In one early study, Klüver and Bucy (1939) damaged the amygdala of an aggressive rhesus monkey. The amygdala has connections to other bodily systems related to fear, including the sympathetic nervous system (which we will see later is important in fear responses), facial responses (which perceive and express emotions), the processing of smells, and the release of neurotransmitters related to stress and aggression (Best, 2009). The amygdala consists of two “almond-shaped” clusters (amygdala comes from the Latin word for “almond”) and is primarily responsible for regulating our perceptions of, and reactions to, aggression and fear. It includes the amygdala, the hypothalamus, and the hippocampus. The limbic system is a brain area, located between the brain stem and the two cerebral hemispheres, that governs emotion and memory. Whereas the primary function of the brain stem is to regulate the most basic aspects of life, including motor functions, the limbic system is largely responsible for memory and emotions, including our responses to reward and punishment. Also, the cerebellum contributes to emotional responses, helps us discriminate between different sounds and textures, and is important in learning (Bower & Parsons, 2003). Consuming alcohol influences the cerebellum, which is why people who are drunk have more difficulty walking in a straight line. People who have damage to the cerebellum have difficulty walking, keeping their balance, and holding their hands steady. It functions to coordinate voluntary movement. The cerebellum (literally, “little brain”) consists of two wrinkled ovals behind the brain stem. This diagram shows the major parts of the limbic system, as well as the pituitary gland, which is controlled by it. When electrical stimulation is applied to the reticular formation of an animal, it immediately becomes fully awake, and when the reticular formation is severed from the higher brain regions, the animal falls into a deep coma. The reticular formation also plays important roles in walking, eating, sexual activity, and sleeping. The job of the reticular formation is to filter out some of the stimuli that are coming into the brain from the spinal cord and to relay the remainder of the signals to other areas of the brain. Running through the medulla and the pons is a long, narrow network of neurons known as the reticular formation. The spherical shape above the medulla is the pons, a structure in the brain stem that helps control the movements of the body, playing a particularly important role in balance and walking. In many cases the medulla alone is sufficient to maintain life-animals that have the remainder of their brains above the medulla severed are still able to eat, breathe, and even move. The brain stem begins where the spinal cord enters the skull and forms the medulla, the area of the brain stem that controls heart rate and breathing. It’s designed to control the most basic functions of life, including breathing, attention, and motor responses ( Figure 3.8 “The Brain Stem and the Thalamus”). The brain stem is the oldest and innermost region of the brain. Humans have a very large and highly developed outer layer known as the cerebral cortex (see Figure 3.7 “Cerebral Cortex”), which makes us particularly adept at these processes. Mammals, including humans, have developed further brain layers that provide more advanced functions-for instance, better memory, more sophisticated social interactions, and the ability to experience emotions. The “old brain” regulates basic survival functions, such as breathing, moving, resting, and feeding, and creates our experiences of emotion. The innermost structures of the brain-the parts nearest the spinal cord-are the oldest part of the brain, and these areas carry out the same the functions they did for our distant ancestors. In each animal the brain is layered, and the basic structures of the brain are similar (see Figure 3.6 “The Major Structures in the Human Brain”). This is because the brains of all animals are very similar in overall form. If you were someone who understood brain anatomy and were to look at the brain of an animal that you had never seen before, you would nevertheless be able to deduce the likely capacities of the animal.
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