Memory is the basis for learning. An animal that can remember its encounter with a predator or noxious plant will avoid that danger in the future. A memory is formed by changes to neural circuits within specific regions of the brain, specifically, by changes in the synaptic connections between neurons in a neural circuit. These changes enable an animal, for example, to recognize other individuals in its social group or to recognize the threat of a predator. In humans and other primates, the hippocampus, a brain structure in the limbic region, plays a special role in memory formation. We can remember much of the information we receive for several minutes. However, unless reinforced by repetition or by paying particular attention, the information is typically forgotten. The hippocampus transforms reinforced short-term memories into long-term memories. Long-term memory is essential for learning.

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The hippocampus forms long-term memories by repetitively relaying information to regions of the cerebral cortex. Although the details remain uncertain, memory and learning depend on establishing particular neural circuits in the hippocampus and cerebral cortex. These circuits can be activated to recall a memory that is triggered by a particular sound, or sight, or other relevant stimulus. Establishing a neural circuit requires establishing a particular set of connections between neurons. Thus, the formation of a memory circuit requires changes in synapses. Some synapses are weakened or removed altogether, and others are formed or strengthened.

The ability to adjust synaptic connections between neurons is called synaptic plasticity (Fig. 36.25). Long-term potentiation (LTP) is an example of synaptic plasticity that is believed to underlie memory and learning. In response to repeated excitation, neurons in excited circuits within the hippocampus release the neurotransmitter glutamate, opening Na+ and Ca²⁺ channels. The increased Ca²⁺ stimulates signaling pathways leading to protein synthesis. New receptors are placed in the postsynaptic cell membrane and new dendrites are formed, strengthening synaptic signaling between the two cells. These changes make the cells more responsive to subsequent stimulation.

Through repeated activation of neurons within the hippocampus and associated regions of the cerebral cortex, neural circuits are established that may be reinforced by LTP to create a memory that can be retrieved over long periods of time. Although memories may be associated with certain regions of the cortex, they also rely on the interaction of nerve cells and networks within multiple brain regions.

Although the brain regions involved in memory and learning vary among different animals, the basic features of how memories are constructed by means of synaptic plasticity within neural circuits are likely broadly shared across animals capable of forming memories. For example, the ability of an octopus to learn and remember how to perform motor tasks rivals that of certain vertebrates. Even the roundworm C. elegans has been found to be capable of memory and learning: Individual roundworms learn to avoid pathogenic bacteria in favor of bacteria that are nutritious.

Quick Check 5 Which three areas of the human brain are important for remembering and recognizing a face?