdala

From nasal respiration to brain dynamic

While breathing is a vital, involuntary physiological function, the mode of respiration, particularly nasal breathing, exerts a profound influence on brain activity and cognitive processes. This review synthesizes existing research on the interactions between nasal respiration and the entrainment of oscillations across brain regions involved in cognition. The rhythmic activation of olfactory sensory neurons during nasal respiration is linked to oscillations in widespread brain regions, including the prefrontal cortex, entorhinal cortex, hippocampus, amygdala, and parietal cortex, as well as the piriform cortex. The phase-locking of neural oscillations to the respiratory cycle, through nasal breathing, enhances brain inter-regional communication and is associated with cognitive abilities like memory. Understanding the nasal breathing impact on brain networks offers opportunities to explore novel methods for targeting the olfactory pathway as a means to enhance emotional and cognitive functions.

Amygdala and cortical gamma-band responses to emotional faces are modulated by attention to valence

The amygdala might support an attentional bias for emotional faces. However, whether and how selective attention toward a specific valence modulates this bias is not fully understood. Likewise, it is unclear whether amygdala and cortical signals respond to emotion and attention in a similar way. We recorded gamma-band activity (GBA, > 30 Hz) intracranially in the amygdalae of 11 patients with epilepsy and collected scalp recordings from 19 healthy participants. We presented angry, neutral, and happy faces randomly, and we denoted one valence as the target. Participants detected happy targets most quickly and accurately. In the amygdala, during attention to negative faces, low gamma-band activity (LGBA, < 90 Hz) increased for angry compared with happy faces from 160 ms. From 220 ms onward, amygdala high gamma-band activity (HGBA, > 90 Hz) was higher for angry and neutral faces than for happy ones. Monitoring neutral faces increased amygdala HGBA for emotions compared with neutral faces from 40 ms. Expressions were not differentiated in GBA while monitoring positive faces. On the scalp, only threat monitoring resulted in expression differentiation. Here, posterior LGBA was increased selectively for angry targets from 60 ms. The data show that GBA differentiation of emotional expressions is modulated by attention to valence: Top-down-controlled threat vigilance coordinates widespread GBA in favor of angry faces. Stimulus-driven emotion differentiation in amygdala GBA occurs during a neutral attentional focus. These findings align with a multi-pathway model of emotion processing and specify the role of GBA in this process.

What do brain oscillations tell about the human sense of smell?

Brain activity may manifest itself as oscillations which are repetitive rhythms of neuronal firing. These local field potentials can be measured via intracranial electroencephalography (iEEG). This review focuses on iEEG used to map human brain structures involved in olfaction. After presenting the methodology of the review, a summary of the brain structures involved in olfaction is given, followed by a review of the literature on human olfactory oscillations in different contexts. A single case is provided as an illustration of the olfactory oscillations. Overall, the timing and sequence of oscillations found in the different structures of the olfactory system seem to play an important role for olfactory perception.

What Does the Human Olfactory System Do, and How Does It Do It?

Historically, the human sense of smell has been regarded as the odd stepchild of the senses, especially compared to the sensory bravado of seeing, touching, and hearing. The idea that the human olfaction has little to contribute to our experience of the world is commonplace, though with the emergence of COVID-19 there has rather been a sea change in this understanding. An ever increasing body of work has convincingly highlighted the keen capabilities of the human nose and the sophistication of the human olfactory system. Here, we provide a concise overview of the neuroscience of human olfaction spanning the last 10-15 years, with focus on the peripheral and central mechanisms that underlie how odor information is processed, packaged, parceled, predicted, and perturbed to serve odor-guided behaviors. We conclude by offering some guideposts for harnessing the next decade of olfactory research in all its shapes and forms.

Smell-induced gamma oscillations in human olfactory cortex are required for accurate perception of odor identity

Studies of neuronal oscillations have contributed substantial insight into the mechanisms of visual, auditory, and somatosensory perception. However, progress in such research in the human olfactory system has lagged behind. As a result, the electrophysiological properties of the human olfactory system are poorly understood, and, in particular, whether stimulus-driven high-frequency oscillations play a role in odor processing is unknown. Here, we used direct intracranial recordings from human piriform cortex during an odor identification task to show that 3 key oscillatory rhythms are an integral part of the human olfactory cortical response to smell: Odor induces theta, beta, and gamma rhythms in human piriform cortex. We further show that these rhythms have distinct relationships with perceptual behavior. Odor-elicited gamma oscillations occur only during trials in which the odor is accurately perceived, and features of gamma oscillations predict odor identification accuracy, suggesting that they are critical for odor identity perception in humans. We also found that the amplitude of high-frequency oscillations is organized by the phase of low-frequency signals shortly following sniff onset, only when odor is present. Our findings reinforce previous work on theta oscillations, suggest that gamma oscillations in human piriform cortex are important for perception of odor identity, and constitute a robust identification of the characteristic electrophysiological response to smell in the human brain. Future work will determine whether the distinct oscillations we identified reflect distinct perceptual features of odor stimuli.

Intracranial recordings from human olfactory cortex reveal a characteristic spectrotemporal response to odors, including theta, beta and gamma oscillations, and show that high-frequency responses are critical for accurate perception of odors.

Human Primary Olfactory Amygdala Subregions Form Distinct Functional Networks, Suggesting Distinct Olfactory Functions

Three subregions of the amygdala receive monosynaptic projections from the olfactory bulb, making them part of the primary olfactory cortex. These primary olfactory areas are located at the anterior-medial aspect of the amygdala and include the medial amygdala (MeA), cortical amygdala (CoA), and the periamygdaloid complex (PAC). The vast majority of research on the amygdala has focused on the larger basolateral and basomedial subregions, which are known to be involved in implicit learning, threat responses, and emotion. Fewer studies have focused on the MeA, CoA, and PAC, with most conducted in rodents. Therefore, our understanding of the functions of these amygdala subregions is limited, particularly in humans. Here, we first conducted a review of existing literature on the MeA, CoA, and PAC. We then used resting-state fMRI and unbiased k-means clustering techniques to show that the anatomical boundaries of human MeA, CoA, and PAC accurately parcellate based on their whole-brain resting connectivity patterns alone, suggesting that their functional networks are distinct, relative both to each other and to the amygdala subregions that do not receive input from the olfactory bulb. Finally, considering that distinct functional networks are suggestive of distinct functions, we examined the whole-brain resting network of each subregion and speculated on potential roles that each region may play in olfactory processing. Based on these analyses, we speculate that the MeA could potentially be involved in the generation of rapid motor responses to olfactory stimuli (including fight/flight), particularly in approach/avoid contexts. The CoA could potentially be involved in olfactory-related reward processing, including learning and memory of approach/avoid responses. The PAC could potentially be involved in the multisensory integration of olfactory information with other sensory systems. These speculations can be used to form the basis of future studies aimed at clarifying the olfactory functions of these under-studied primary olfactory areas.

Nasal Respiration Entrains Human Limbic Oscillations and Modulates Cognitive Function

The need to breathe links the mammalian olfactory system inextricably to the respiratory rhythms that draw air through the nose. In rodents and other small animals, slow oscillations of local field potential activity are driven at the rate of breathing (∼2-12 Hz) in olfactory bulb and cortex, and faster oscillatory bursts are coupled to specific phases of the respiratory cycle. These dynamic rhythms are thought to regulate cortical excitability and coordinate network interactions, helping to shape olfactory coding, memory, and behavior. However, while respiratory oscillations are a ubiquitous hallmark of olfactory system function in animals, direct evidence for such patterns is lacking in humans. In this study, we acquired intracranial EEG data from rare patients (Ps) with medically refractory epilepsy, enabling us to test the hypothesis that cortical oscillatory activity would be entrained to the human respiratory cycle, albeit at the much slower rhythm of ∼0.16-0.33 Hz. Our results reveal that natural breathing synchronizes electrical activity in human piriform (olfactory) cortex, as well as in limbic-related brain areas, including amygdala and hippocampus. Notably, oscillatory power peaked during inspiration and dissipated when breathing was diverted from nose to mouth. Parallel behavioral experiments showed that breathing phase enhances fear discrimination and memory retrieval. Our findings provide a unique framework for understanding the pivotal role of nasal breathing in coordinating neuronal oscillations to support stimulus processing and behavior.

SIGNIFICANCE STATEMENT

Animal studies have long shown that olfactory oscillatory activity emerges in line with the natural rhythm of breathing, even in the absence of an odor stimulus. Whether the breathing cycle induces cortical oscillations in the human brain is poorly understood. In this study, we collected intracranial EEG data from rare patients with medically intractable epilepsy, and found evidence for respiratory entrainment of local field potential activity in human piriform cortex, amygdala, and hippocampus. These effects diminished when breathing was diverted to the mouth, highlighting the importance of nasal airflow for generating respiratory oscillations. Finally, behavioral data in healthy subjects suggest that breathing phase systematically influences cognitive tasks related to amygdala and hippocampal functions.

Theta Oscillations Rapidly Convey Odor-Specific Content in Human Piriform Cortex

Olfactory oscillations are pervasive throughout vertebrate and invertebrate nervous systems. Such observations have long implied that rhythmic activity patterns play a fundamental role in odor coding. Using intracranial EEG recordings from rare patients with medically resistant epilepsy, we find that theta oscillations are a distinct electrophysiological signature of olfactory processing in the human brain. Across seven patients, odor stimulation enhanced theta power in human piriform cortex, with robust effects at the level of single trials. Importantly, classification analysis revealed that piriform oscillatory activity conveys olfactory-specific information that can be decoded within 110-518 ms of a sniff, and maximally within the theta frequency band. This temporal window was also associated with increased theta-specific phase coupling between piriform cortex and hippocampus. Together these findings suggest that human piriform cortex has access to olfactory content in the time-frequency domain and can utilize these signals to rapidly differentiate odor stimuli.

Olfaction: anatomy, physiology, and disease

The olfactory system is an essential part of human physiology, with a rich evolutionary history. Although humans are less dependent on chemosensory input than are other mammals (Niimura 2009, Hum. Genomics 4:107-118), olfactory function still plays a critical role in health and behavior. The detection of hazards in the environment, generating feelings of pleasure, promoting adequate nutrition, influencing sexuality, and maintenance of mood are described roles of the olfactory system, while other novel functions are being elucidated. A growing body of evidence has implicated a role for olfaction in such diverse physiologic processes as kin recognition and mating (Jacob et al. 2002a, Nat. Genet. 30:175-179; Horth 2007, Genomics 90:159-175; Havlicek and Roberts 2009, Psychoneuroendocrinology 34:497-512), pheromone detection (Jacob et al. 200b, Horm. Behav. 42:274-283; Wyart et al. 2007, J. Neurosci. 27:1261-1265), mother-infant bonding (Doucet et al. 2009, PLoS One 4:e7579), food preferences (Mennella et al. 2001, Pediatrics 107:E88), central nervous system physiology (Welge-Lüssen 2009, B-ENT 5:129-132), and even longevity (Murphy 2009, JAMA 288:2307-2312). The olfactory system, although phylogenetically ancient, has historically received less attention than other special senses, perhaps due to challenges related to its study in humans. In this article, we review the anatomic pathways of olfaction, from peripheral nasal airflow leading to odorant detection, to epithelial recognition of these odorants and related signal transduction, and finally to central processing. Olfactory dysfunction, which can be defined as conductive, sensorineural, or central (typically related to neurodegenerative disorders), is a clinically significant problem, with a high burden on quality of life that is likely to grow in prevalence due to demographic shifts and increased environmental exposures.

Effect of aging on hedonic appreciation of pleasant and unpleasant odors

Does hedonic appreciation evolve differently for pleasant odors and unpleasant odors during normal aging? To answer this question we combined psychophysics and electro-encephalographic recordings in young and old adults. A first study showed that pleasant odorants (but not unpleasant ones) were rated as less pleasant by old adults. A second study validated this decrease in hedonic appreciation for agreeable odors and further showed that smelling these odorants decreased beta event-related synchronization in aged participants. In conclusion, the study offers new insights into the evolution of odor hedonic perception during normal aging, highlighting for the first time a change in processing pleasant odors.

Fear and happiness in the eyes: an intra-cerebral event-related potential study from the human amygdala

We present the response pattern of intracranial event-related potentials (ERPs) recorded from depth-electrodes in the human amygdala (four patients) to faces or face parts encoding fearful, happy or neutral expressions. The amygdala showed increased amplitude ERPs (from 200 to 400 ms post-stimulus) in response to the eye region of the face compared to whole faces and to the mouth region. In particular, a strong emotional valence effect was observed, both at group and at single-subject level, with a preferential response to fearful eyes respect to every other stimulus category from 200 to 400 ms after stimulus presentation. A preferential response to smiling eyes compared to happy faces and smiling mouths was also observed at group level from 300 to 400 ms post-stimulus presentation. A complementary time-frequency analysis was performed showing that an increase in the theta frequency band (4-7 Hz) accounted for the main event-related band power (ERBP) change during the 200-500 ms post stimulus interval. The analysis of the ERBPs changes according to their emotional valence showed a strong increase in theta ERBP to fearful eyes, which was higher respect to any other facial stimulus. Moreover, theta ERBP increase to "smiling eyes" was larger respect with that evoked by smiling mouths and whole happy faces. Minimal post-stimulus ERBPs changes were evoked by neutral stimuli. These data are consistent with a special role of the amygdala in processing facial signals, both with negative and positive valence, conveyed by the eye region of the face.

The role of olfactory stimulus in adult mammalian neurogenesis

Neurogenesis occurs in the adult mammalian brain in discrete regions related to olfactory sensory signaling and integration. The olfactory receptor cell population is in constant turn-over through local progenitor cells. Also, newborn neurons are added to the olfactory bulbs through a major migratory route from the subventricular zone, the rostral migratory stream. The olfactory bulbs project to different brain structures, including: piriform cortex, amygdala, entorhinal cortex, striatum and hippocampus. These structures play important roles in odor identification, feeding behavior, social interactions, reproductive behavior, behavioral reinforcement, emotional responses, learning and memory. In all of these regions neurogenesis has been described in normal and in manipulated mammalian brain. These data are reviewed in the context of a sensory-behavioral hypothesis on adult neurogenesis that olfactory input modulates neurogenesis in many different regions of the brain.

Changes in olfactory function in pregnancy and postpartum

OBJECTIVE

To track changes in olfactory performance during pregnancy and the postpartum.

METHODS

In a prospective study the olfactory function of 38 pregnant women was assessed at about 12, 21, and 36 weeks of pregnancy and 7 weeks after delivery. A control group of 46 nonpregnant women were also asked to rate the intensity and hedonic tone of 10 "natural" odors.

RESULTS

There was no difference in olfactory performance between the women in the first trimester of pregnancy and the controls, but at approximately 36 weeks of pregnancy the pregnant women experienced a decreased odor threshold compared with the nonpregnant controls, and this decrease was still present after delivery. There was no significant difference between the groups in capacity for odor discrimination or odor identification. "Objective" decreases were observed even though pregnant women rated their olfactory sensitivity significantly higher than the controls. Although the 2 groups did not differ with regard to intensity ratings of the 10 "natural" odors, there were differences with regard to the hedonic ratings.

CONCLUSION

Pregnancy is accompanied by changes in olfactory performance. Changes in hedonic odor ratings indicate a potential embryo-protective mechanism. The discrepancy between "objective" and "subjective" olfactory function may relate to changes in the cognitive processing of chemosensory information during pregnancy.

Evaluative conditioning is intact after unilateral resection of the anterior temporal lobe in humans

Several lesion and functional imaging studies conducted in animals and humans suggest that structures within the amygdaloid nuclear complex (ANC) are important for the occurrence of fear conditioning. Whether this brain structure is also critical for evaluative conditioning, has been investigated less frequently. In the current experiment, a group of participants with unilateral resection of the anterior temporal lobe and a control group received a differential evaluative flavor-taste conditioning task. In the pre-acquisition phase, two fruit flavors (the conditioned stimuli (CSs)) were presented and participants were instructed to evaluate both. In the subsequent acquisition phase, one of these fruit flavors (CS+) was presented together with a bad tasting substance Tween20 (polysorbate 20, the US), while the other flavor (CS-) was never paired with Tween20. Finally, in the post-acquisition phase, the two flavors were presented again without Tween20 and participants were asked to evaluate both of them for a last time. The control group as well as the lesion group rated the CS+ in the post-acquisition phase less favorable than in the pre-acquisition phase, while the ratings of the CS- remained the same in both phases. We clearly demonstrated evaluative conditioning in both test groups. Because the lesion group had still one intact ANC it would be premature, however, to conclude that the ANC is not involved in evaluative conditioning. We conclude that despite evidence for impaired fear conditioning, unilateral damage to the ANC does not impair evaluative conditioning.

Inspiratory phase-locked alpha oscillation in human olfaction: source generators estimated by a dipole tracing method

Olfactory perception and related emotions are largely dependent on inspiration. We acquired simultaneous respiration and electroencephalographic recordings during pleasant odour and unpleasant odour stimulation. We sought to identify changes in respiratory pattern, inspiratory-related potentials and location of dipoles estimated from the potentials. Electroencephalographic recording was triggered by inspiration onset. Respiratory frequency decreased at pleasant odour recognition, and it increased at unpleasant odour detection and recognition. O2 consumption records showed that these changes were not due to metabolic demand. During olfactory stimulation, inspiratory phase-locked alpha oscillation (I-alpha) was found in the averaged potential triggered by inspiration onset. I-alpha was observed at both pleasant odour and unpleasant odour detection and recognition, but it was not seen in the inspiration-triggered potentials of normal air breathing. Electroencephalographic dipole tracing identified the location of dipoles from the I-alpha in the limbic area and the cortex; the entorhinal cortex, hippocampus, amygdala, premotor area and centroposterior orbitofrontal cortex subserve odour detection, and the rostromedial orbitofrontal cortex subserves odour recognition. We suggest that the I-alpha in our study originated from the olfactory cortex in the forebrain and was phase-locked to inspiration.

A stimulation method using odors suitable for PET and fMRI studies with recording of physiological and behavioral signals

A design for a semi-automatic olfactometric system is described for PET and fMRI experiments. The olfactometer presents several advantages because it enables the use of an 'infinite' number of odorants and the synchronization of stimuli with breathing. These advantages mean that the subject is recorded while breathing normally during olfactory judgment tasks. In addition, the design includes a system for recording the behavioral (rating scale) and physiological (breathing, electrodermal reaction (ED), plethysmography (PL)) signals given by the subject. Both systems present the advantage of being compatible with fMRI magnetic fields since no ferrous material is used in the Faraday cage and signals are transmitted via an optical transmission interface to an acquisition system.

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.

Intracranial EEG and human brain mapping

This review is an attempt to highlight the value of human intracranial recordings (intracranial electro-encephalography, iEEG) for human brain mapping, based on their technical characteristics and based on the corpus of results they have already yielded. The advantages and limitations of iEEG recordings are introduced in detail, with an estimation of their spatial and temporal resolution for both monopolar and bipolar recordings. The contribution of iEEG studies to the general field of human brain mapping is discussed through a review of the effects observed in the iEEG while patients perform cognitive tasks. Those effects range from the generation of well-localized evoked potentials to the formation of large-scale interactions between distributed brain structures, via long-range synchrony in particular. A framework is introduced to organize those iEEG studies according to the level of complexity of the spatio-temporal patterns of neural activity found to correlate with cognition. This review emphasizes the value of iEEG for the study of large-scale interactions, and describes in detail the few studies that have already addressed this point.

Both of us disgusted in My insula: the common neural basis of seeing and feeling disgust

What neural mechanism underlies the capacity to understand the emotions of others? Does this mechanism involve brain areas normally involved in experiencing the same emotion? We performed an fMRI study in which participants inhaled odorants producing a strong feeling of disgust. The same participants observed video clips showing the emotional facial expression of disgust. Observing such faces and feeling disgust activated the same sites in the anterior insula and to a lesser extent in the anterior cingulate cortex. Thus, as observing hand actions activates the observer's motor representation of that action, observing an emotion activates the neural representation of that emotion. This finding provides a unifying mechanism for understanding the behaviors of others.

A specific role for the human amygdala in olfactory memory

The medial temporal lobe is known to play a role in the processing of olfaction and memory. The specific contribution of the human amygdala to memory for odors has not been addressed, however. The role of this region in memory for odors was assessed in patients with unilateral amygdala damage due to temporal lobectomy (n = 20; 11 left, 9 right), one patient with selective bilateral amygdala damage, and in 20 age-matched normal controls. Fifteen odors were presented, followed 1 h later by an odor-name matching test and an odor-odor recognition test. Signal detection analyses showed that both unilateral groups were impaired in their memory for matching odors with names, these patients were not significantly impaired on odor-odor recognition. Bilateral amygdala damage resulted in severe impairment in both odor-name matching as well as in odor-odor recognition memory. Importantly, none of the patients were impaired on an auditory verbal learning task, suggesting that these findings reflect a specific impairment in olfactory memory, and not merely a more general memory deficit. Taken together, the data provide neuropsychological evidence that the human amygdala is essential for olfactory memory.

Convergent and divergent effects of odors and emotions in depression

The aim of the present study was to investigate the similarities and differences in the olfactory and visual processing of emotional stimuli in healthy subjects and in patients with major depressive disorder (MDD). Twenty-five inpatients were investigated after admission to the psychiatric clinic. Fifteen of them participated a second time, shortly before their discharge from the hospital. A group of healthy subjects, matched according to age and sex, served as a control. Chemsosensory event-related potentials (CSERPs) were recorded using the constant flow method. In addition, event-related potentials (ERPs), in response to colors and emotional slides, were obtained to control modality and emotion-specific effects. The subjects' task was to discriminate the colors (red/yellow) and odors (phenyl-ethylalcohol = rose/ isobutyraldehyde = rotten butter) according to their quality and to judge the valence of the emotional slides (IAPS slides). The EEG was recorded from 32 scalp locations. At the beginning of the therapy, visual stimulus processing was attenuated in depressive subjects at a relatively late processing level (reduced amplitudes of the P3 and pSW in response to colors and emotional slides), whereas olfactory stimulus processing had already been affected at an early level (reduced amplitudes of the P2 and P3-1 peaks in MDD patients). However, after successful medical treatment, ERPs did not differentiate between depressive patients and healthy controls. We discuss whether functional deviations within the primary olfactory cortex are responsible for the lower olfactory sensitivity, as well as for the altered emotional stimulus processing in MDD patients.

Functional heterogeneity in human olfactory cortex: an event-related functional magnetic resonance imaging study

Studies of patients with focal brain injury indicate that smell perception involves caudal orbitofrontal and medial temporal cortices, but a more precise functional organization has not been characterized. In addition, although it is believed that odors are potent triggers of emotion, support for an anatomical association is scant. We sought to define the neural substrates of human olfactory information processing and determine how these are modulated by affective properties of odors. We used event-related functional magnetic resonance imaging (fMRI) in an olfactory version of a classical conditioning paradigm, whereby neutral faces were paired with pleasant, neutral, or unpleasant odors, under 50% reinforcement. By comparing paired (odor/face) and unpaired (face only) conditions, odor-evoked neural activations could be isolated specifically. In primary olfactory (piriform) cortex, spatially and temporally dissociable responses were identified along a rostrocaudal axis. A nonhabituating response in posterior piriform cortex was tuned to all odors, whereas activity in anterior piriform cortex reflected sensitivity to odor affect. Bilateral amygdala activation was elicited by all odors, regardless of valence. In posterior orbitofrontal cortex, neural responses evoked by pleasant and unpleasant odors were segregated within medial and lateral segments, respectively. The results indicate functional heterogeneity in areas critical to human olfaction. They also show that brain regions mediating emotional processing are differentially activated by odor valence, providing evidence for a close anatomical coupling between olfactory and emotional processes.

Neural systems for recognizing emotion

Recognition of emotion draws on a distributed set of structures that include the occipitotemporal neocortex, amygdala, orbitofrontal cortex and right frontoparietal cortices. Recognition of fear may draw especially on the amygdala and the detection of disgust may rely on the insula and basal ganglia. Two important mechanisms for recognition of emotions are the construction of a simulation of the observed emotion in the perceiver, and the modulation of sensory cortices via top-down influences.