2.11 Identify structures of the brain that are involved in emotion, learning, memory, and motivation.
The forebrain includes the two cerebral hemispheres of the brain, including the cortex, which is discussed in detail later in this chapter, and a number of important structures located under the cortex in each hemisphere. These subcortical structures (the prefix sub means “under” or “below”) play a part in our thinking and behavior. While there are subcortical structures that influence motor control and the learning of motor skills, the basal ganglia, and white matter fiber pathways that connect the cortex to other parts of the brain and spinal cord, we will focus on the subcortical structures that have been collectively referred to as the limbic system. (See Figure 2.15.)
The limbic system (the word limbic means “marginal” and these structures are found in the inner margin of the upper brain) includes the thalamus, hypothalamus, hippocampus, amygdala, and the cingulate cortex. In general, the limbic system is involved in emotions, motivation, and learning.
The thalamus (“inner chamber”) is in some ways similar to a triagemore info nurse. This somewhat round structure in the center of the brain acts as a kind of relay station for incoming sensory information. Like a nurse, the thalamus might perform some processing of that sensory information before sending it on to the part of the cortex that deals with that kind of sensation—hearing, sight, touch, or taste. Damage to the thalamus might result in the loss or partial loss of any or all of those sensations.
The sense of smell is unique in that signals from the neurons in the sinus cavity go directly into special parts of the brain called olfactory bulbs, just under the front part of the brain. Smell is the only sense that does not have to first pass through the thalamus.
A very small but extremely powerful part of the brain is located just below and in front of the thalamus (see Figure 2.15). The hypothalamus (“below the inner chamber”) regulates body temperature, thirst, hunger, sleeping and waking, sexual activity, and emotions. It sits right above the pituitary gland. The hypothalamus controls the pituitary, so the ultimate regulation of hormones lies with the hypothalamus.
Like many structures in the brain, the hippocampus was named based on its appearance. Hippocampus is the Greek word for “seahorse” and it was given to this brain structure because the first scientists who dissected the brain thought it looked like a seahorse. The hippocampus is located within the medial temporal lobe on each side of the brain (medial means “toward the middle”). Research has shown that the hippocampus is instrumental in forming long-term (permanent) declarative memories that are then stored elsewhere in the brain (Squire & Kandel, 2009). See Learning Objective 6.12. As mentioned earlier, ACh, the neurotransmitter involved in muscle control, is also involved in the memory function of the hippocampus. People who have Alzheimer’s disease, for example, have much lower levels of ACh in that structure than is normal, and the drugs given to these people boost the levels of ACh.
The amygdala (“almond”) is another area of the brain named for its shape and appearance. It is located near the hippocampus. The amygdala is involved in fear responses and memory of fear. Information from the senses goes to the amygdala before the upper part of the brain is even involved, so that people can respond to danger very quickly, sometimes before they are consciously aware of what is happening. In 1939 researchers found that monkeys with large amounts of their temporal lobes removed—including the amygdala—were completely unafraid of snakes and humans, both of which are normally fear-provoking stimuli (Klüver & Bucy, 1939). This effect came to be known as the Klüver-Bucy syndrome. Rats that have damaged amygdala structures will also show no fear when placed next to a cat (Maren & Fanselow, 1996). Case studies of humans with damage to the amygdala also show a link to decreased fear response (Adophs et al., 2005). Although the amygdala plays a vital role in forming emotional memories, it is still unclear if the memories are stored in the amygdala (Squire & Kandel, 2009).
The cingulate cortex is the limbic structure that is actually found in the cortex. It is found right above the corpus callosum in the frontal and parietal lobes and plays an important role in both emotional and cognitive processing. The cingulate cortex can be divided into up to four regions that play different roles in processing emotional, cognitive, and autonomic information (Vogt & Palomero-Gallagher, 2012). It has been shown to be active during a variety of cognitive tasks such as selective attention, written word recognition, and working memory (Cabeza & Nyberg, 2000) and has been implicated in a variety of psychological and mental disorders, including attention-deficit/hyperactivity disorder (Bush et al., 1999; Bush et al., 2008), schizophrenia, major depressive disorder, and bipolar disorder (Fornito et al., 2009; Maletic et al., 2007). The next section further explores the cortex and its functions.