Emotion: Learn It 3—The Biology of Emotions

 The Biology of Emotions

Earlier, you learned about the limbic system, which is an area of the brain involved in emotion and memory (Figure 1). The limbic system includes the hypothalamus, thalamus, amygdala, and the hippocampus. An illustration of the brain labels the locations of the “thalamus,” “hypothalamus,” “amygdala,” and “hippocampus.”Figure 1. The limbic system, which includes the hypothalamus, thalamus, amygdala, and the hippocampus, is involved in mediating emotional response and memory.

Each structure of the limbic system plays a distinct role in emotion:

  • The hypothalamus plays a role in the activation of the sympathetic nervous system that is a part of any given emotional reaction.
  • The thalamus serves as a sensory relay center whose neurons project to both the amygdala and the higher cortical regions for further processing.
  • The amygdala plays a role in processing emotional information and sending that information on to cortical structures (Fossati, 2012).
  • The hippocampus integrates emotional experiences with cognition (Femenía, Gómez-Galán, Lindskog, & Magara, 2012).

Let’s take a closer look at the amygdala and the hippocampus and their roles in emotion.

structure and function of the amygdala

The amygdala has received a great deal of attention from researchers interested in understanding the biological basis for emotions, especially the emotions of fear and anxiety (Blackford & Pine, 2012; Goosens & Maren, 2002; Maren, Phan, & Liberzon, 2013). The amygdala is composed of various subnuclei (neural subcomponents), including the basolateral complex and the central nucleus.

The basolateral complex has dense connections with a variety of sensory areas of the brain. It is critical for classical conditioning and for attaching emotional value to learning processes and memory.

The central nucleus plays a role in attention, and it has connections with the hypothalamus and various brainstem areas to regulate the autonomic nervous and endocrine systems’ activity (Pessoa, 2010).

An illustration of the brain labels the locations of the “basolateral complex” and “central nucleus” within the “amygdala.”
Figure 2. The anatomy of the basolateral complex and central nucleus of the amygdala are illustrated in this diagram.

A story from animal research: context matters

One striking set of studies shows that the amygdala’s role in learning fear can depend heavily on context.

Researchers found that rat pups who experienced an odor paired with a mild shock without their mother present showed increased amygdala activation and later avoided that odor—suggesting they learned fear.

But when the mother was present, the pups actually showed a preference for the odor cue, even though it had been paired with a shock. In that condition, the amygdala did not show the same increase in activation. In other words, the mother’s presence changed how the pups’ brains responded—and what they learned (Moriceau & Sullivan, 2006).

Early experience and later emotional patterns

Related work suggests that adverse early experiences can alter amygdala functioning in ways that shape later behavior. In one study, rat pups exposed to stressful or abusive-like conditions early in life later showed more “depressive-like” behavior during adolescence, along with increased amygdala activation (Raineki et al., 2012).

While we should be cautious about directly translating rat findings to humans, this research supports a broader point: early experiences can influence how brain systems involved in threat and emotion develop over time.

Links to human mood and anxiety disorders

Human research also finds associations between amygdala structure/function and disorders involving mood and anxiety. Differences in amygdala activity have been observed in adolescents who are at risk for—or diagnosed with—certain anxiety or mood disorders. Some researchers have also explored whether patterns of amygdala activation could help distinguish between conditions such as bipolar disorder and major depressive disorder (though this remains an area of ongoing research).

Hippocampus

The hippocampus is also important in emotional processing, especially because it supports memory and context. That context matters for emotion: it helps your brain answer questions like, “Is this situation safe?” or “Have I been here before?”

Research links hippocampal structure and function to several mood and anxiety disorders. For example, studies of people with posttraumatic stress disorder (PTSD) often find reduced volume in parts of the hippocampus. Some researchers suggest these patterns may relate to changes in neurogenesis and dendritic branching (the growth of new neurons and new neural connections).

It’s important to note that many of these studies are correlational, so they cannot prove cause and effect. However, some work suggests that after treatment (including medication and/or cognitive-behavioral therapy), people with PTSD may show symptom improvement alongside changes in hippocampal functioning and, in some cases, volume measures.