The midbrain section of the neural tube that human brains develop from grows to become what has been called the limbic system (Figure 6.28), which is generally described as including the:
· Thalamus. A sensory relay station for sight, sound, and touch input.
· Hypothalamus. Connects to the peripheral nervous system that controls body states.
· Hippocampus. Acts to lay down memories.
· Amygdala. Analyzes sense and body state input to generate emotions.
· Cingulate gyrus. Links to the cerebral cortex.
These structures handle emotions, motivation, and memory in humans and other animals in ways that are still being understood but clinical studies give some clues. Hippocampus damage can result in amnesia, the inability to recall memories or lay down new ones. Cingulate gyrus damage is involved in depression and schizophrenia. Amygdala damage reduces the ability to process emotions based on facial and other signals, and is a neural marker of autism. The hypothalamus links to the peripheral nervous system, a second brain of a 100 million nerves outside the head that handles body hormones and digestion. The limbic system then relates body states to sensory situations, lays down memories of them, and generates emotions that motivate behavior, so it can be called a feedback control center in its own right.
Locus of control theory describes the midbrain as a control center specializing in body states, with its own sensory-visceral input, learning, memories, and state output. For example, while hindbrain muscle memory remembers motor tasks like riding a bicycle, midbrain memories recall past events like whether we left a stove on. This episodic memory includes time and space tags plus any feelings at the time, so remembering a funeral can re-experience its sadness. The past event timeline of midbrain memories allows emotional learning that links situations to good or bad consequences, to allow cause and effect conclusions.
Like the hindbrain, the midbrain limbic system has its own dedicated nerve paths, so it can react to facial data in less than a tenth of a second by a subcortical visual path before the cortex can respond (Adolphs, 2008). Sense data from the thalamus goes direct to the amygdala by a short route, as well as taking a longer route to the cortex (Figure 6.29), so the amygdala can initiate emotional responses like sweaty hands, dry mouth, and tense muscles before the visual cortex can identify seen objects. The thalamus still passes data to the cortex for a more complex but slower decision, but the two paths are separate. Like the hindbrain, the midbrain has its own neural links, that evolved long before the cortex developed the ability to think, which it can use to initiate responses.
The limbic system also has its own space apart from the cerebellum. The hindbrain provides a navigational map based on vector directions and distances between key points, but the midbrain map gives a visual record of location details, including what is there, when it was accessed, and the likely benefits or dangers. Two maps are better than one, so navigation maps help hunters in featureless terrains like deserts and oceans, while location maps help food gatherers remember what is where, and when it is available. This midbrain map provides the details needed to return to a point in space (O’Keefe & Nadel, 1978), so birds like nutcrackers can hide about 30,000 seeds at locations over a 200 square mile area and still recover them six months later.
The limbic system then isn’t a brain that takes over but an emotional control center, just as the cerebellum is a movement control center, so they operate in parallel rather than overlay. For example, the hindbrain can initiate a fight response that the midbrain prepares the body for, or the midbrain can initiate a desire to flee which the hindbrain then responds to. Each represents the world differently, as one reveals what we can do based body position and response options, while the other reveals what we want to do based on past experiences and what the body needs.
If the midbrain directs emotional responses, what then is an emotion? An emotion can be seen as a neural representation of a body state that can associate with situations. For example, fear is the state of increased heart rate, breathing, adrenaline, blood pressure, and blood sugar that arises in a threat situation. Having experienced this visceral state, for any reason, the limbic system can recall and re-activate it, to prepare the body for fight or flight, so fear evolved over time because being primed to act increases survival.
Applying this logic to other basic emotions suggests they also evolved to support survival. Emotions like lust, anger, and greed now seem unnecessary urges but anger is useful in a fight, lust benefits the species, and greed lets us feast to survive a later famine. Dependence might not help an adult but children who stay by their parents survive more, and even laziness has a value, to motivate tired animals to rest and recover. All emotions have survival value in the right situations.
Midbrain emotions are then a tool kit of body-state responses that help survival when tailored to situations. Their evolution was a big advance at the time but like all toolkits, one must use the right tool for the job. If a carpenter uses a hammer to shorten a plank instead of a saw, it isn’t the toolkit’s fault, and likewise, what we now call negative emotions are only so if used incorrectly.
The limbic amygdala is the key processing center for emotions, that manages detection of danger, emotional learning, and interprets facial expressions like anger (Hooker et al., 2006). For example, a skin sensation that is like a crawling insect can produce a sudden emotional and physical response initiated by the amygdala. It also allows the brain to assess another’s intent, to project what I would do on another. For example, many birds cache their food to hide it for use later, but if they see another bird watching them do it, they return later to re-hide it (Clayton et al., 2007). In humans, this ability to understand another allows higher emotions like empathy, the ability to feel what another feels, which could be the basis of what we call love.
The hindbrain evolved because doing something is better than doing nothing, but if a predator is nearby, it’s often better to stay still. As the emotional center evolved to manage threat responses, it had to override the tendency to move to effect this response. When a mammal sees a predator, the instinct to run away is stopped by the emotion of fear, as the amygdala activates its connections to the brainstem and cerebellum to freeze an animal in its tracks (Ressler, 2010). Mammals have this paralysis by fright response but fish don’t because their midbrain is less evolved. One center can then override another, but by suppressing rather than replacing it.
The natural next step of brain evolution was to develop the forebrain control center to give humans language and thought. We experience one being but our brain evolved in stages, so the forebrain can represent reality using thoughts as well as actions and feelings. The classical division of human nature into will, emotions, and intellect then began when brains specialized in output, state, and input functions, so the next section addresses how our intellect develops.