The amygdala-centered network
The amygdala-centered network constitutes the neurobiological substrate for the integration of sensory input and emotional arousal to decode the significance of the stimulus for the organism, and includes the amygdala, areas of the olfactory, orbitofrontal, insular, anterior and midcingulate cortex as well as the ventral striatopallidum (Amaral et al.
1992; Catani et al.
2013; Geschwind
1965; Mesulam
2000).
The role of the amygdala is to assess a sensory stimulus on the basis of its intrinsic hedonic properties and possible association with other previously acquired primary reinforcers, as well as based on the organism’s current motivational state to determine its valence and modulate its neural impact on the organism to induce an adequate emotional state (Mesulam
2000). Fear is probably the emotion category most often associated with the amygdala, and was first described in the seminal work by LeDoux (
1994). Although the amygdala appears to play a more extensive role in negatively valenced emotions, it is also significantly involved in the processing of positively valenced stimuli (Cunningham and Kirkland
2014; Hamann et al.
2002; Wang et al.
2017), and depressed patients show higher amygdala responses to negative stimuli and lower amygdala responses to positive stimuli than do healthy controls (Groenewold et al.
2013). A meta-analytic functional connectivity-based parcellation of the amygdala revealed three clusters comparable in shape and relative position with the cytoarchitectonically identified laterobasal, centromedial, and superficial nuclei groups (Amunts et al.
2005; Bzdok et al.
2013). Functional profiling of the three clusters showed the “laterobasal cluster” to be associated with coordinating high-level sensory input, the “centromedial cluster” to mediate attentional, vegetative, and motor responses, and the “superficial cluster” to be involved in the processing of olfactory stimuli (Bzdok et al.
2013).
The amygdala is able to integrate and process multimodal information, a primordial requisite for the modulation of higher cognitive functions such as emotional behavior, and this integrative function is subserved by its connectivity with numerous cortical and subcortical structures belonging to multiple functional systems. Furthermore, individual neurons in the amygdala not only respond to all types of unimodal sensory or viscerosensory stimuli, but also to multimodal sensory stimuli, to reward or punishment-related reinforcers, and to stimuli with a cognitive significance (Yilmazer-Hanke
2012). The primate amygdala is connected with primary/higher order unimodal areas belonging to all sensory systems, with multimodal areas of the orbitofrontal, anterior cingulate, insular, and temporal cortex, including the hippocampal complex (Aggleton et al.
1980,
2015; Aggleton and Saunders
2000; Amaral
1986; Amaral et al.
1992; Carmichael and Price
1995; Freese and Amaral
2005; Price
2003; Young et al.
1994). It is also connected with numerous subcortical structures, including the basal forebrain, thalamus, hypothalamus, periaqueductal central gray, and the peripeduncular nucleus (Aggleton et al.
1980; Price
2003). These connections are mostly reciprocal, and connectivity between the amygdala and unimodal sensory regions is organized in such a way that efferents arise from the higher order sensory areas, whereas the amygdala targets the primary or secondary sensory areas (Amaral et al.
1992; Turner et al.
1980). Visual input arises specifically from areas of the ventral visual stream, and gustatory and somatosensory information reaches the amygdala through a relay in the insula (Aggleton
1993; Mesulam and Mufson
1985).
The orbitofrontal cortex is involved in the integration of value-related olfactory and gustatory information with viscerosensory information (processed in the anterior insula), and in the transfer of this information to the pACC (Rolls
2019). The lateral orbitofrontal cortex showed a stronger functional connectivity with the gyral components of pACC area p24 (i.e., areas p24a and p24b), whereas medial orbitofrontal areas are more tightly associated with p24c (i.e., the sulcal component of area p24) and with p32 (Palomero-Gallagher et al.
2019). The orbitofrontal cortex, together with subgenual cingulate area 25 (a key node of the cortical autonomic network; Gianaros et al.
2005; Kimmerly et al.
2005; Wong et al.
2007), also modulates autonomic and visceral functions in response to the valence of the stimulus, and does so via connections with the anterior insula, periaqueductal gray and hypothalamus (Critchley and Harrison
2013; Öngür and Price
2000; Palomero-Gallagher et al.
2015; Rempel-Clower and Barbas
1998).
The insula plays a major role in functional integration, and is thought to constitute a correlate of consciousness (Craig
2009). It is a structurally and functionally segregated brain region involved in olfactory, gustatory, sensorimotor and cognitive processes, including emotion processing (Kurth et al.
2010; Mesulam and Mufson
1985). Interestingly, a meta-analysis of functional imaging studies revealed an overlap of activations related to the olfacto-gustatory, emotional and cognitive domains in the anterior-dorsal insula, which thus constitutes a key region in the human brain for the integration of olfaction, emotion and memory (Kurth et al.
2010). Furthermore, activation levels in the anterior insular cortex serve as correlates of the intensity of the experienced emotion, regardless of its valence (Zhu et al.
2019).
The amygdala targets the subgenual and pregenual parts of the anterior cingulate cortex (sACC and pACC, respectively) via the uncinate fasciculus, and cingulate regions are interconnected via the cingulate bundle (Dejerine
1895). The two ACC regions and the anterior midcingulate cortex (aMCC) monitor sensory stimuli, whereby ACC areas monitor emotional stimuli with respect to their pleasantness or unpleasantness (Oane et al.
2020; Palomero-Gallagher et al.
2015,
2019; Vogt and Miller
1983), and areas of the aMCC region play a crucial role in both the perception and anticipation of pain (Porro and Lui
2009; Vogt et al.
1996; Vogt and Sikes
2009b).
It is widely accepted that areas of the sACC subserve the processing of negatively valenced stimuli (Etkin et al.
2011; George et al.
1995; Karama et al.
2011; Liotti et al.
2000; Mechias et al.
2010; Smith et al.
2011). The processing of sadness and fear activates cytoarchitectonic areas s24 and s32, respectively (Palomero-Gallagher et al.
2015). Interestingly, pACC area p32 is associated with the domains of anxiety and fear, though these activations were elicited by tasks requiring the induction of emotions and theory of mind processes, and not by the experience of the emotion itself (Palomero-Gallagher et al.
2015). This association of area p32 with the subject’s ability to experience empathy highlights the unique position of the cingulate cortex as a link between the emotional and memory domains, thus enabling cognitive influences on emotion (Palomero-Gallagher et al.
2015; Stevens et al.
2011). Notably, although some studies studying the neural substrate for the subjective feeling of happiness reported activations within ACC (e.g., Habel et al.
2005; Phillips et al.
1998), no meta-analytic approaches have been able to identify a significant association between the pACC (or any of its areas) and the processing of positively valenced emotions (Kirby and Robinson
2017; Palomero-Gallagher et al.
2015; Phan et al.
2002; Torta and Cauda
2011; Vytal and Hamann
2010).
pACC receives gustatory and viscerosensory input from the orbitofrontal cortex, and is able to integrate visceral sensations via its reciprocal connections with the insula (Qadir et al.
2018; Taylor et al.
2009), and a recent cytoarchitectonically informed meta-analysis found the gyral components of pACC area p24 to be significantly associated with the behavioral domains of gustation and interoception (Palomero-Gallagher et al.
2019). These areas also co-activate with areas of the affective network (George et al.
1995; Lévesque et al.
2003), highlighting the importance of reward value in the generation of emotions (Glascher et al.
2012; Grabenhorst and Rolls
2011). Area p32 of pACC, and also area s24 of sACC, are involved in estimating the emotional valence of faces via visual input arising from areas of the ventral stream (Palomero-Gallagher et al.
2015,
2019). The pACC is also involved in conflict monitoring, and the sulcal component of area p24 is associated with action inhibition, and co-activates with components of the salience network (Palomero-Gallagher et al.
2019). Thus, the pACC integrates information from the dorsolateral prefrontal cortex concerning the selection and maintenance of options to current or proposed behaviors to provide the motivation to carry out selected behavior (Holroyd and Yeung
2012). Furthermore, it was shown that face-evoked responses in the anterior insula and anterior cingulate cortex contain information which is shaped by social interaction, and it was hypothesized that this provides a substrate of how social inclusion shapes future behavior and interaction, while the recognition of individual faces is supported by the visual cortex (Eger et al.
2013).
As part of both the amygdala- and the hippocampus-centered network, the ACC region is also able, either via its direct reciprocal connections with the rostral hippocampus, or in a relay through the thalamus, to modulate the consolidation and retrieval of memory (Aggleton
2012; Navawongse and Eichenbaum
2013; Xu and Sudhof
2013). Given that memories of emotionally valenced stimuli are easier to recollect than those of neutral ones, the ACC is thought to facilitate retrieval of related and competing memories by creating contextual representations of these experiences during the consolidation phase (Bian et al.
2019).
The aMCC receives input from ACC regions and also via the medial pain system and is thus in an ideal position to modulate avoidance behavior in response to noxius stimuli (Vogt
2005), whereby activations were found to be proportional to the degree of pain experienced (Derbyshire et al.
1998; Vogt et al.
1996). The aMCC is also activated during the processing of negatively valenced stimuli, and involved in the expression of fear responses (Pereira et al.
2010; Vogt et al.
2003). The MCC region projects to the supplementary areas, and the sulcal component of aMCC also contains a cingulate motor area (Morecraft and Tanji
2009; Vogt and Sikes
2009b), which projects directly to the facial motor nucleus and to portions of the spinal cord that control finger and hand movements. Thus, a brain network subserving emotion is able to directly generate and modulate facial, limb, or vocal reactions in response to a perceived stimulus. Furthermore, aMCC is thought to coordinate skeletomotor reflex responses in fear avoidance strategies (Vogt et al.
2003).
The ventral striatopallidum encompasses the ventral portions of the caudate nucleus, putamen and globus pallidus, as well as the nucleus accumbens and the olfactory tubercle (Mesulam
2000). It receives direct input from the amygdala, but is also connected with the orbitofrontal cortex and the ACC, and is a central component of the reward circuit, and in the generation of emotional motor activity (Nieuwenhuys et al.
2008).
The hippocampus-centered network
The hippocampus-centered network mediates the integration of information processed by multiple large-scale brain networks involved in the different memory types to incorporate cognition into emotion processing. It includes the hippocampal complex, entorhinal and retrosplenial cortex (RSC), areas of the anterior (discussed above) and posterior cingulate cortex, as well as the thalamus (Mesulam
2000).
The hippocampal formation is a key structure in the consolidation and retrieval of declarative, spatial and emotional memory (Bird and Burgess
2008; Fanselow and Dong
2010; Strange et al.
2014), and the entorhinal cortex represents the nodal point in neocortico-hippocampal circuits (Insausti and Amaral
2008). The hippocampal formation consists of the hippocampus proper, with the Cornu Ammonis regions CA1–CA4 and the fascia dentata, and the subicular complex, with the prosubiculum, subiculum, presubiculum, and parasubiculum (Palomero-Gallagher et al.
2020). The hippocampus is situated at the top of a highly complex interconnected and hierarchically organized network participating in memory functions (for a comprehensive review see Aggleton
2012), and its reciprocal connections with the amygdala are of particular importance for affective and social learning (Insausti and Amaral
2012; Yilmazer-Hanke
2012).
The dorso-ventral axis of the rodent hippocampus, which is homolog to a posterior-to-anterior axis in primates, is structurally and functionally segregated (Fanselow and Dong
2010; Strange et al.
2014). The dorsal hippocampus is more densely connected with the RSC, mammillary bodies, and anterior thalamus, and is mainly involved in cognitive functions such as navigation and exploration (Fanselow and Dong
2010; Jones and Witter
2007; Moser et al.
1993; Risold and Swanson
1997; Strange et al.
2014; Witter
1993). The ventral hippocampus is more strongly connected to the amygdala, nucleus accumbens and hypothalamus, and is involved in motivated behavior and autonomic responses (Canteras and Swanson
1992; Fanselow and Dong
2010; Groenewegen et al.
1987; Henke
1990; Strange et al.
2014; van Groen and Wyss
1990). The primate hippocampus presents a comparable heterogeneity in structural connectivity, with a rostro-caudal decrease in connectivity with the amygdala, nucleus accumbens and prefrontal cortex, and a rostro-caudal increase in connectivity with the posterior cingulate cortex (PCC) and RSC (Aggleton
2012; Friedman et al.
2002; Fudge et al.
2012; Kobayashi and Amaral
2003,
2007).
In humans, the posterior hippocampus is activated during declarative and spatial memory tasks (Greicius et al.
2003; Maguire et al.
1997). Resting state functional connectivity analyses found the posterior hippocampus to be more highly connected to the RSC and lateral parietal cortex, i.e., areas involved in visuospatial cognition, whereas the anterior hippocampus was more strongly connected to the temporal, orbitofrontal and anterior cingulate cortex, i.e., areas associated with motivational behavior (Adnan et al.
2016; Vogel et al.
2020). There is also evidence of anatomical connectivity between the anterior hippocampus and the fusiform gyrus (Duvernoy
2005), a part of the visual system particularly involved in the identification of faces (Kanwisher and Yovel
2006), words (Cohen and Dehaene
2004) and places (Epstein et al.
1999; Epstein
2008), and single neurons in the human hippocampus have not only been found to respond differentially to faces and objects, but also to respond preferentially to specific emotional expressions (Fried et al.
1997). Interestingly, genes expressed in the posterior hippocampus correlate with cortical regions involved in memory processes, whereas gene expression in the anterior hippocampus correlates with regions involved in emotion (Vogel et al.
2020).
The hippocampal complex and entorhinal cortex are interconnected with the RSC and with PCC area 23, though connections are much denser with the former than with the latter region (Kobayashi and Amaral
2003). RSC is also densely interconnected with areas 24 and 23 of the ACC and PCC, respectively (Kobayashi and Amaral
2003,
2007). The RSC is reciprocally connected to dorsolateral prefrontal areas 9 and 46, and thus constitutes a link between the hippocampus and brain regions involved in executive functions (Kobayashi and Amaral
2003,
2007). It receives early visual input from areas v2 and v4 of the ventral stream and is also interconnected with inferior parietal area 7a (Kobayashi and Amaral
2003,
2007), which mediates visuomotor coordination (Rozzi et al.
2008). Reciprocal connections between the anterior thalamic nucleus and both the hippocampus and RSC facilitate the integration of visual and body-based orientation cues (Miller et al.
2014; Shine et al.
2016), the episodic retrieval of familiar places and objects (Sugiura et al.
2005), and provide an anatomical substrate for fear conditioning processes whereby the RSC is critically involved in tasks during which subjects must form appropriate associations among diverse cues and outcomes to perform optimally (Corcoran et al.
2016; Keene and Bucci
2008a,
b). Thus, the RSC is in a position to modulate both the storage and retrieval of spatial and contextual information, in particular that related to fear.
The PCC is primarily involved in visuospatial, sensorimotor and long-term memory functions, and in the framework of emotion processing, plays a role in the assessment of the self-relevance of emotional events and stimuli (Vogt and Laureys
2009a). The PCC has reciprocal connections with sACC (Vogt and Pandya
1987), and is also targeted by the hippocampal complex and the RSC (Kobayashi and Amaral
2003,
2007). Furthermore, the PCC receives input from auditory association areas and has extensive connections with the inferior parietal cortex (Vogt and Pandya
1987) through which it receives input from areas belonging to the dorsal visual stream and involved in movement and spatial orientation (Kravitz et al.
2011; Ungerleider and Mishkin
1982). The convergence of visual and auditory stimuli together with information coded for valence in ACC enable the self-referential processing of stimuli and experiences.