involve a pathway using only a few neurons. Being startled by a loud noise or touching a hot stove are examples of processes that have short neuronal pathways. Other processes use a more complex series of connections. More voluntary and complex processes use a much larger series of neuronal connections referred to as networks.
Researchers are now turning to examine how specific brain areas work together as networks. This search has also extended to psychopathology. Psychopathology can be seen in terms of problems involving either particular brain areas or the connections between areas that make up the network.
We all experience the brain organizing itself in terms of various networks throughout our day. One of the most familiar is sleep. Another is waiting for a lecture to start, when we just let our mind wander. Both of these cases are not responses to external stimuli but are self-organizing processes that occur. These types of processes are controlled by a large number of neurons working together in the form of a network.
Networks allow our brains to process information efficiently (Laughlin & Sejnowski, 2003; Sporns, 2011). Overall, cortical networks are influenced by experience and designed to be efficient in terms of connections between neurons in the network. This efficiency allows for less use of energy. One way energy is conserved is through not having every neuron connect with every other neuron.
Neurons Connect in a Network
How are neurons connected in a network? The answer may seem strange. Neurons are neither totally random in their connections with other neurons nor totally patterned. It appears that neurons are connected to one another in the same way that all humans on this planet are socially connected.
In the 1960s, the social psychologist Stanley Milgram (Travers & Milgram, 1969) asked the question, “What is the probability that any two people randomly selected from a large population of individuals such as the United States would know each other?” He answered this question by giving an individual a letter addressed to another person somewhere in the United States. This individual was to send the letter to someone he knew who might know the other person. In turn, this person was to send the letter to someone she knew who might know the person. Surprisingly, it only required five or six different people for the letter to go from the first individual to the final individual. This phenomenon has been referred to as the small world problem; more recently, the phrase six degrees of separation has been used.
Various studies have shown that the neurons in the brain can also be considered within a small world framework (Sporns, 2011). Neurons have numerous short-distance local connections, which taken together can be considered as a hub or module. From these hubs extend more long-distance connections to other hubs.
small world framework: a model of brain connectivity based on the idea that the ability to socially contact any two random individuals in the world can be accomplished in a limited number of connections
Local hubs can be made up of neurons that connect with each other over very short distances. Such connections are seen in gray matter. Underlying this are the axons, which transfer information throughout the brain. Their myelin sheaths are lighter in color, and thus, these areas are referred to as white matter. Myelin is made up of fats and proteins. It wraps around axons like insulation does around electrical cables and results in an increased speed in information transmissions. About 44% of the human brain is white matter. White matter generally represents longer connections between neurons. This allows for cortical networks over larger areas of the brain. Knowing this, it is possible to examine the network connections in individuals with a particular disorder and their matched controls. For example, individuals with schizophrenia have been shown to have disrupted global networks of the brain (O. Wang et al., 2012).
Figure 2.22 Major Networks of the Brain
Source: Raichle, M. E. (2015, March 30). The restless brain: how intrinsic activity organizes brain function. Philosophical Transactions of the Royal Society B.
central executive network: the neural network involved in performing such tasks as planning, goal setting, directing attention, performing, inhibiting the management of actions, and the coding of representations in working memory
salience network: the neural network involved in monitoring and noting important changes in biological and cognitive systems
Networks have been studied in terms of a variety of cognitive and emotional tasks (Bressler & Menon, 2010; Raichle, 2015). These include separate networks involved in the processing of visual or auditory information, sensorimotor processes, attentional processes, executive control, salience, and default mode (see Figure 2.22).
Three of these networks have been examined in terms of psychopathology (Menon, 2011). These are the baseline or default network (also called the intrinsic network), the central executive network, and the salience network. The default network is active when an individual is not performing a particular task, such as when one’s mind wanders or is processing internal information. The central executive network is involved in higher-order cognitive and attentional tasks. The salience network is important for monitoring critical external events as well as internal states. As will be described throughout this text, psychopathological disorders such as schizophrenia, depression, anxiety, dementia, and autism have been shown to involve problems in turning networks on or off as well as problems in the connections within the network itself.
The historical considerations of psychopathology emphasized careful observation and interaction with the afflicted individuals as important methods for understanding the nature of the disorder. However, with progress in the neurosciences, brain imaging, and genetics, other levels of analysis have become possible. The new levels offer different perspectives for the field of mental illness, but because many of these discoveries are so new, it is not surprising that our understanding of the field of mental illness is currently in flux. Neuroscience research has been used to find more objective markers in the diagnosis and treatment of mental disorders. It has also helped describe cognitive, emotional, and motor processes in both health and illness. This has resulted in a better articulation of what underlies these processes such as problems in setting goals, having relationships with others, thinking, and feeling, as well as deficits in the memory system and the reward system.
What Is the Brain’s Default (Intrinsic) Network?
What does your brain do when you are just sitting and waiting or daydreaming or talking to yourself? This is a question that is just now beginning to be explored. In psychology, most of the research you read about involves a person doing something. Reacting to emotional pictures or solving cognitive problems are common examples. In these cases, one’s attention is focused on a task in the external world.
In the same way that the brain is organized to process spatial and verbal material differently and involve different cortical networks, it also appears that different circuits are involved with internal versus external information. A variety of studies have examined brain imaging procedures in which individuals performed internal tasks versus external tasks (e.g., Ray & Cole, 1985).
However, we all know that even without an external task to do, our mind is constantly working. It jumps from one thought to another. The psychologist William James called this process the stream of consciousness. Recent researchers refer to this process as mind wandering.
Those neural networks that are active during internal processing have come to be referred to as the brain’s default or intrinsic network (Buckner, Andrews-Hanna, & Schacter, 2008; Raichle, 2011; Raichle & Snyder, 2007). It has been suggested that intrinsic is a better term than default, since a variety of internal tasks use this network (C. Kelly, Biswal, Craddock, Castellanos, & Milham, 2012). The default network is separate from, but one that can be understood as similar to, other networks such as those involved in visual perception or motor activities. It is made up of a set of interacting brain regions. Those areas involved are pictured in Figure