Before we go on any further in our journey to understand the brain, we must first take some time looking at the anatomy of the brain. Pessoa underscores the importance of this by referencing biology's axiom, that function is inherently tied to structure. In other words, different structures suggest different functions. Axioms are essentially first principles or fundamental premises on which a science is built. If this is biology's (and neuroscience's) axiom, it is no wonder that neuroscientists are commonly in the business of finding functions performed by specific brain regions. When Brodmann created his map of the brain, he too demarcated boundaries in the brain based on cell shape and size, as well as spatial differences in cell distributions and densities. To him, each of these regions had its distinct structures and consequently, also had its unique functions.
To what extent is biology's axiom true? Are structural properties always uniquely tied to functional ones? Could brains not exhibit forms of redundancy, where different structures perform the same function, or the same structures perform different functions? For now, we'll put these questions on hold. Regardless of the answers, there's no denying that at least some basic knowledge of neuroanatomy will be of use to us. Here, the chapter focuses on 4 main parts of the brain: cortical structures, subcortical structures, neurons, and white matter tracts.
The Cortex
The cortex is what you see when you look at just about any image of the brain. Translated to 'bark' in Latin, the cortex is the thin outer part of the brain that envelopes the internal and subcortical structures. It is highly folded and wrinkled with numerous bumps (gyri) and depressions (sulci), a structural characteristic that allows a large area of cortex to fit inside our skull. The cortex itself is made up of around 3 - 6 layers of cells, known as grey matter (see Fig. 1B). These cells include neurons as well as other cells which support the cellular environment. In mammals, the 6-cell-layered cortex is also called the neocortex (as it is the part of the cortex that developed most recently through evolution).
Fig. 1 A: Cortical lobes; B: Grey and white matter (Pessoa, 2022)
Zooming out, the cortex can be separated into different cortical areas. Visible cortical areas (see Fig. 1A) include the frontal lobe (front of brain), occipital lobe (back of brain), temporal lobe (sides of brain), and parietal lobe (between the frontal and occipital lobes). Famous ascribed functions of these regions include: attention and critical thinking by the prefrontal cortex in the frontal lobe, vision by the primary visual cortex in the occipital lobe, audition by the primary auditory cortex in the temporal lobes, and touch by the somatosensory cortex in the parietal lobe.
There are cortical areas hidden within the brain, too. These include the cingulate cortex (located between the two hemispheres, see Fig. 2), insula cortex (deep within the lateral sulci of the brain, see Fig. 3), and hippocampus (which, contrary to popular knowledge, is a cortical and NOT subcortical structure, see Fig. 5).
Fig. 2 Darker shade represents the cingulate cortex (Pessoa, 2022)
Fig. 3 The insula cortex (Pessoa, 2022)
Embryonic Development of the Brain
At one point, Pessoa makes a brief but nontrivial aside to how the nervous system develops in embryos. The nervous system starts out as a smooth, regular, and undifferentiated cylinder called a neural tube (see Fig. 4). As the embryo develops, the neural tube starts to crease and bulge at different points, resulting in the formation of 4 distinct compartments. These compartments correspond to what will become different parts of the nervous system, including the forebrain, midbrain, hindbrain, and spinal cord. Crucially, the forebrain will differentiate into the cortex and some other subcortical structures. The other compartments do not contribute to the cortex at all.
Fig. 4 The neural tube (Pessoa, 2022)
The Subcortex
Subcortical structures are masses of cells located at the base and towards the middle of the brain. Some examples of these include the thalamus, hypothalamus, amygdala, and striatum. Here, the thalamus (see Fig. 5) is thought to function as a relay station that connects sensory periphery structures (e.g., receptors at the eyes, ears, skin, tongue) to their respective sensory cortical areas (note: the thalamus does NOT connect olfactory structures to the brain). For example, the thalamus receives signals from the retina and relays them to the primary visual cortex, and receives nerve impulses from the inner ear before transmitting them to the primary auditory cortex. This still reflects a limited understanding of the thalamus. According to Pessoa and his network-based view of the brain, the thalamus is actually intricately connected with both cortical and subcortical areas, and might actually have a function beyond that of a relay station for the senses. This idea will be revisited in later chapters.
Fig. 5 Subcortical structures + the hippocampus (Pessoa, 2022)
The striatum (see Fig. 6) is another subcortical structure that consists of other components like the caudate and putamen. Aside from the primary visual cortex, the striatum is richly connected with all other cortical areas. Additionally, it links to other subcortical areas that are involved in motor control. Fittingly, then, the striatum with its nearby structures makes up the basal ganglia, also known as the brain's motor control system. Like the thalamus, it turns out that the striatum is actually involved in many more functions beyond the obvious one ascribed to it.
Fig. 6 The striatum (Pessoa, 2022)
Finally, we have the brainstem, which includes the midbrain and hindbrain. It is located at the back and at the base of the brain, resembling a stem on top of which rests the brain. Here, the brainstem is commonly associated with basic and autonomous functions, including breathing and heart rate control.
Neurons
Neurons (see Fig. 7) are specialised cells that form the basic building blocks of the nervous system. They consist of 2 main features. Firstly, neurons have a soma, which is their main cell body that contains the cell nucleus. Secondly, neurons have thin, tube-like extensions that radiate outward from their cell body. There are 2 kinds of these extensions. On the one hand, axons are long, single extensions that carry electrical signals away from the cell body. Axons are myelinated, that is, there are white clumps of fat deposits (called myelin sheaths) along the whole nerve extension that help facilitate faster and more efficient signal transmissions. On the other hand, dendrites are shorter and more numerous branches of extensions that carry nerve impulses toward the cell body. One neuron's axon is typically connected to the dendrites of multiple other neurons. Note that they are not actually physically connected -- there is a small gap between the ends of axons and dendrites called a synapse.
Fig. 7 Neurons (Pessoa, 2022)
Neurons communicate with each other through these synapses. Nerve communication starts when the electrical voltage within a neuron crosses its threshold value, after which an action potential (i.e., an electrical spike or nerve impulse) is triggered. This electrical signal then travels down the long, myelinated axon, at the end of which triggers the release of chemicals called neurotransmitters (e.g., dopamine, serotonin). Neurotransmitters are released into the synaptic space between the presynaptic neuron (i.e., where the action potential originates) and the postsynaptic neuron (i.e., the neuron receiving the signal).
At this point, the neurotransmitters bind to cell surface receptors on the dendrites of the postsynaptic neuron, thereby altering the permeability of its cell surface membrane and resulting in the differential inflow and outflow of various types of ions. This process can then excite the postsynaptic neuron, following which another action potential is generated, and the process repeats itself in neurons downstream. Note that neurotransmitters can either activate or inhibit nerve activity in neurons. Having an inhibition mechanism prevents overexcitation of neurons in the brain, which left unchecked, can lead to epileptic seizures.
White Matter Tracts
Axons are typically longer than dendrites. In most cases, they are still short enough to allow neuronal communication locally or between adjacent regions. However, there also exist axons in the brain that extend out longer and bunch up together to form white matter tracts (see Fig. 8) that allow for cross-brain signal transmission. According to Pessoa, 20 of these tracts that connect different brain regions have been identified so far.
Fig. 8 White matter tracts (Pessoa, 2022)
Networks, not Regions
The introduction of white matter tracts allows us to end on a conceptual note. These tracts allow for hugely complex levels of connectivity throughout the brain. How might this help us rethink the original axiom in biology introduced at the start? Here, Pessoa reiterates the dominant view in neuroscience: function is a result of specialised brain structure. This is reflected in Brodmann's conception of the brain -- he divided the brain solely based on differences in cell type and density without much consideration for the connectivity introduced by white matter. Here, if a lesion observed in a region of the brain led to a change in a specific functional property, one would conclude that that region of the brain is the structural basis for the particular function.
In contrast, we have an alternative view espoused by so-called associationists. Given the same observation of structural lesion and functional change, associationists come to a very different conclusion -- that the change in function might actually be due to impairments in other brain regions that are nowhere close to the observed lesion! How might this be possible? Pessoa introduces 2 possible mechanisms: diaschisis and disconnection.
Diaschisis (Greek for 'shocked throughout') is the idea that a particular brain area might be impaired due to its neuronal connection to the site of an observed lesion. Subsequently, this resulted in a change in the function of that particular brain area. Meanwhile, if a function is a result of the communication between multiple brain regions, this can be disrupted via disconnection, where damage to the tract connecting them prevents this communication from occurring in the first place.
I have some questions related to diaschisis. Based on my understanding, Pessoa is claiming that the damaged brain area might not be involved in a particular function, and another region distant from the damaged part is implicated instead. How does this support the associationists' perspective more than the dominant one? Is this not still based on the assumption that a function is a specialised result of brain structure? The only difference in the case of diaschisis, to me at least, is that the connection between function and structure is less obvious and intuitive than it otherwise appears.
Of course, I might have entirely missed the point here, too. Perhaps the dominant strongly emphasises a function-to-isolated-structure mapping, whereas the associationists are arguing that while a structure has a direct contribution to function, this function-to-structure mapping is contingent on the connections between said structure and other brain regions.
Concluding remarks
At this point, we're still laying out the current scope and ideas of the field and haven't actually provided concrete evidence for the alternative to the status quo. What is clear, however, is that it is possible that we might be doing neuroscience wrong this whole time. Instead of carrying out functions in isolated brain regions, the argument here is that our capacities and behaviours are actually a result of the complex interactions within and between regions. Consequently, it doesn't make sense to confine different functions to rigid and isolated boxes of structure. Instead, Pessoa pushes for a complex, systems- and networks-based analysis of the brain, which will be made more evident throughout the rest of the book.
References
Pessoa, L. (2022). The entangled brain. Journal of Cognitive Neuroscience, 35(3), 349–360. https://doi.org/10.1162/jocn_a_01908
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