Experiments show that the higher intellectual processes—such as reasoning and memory—are based in the cerebral cortex (outer layer of the cerebral hemispheres of the brain). If certain parts of the cerebral cortex are stimulated electrically, there will be recall of experiences. If large areas of the cortex are destroyed, memory is impaired. Within the lateral ventricles of the cerebral hemispheres lies the limbic system. It consists of the hippocampus and the amygdala, which are responsible for storing specific memories. The hippocampus stores memories associated with places and facts; the amygdala stores memories associated with emotions and skills. Research indicates that glutamate—a neurotransmitter, or chemical messenger, in the hippocampus—plays an important role in memory retention. Glutamate serves two functions: (1) it activates certain receptors, called NMD A receptors, so they can activate calcium ions, which are crucial to forming memories, and (2) it permanently strengthens synapses, the connections between neurons, which are vital for memory formation. Learning depends on adding to or altering the strength of the synapses in the brain.
Exactly how information is retained, physiologically, has not been determined. One theory states that memory is localized— it is stored in specific regions in the brain. A particular memory is retrieved by a complex series of steps that activate a unique pattern of electrical activity in the brain. This theory is supported by the fact that certain brain injuries are specific in their effects on speech and memory. Another theory states that memory is diffuse—it is a collective action of countless cells. When a memory is retrieved, electrical activity spreads across the brain. The memory processes overlap and destruction of one area does not always erase the entire memory.