Basic Structures and Functions
Neurons and glia form structures within the brain, ranging from smaller networks known as nuclei to more complex systems described as circuits, regions, and hemispheres. More broadly, neuroanatomists distinguish three global areas of the brain, described as the lower region of the brainstem, which regulates fundamental physiological functions; the middle region of the limbic system, which mediates our experience of emotion and memory; and the upper region of the cerebral cortex, which controls perception, attention, thinking, and reasoning; these areas correspond to the “bottom,” “middle,” and “top” levels of the brain, following the hierarchical model of neural organization proposed by John Hughlings Jackson, the British neurologist, at the end of the 19th century'.
Although the nuclei, circuits, regions, and hemispheres serve different types of processing functions, generating distinct forms of knowledge and understanding, they are closely linked and form a functional system. The brain is highly interconnected, and specialized areas interact with hundreds of other regions. Charles Sherrington, one of the founders of neurophysiology, a colleague and lifelong friend of Cajal, imagined the awakening brain as “an enchanted loom where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern though never an abiding one; a shifting harmony of subpatterns” (1943, p. 178). Oliver Sacks offered a musical analogy', comparing the ongoing integration of regions to “something like a vastly complicated orchestra with thousands of instruments, an orchestra that conducts itself, with an ever-changing score and repertory'” (2010, p. 104). Shortly after the turn of the century' researchers introduced the term “connectome” to represent the structural and functional interconnections believed to shape global states of brain function; neuroscientists describe the oscillation of neural firing in waves of energy' patterns that link different regions into a functional whole as “connectome harmonics” (see Siegel, 2020, for expanded accounts of neural integration and the connectome).
The Lower Region
The brain stem, located at the bottom of the brain, regulates fundamental physiological processes, including temperature, respiration, reflexes, heart rate, and blood pressure. From an evolutionary' perspective it is the oldest structure of the brain, often called the reptilian brain, and we can think of it as the neural substrate of the somatic realm of experience. Hughlings Jackson characterized this region as the physiological bottom of the mind (1931).
The autonomic nervous system governs states of alertness and arousal and the dynamics of the fight, flight, or freeze response. High levels of arousal activate the sympathetic network, releasing adrenalin, preparing us for fight or flight. The parasympathetic network activates the release of acetycholine, lowering lowers levels of arousal, promoting self-preservative functions. Drawing on the work of Charles Darwin, Steven Porges has explored the way's in which the double-branched cranial nerve known as the vagus, connecting the brain with the heart, lungs, stomach, and intestines, governs our reactions to threat (2011). When we register danger, we instinctively attempt to engage others, searching for help, support, and reassurance. If others fail to respond the “vagal brake” is released, mobilizing the sympathetic branch, preparing us for fight or flight. If the threat is acute, the more primitive dorsal vagal system is activated, leading to the immobilization of “freeze” or “collapse” reactions. We can think of the dissociative states that follow trauma as the psychological complement to this physical state. As we restore a sense of safety, the myelinated ventral vagus activates the “vagal brake” that slows the sympathetic nervous system, calming the body, restoring equilibrium (see Van der Kolk, 2014, for expanded account of polyvagal theory and trauma).
The lower portion of the brain contains the hypothalamus and the pituitary’, which regulate physiological homeostasis through neuronal firing and hormonal release. Researchers continue to explore the complex interactions between the lower brain functions and other systems of the body, advancing our understanding of the ways in which the autonomic nervous system and the neuroimmune system mediate states of self, attachment and relational life, reactions to stress and trauma, and illness experience (see Cacioppo, Cacioppo, Capitanio & Cole, 2015; McEwen, 2008; Sapolsky' 2017; Schore, 2019a, 2019b; Siegel, 2020; Sternberg, 2001, 2009).
The body-sensing regions of the lower structures of the brain create neural representations of somatic states thought to influence feelings, thoughts, and behaviors. Antonio Damasio has explored the relationships between the body, mind, and emotion in his pioneering studies of homeostasis, subjectivity', and the experience of the self, deepening our understanding of the role of subcortical and cortical structures in mapping somatic states in the brain (Damasio, 1994, 1999, 2010, 2018; Damasio & Caravalho, 2013).
Drawing on the work of William James, he proposes that the core of self-awareness is closely linked to physical sensations originating in the brain stem that register our experience of the body' through signals from organs, tissues, muscles, and the joints of the skeleton. Body and mind are inseparable. We experience our sense of aliveness through “somatic markers” and primordial feelings—“wordless, unadorned, and connected to nothing but sheer existence” (Damasio, 2010, p. 21). The making of the mind and the creation of feeling is mediated by' the interaction of the nervous system and the rest of the body (see Damasio, 2018, for an expanded account of homeostasis, mind, subjectivity', and culture; see parallels with the depth psychology' of C. G. Jung in Chapter 4).
The Middle Region
The limbic structures of the brain, encompassing the amygdala and the hippocampus, lie between the brain stem and the cerebral cortex. The neural networks of this area regulate sources of arousal in the brain stem and higher-level processing in the cerebral cortex, integrating basic mental processes,
Orienting Perspectives in Neuroscience 41 playing a major role in our experience of perception and attention, the activation of emotion, and the appraisal of meaning, learning, and memory. This region evolved with the emergence of mammals, and it is instrumental in the establishment of the attachment bond that facilitates caretaking and relational life; it is thought to underlie our sense of a “unitary' self” (Markowitz & Stanilou, 2011).
The connectivity of the amygdala system allows it to link distant regions throughout the brain, influencing global states of functioning. Under conditions of threat, the amygdala signals the brainstem to activate the sympathetic nervous system that mediates the fight or flight reaction. Although accounts of the brain often describe the amygdala as the “fear center,” it is responsive to positive experience as well, generating a range of emotional reactions (see Todd & Anderson, 2009, for expanded account of the adaptive functions of the amygdala system). As we will see shortly, the amygdala matures before the hippocampus, and it is instrumental in the formation of implicit memory', registering early experience as unconscious, pre-symbolic emotional memories that exert a global and disproportionate influence on the dynamics of sensation, emotion, cognition, and behavior (LeDoux, 2015; Sapolsky, 2017). The hippocampus begins to mediate the organization of explicit memory' and conscious processing of life events in the second year of life, allowing us to place our experience in context and time as we develop capacities for autobiographical memory.
The Upper Region
The cerebral cortex, the “top” part of the brain, is the last structure to develop and it continues to mature across the course of life through ongoing experience, activity', and learning. From an evolutionary' perspective it is the most recent structure, sometimes called the neo-mammalian brain. This region guides our efforts to process information and carry' out the activities of everyday' life, governing the executive functions of the brain, mediating capacities for perception, self-awareness, regulation of emotion, focused attention, thought, reasoning, judgment, decision-making, problem-solving, planning, and strategic action (for an expanded review of the structures and functions of the upper region see Cozolino, 2017; Sapolsky, 2017; and Siegel, 2020).
Over the course of evolution the two halves of the cerebral cortex have come to serve different functions. Recent studies of brain laterality' continue to document fundamental differences between the right and left hemispheres. Though the regions are closely' linked through the corpus callosum, they differ in their organization and modes of processing experience, moving some researchers to speak of a “conscious left brain system” and an “unconscious right brain system” (Schore, 2019b).
Iain McGilchrist reviews the history of brain laterality research in his pioneering work. The master and his emissary, concluding that the right and left hemispheres create radically' different versions of the world, shaping divergent ways of attending to experience as we negotiate and synthesize opposing realities (2009). In the realm of the right hemisphere, he proposes in a recent essay, we experience “the live, complex, embodied world of individual, always unique, beings, forever in flux, a net of interdependencies, forming and reforming wholes...” (2019, p. 22). In the domain of the left hemisphere, we experience what he describes as a “re-presented” version of our experience, containing “static, separable bounded, but essential fragmented entities, grouped into classes on which predictions can be based. This kind of attention isolates, fixes, and makes each thing explicit by bringing it under the spotlight of attention...” (2019, p. 22). In developing his formulations he emphasizes: “These are not different ways of thinking about the world: they are different ways of being in the world” (2019, p. 22).
Researchers distinguish a “left-brain surface, verbal, conscious, analytical explicit self” and a “right-brain deeper, nonverbal, nonconscious, holistic, emotional, corporeal, subjective implicit self” (Schore, 2019b, p. 185). The left hemisphere, associated with capacities for logic, linearity, literal thinking, and language, is thought to be instrumental in sequential processing, focused attention, and top-down processing of experience (Rogers, 2014; Schore, 2019b). The right hemisphere is central in processing emotion and nonverbal, holistic, visuospatial domains of experience; the region is closely linked with empathic functions, modulation of stress, and integrated maps of the whole body, dominant in bottom-up systems that regulate states of self.
As McGilchrist explains, the right hemisphere helps us see things in their uniqueness, embedded in the concrete particularities of the real world, while the left hemisphere, given to abstraction and reduction, helps us classify and categorize things (2019). He points to research findings supporting the primacy of the dynamics of the unconscious and emotion over conscious will, proposing: “The right hemisphere both grounds our experience of the world at the bottom end, so to speak, and makes sense of it, at the top end” (2015, p. 100; for an expanded review and discussion of research on bilaterality see Cozolino, 2017; Schore, 2019a, 2019b; and Siegel, 2020). We explore the fimctions of each hemisphere further in our discussion of horizontal integration in Chapter 3.
The two hemispheres of the cerebral cortex are divided into four lobes, named after the bones that cover them. By way of overview, the frontal lobes govern executive functions, moral reasoning, regulation of emotion, and movement. The parietal lobes connect sensory experience and motor fimctions with vision, hearing, and balance, creating an embodied sense of self in the world. The right parietal lobe plays a central role in helping us create spatial maps and mental models of the world—our relationship to people, objects, and threats within our immediate surrounds. The left parietal lobe is closely linked with capacities for abstraction, word-finding, metaphor, and arithmetic. The occipital lobes regulate our visual processing of experience and contain distinct areas specialized for different aspects of vision, such as form, color, and motion. The temporal lobes mediate higher perceptual functions, including auditor}'
Orienting Perspectives in Neuroscience 43 processing, receptive language, and memory functions. The upper part of the left temporal lobe contains a region known as Wernicke’s area that mediates the comprehension of meaning and the semantic aspects of language. Each lobe serves distinct functions, though there is considerable interaction among them (for expanded accounts see Kandel, 2012; Ramachandran, 2011; and Siegel, 2020).
The prefrontal cortex encompasses two systems, broadly described as the dorsolateral prefrontal cortex and the orbitomedial prefrontal cortex.The systems differ in their biochemistry, architecture, connectivity', and functions (see Cozolino, 2017, for an expanded review).
The dorsolateral region, the conscious realm of the rational mind, is specialized for cognitive intelligence. The networks of this area are closely linked with the hippocampus and the left hemisphere, instrumental in our efforts to focus attention, process experience, formulate our understanding of circumstances, reason, problem-solve, and plan. Robert Sapolsky describes the dorsolateral region as “the decider of deciders,” the most utilitarian part of the prefrontal cortex (2017).
The structures of the orbitoprefrontal cortex, closely linked to the amygdala and right hemisphere, mediate the dynamics of attachment, subjective experience, emotional regulation, and communication. The networks of this system are thought to play a central role in interoception, self-awareness, and menta-lization, influencing the ways in which we perceive and interpret the feelings, thoughts, and intentions of others (Fonagy, Gergely, Jurist & Target, 2002). Clinical scholars continue to explore the ways in which the structures and functions of this area influence the dynamics of attachment, relational life, and therapeutic action (see Schore, 2019a, 2019b; Siegel, 2020; Wallin, 2007). The middle prefrontal systems integrate divergent structures of the brain, balancing the cortical activity' of cognition with sensory' and emotional experience through the lower regions of the brain and body proper (Cozolino, 2017; Damasio, 2018; Schore, 2019b; Siegel 2020).
Neural circuits process and synthesize our experience of sensation, emotion, imagery', cognition, and behavior, mediating states of self and functional outcomes. In optimal functioning, as discussed earlier, we assume that energy’ and information flow in multiple directions throughout the brain. As Damasio writes:
Mind and behavior are moment-to-moment results of the operation of galaxies of nuclei and cortical parcels articulated by convergent and divergent neural projections. If the galaxies are well-organized and work harmoniously, the owner makes poetry. If not, madness ensues.
(2010, p. 312)