Memory and Learning Require the Cerebral Cortex and Limbic System

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Memory and learning are inextricably linked because part of the learning process involves the assimilation of new information and its commitment to memory. The most likely sites of learning in the human brain are the large association areas of the cerebral cortex, in coordination with subcortical structures deep in the temporal lobe, including the hippocampus and amygdala. The association areas draw on sensory information received from the primary visual, auditory, somatic sensory, and olfactory cortices and on emotional feelings transmitted via the limbic system. This information is integrated with previously learned skills and stored memory, which presumably also reside in the association areas.

The learning process itself is poorly understood, but it can be studied experimentally at the synaptic level in iso lated slices of mammalian brain or in more simple invertebrate nervous systems. Synapses subjected to repeated presynaptic neuronal stimulation show changes in the excitability of postsynaptic neurons. These changes include the facilitation of neuronal firing, altered patterns of neurotransmitter release, second messenger formation, and, in intact organisms, evidence that learning occurred. The phenomenon of increased excitability and altered chemical state on repeated synaptic stimulation is known as long-term potentiation, a persistence beyond the cessation of electrical stimulation, as is expected of learning and memory. An early event in long-term potentiation is a series of protein phosphorylations induced by receptor-activated second messengers and leading to activation of a host of intracellular proteins and altered excitability. In addition to biochemical changes in synaptic efficacy associated with learning at the cellular level, structural alterations occur. The number of connections between sets of neurons increases as a result of experience.

Much of our knowledge about human memory formation and retrieval is based on studies of patients in whom stroke, brain injury, or surgery resulted in memory disorders. Such knowledge is then examined in more rigorous experiments in nonhuman primates capable of cognitive functions. From these combined approaches, we know that the prefrontal cortex is essential for coordinating the formation of memory, starting from a learning experience in the cerebral cortex, then processing the information and communicating it to the subcortical limbic structures. The prefrontal cortex receives sensory input from the parietal, occipital, and temporal lobes and emotional input from the limbic system. Drawing on skills such as language and mathematical ability, the pre-frontal cortex integrates these inputs in light of previously acquired learning. The prefrontal cortex can thus be considered the site of working memory, where new experiences are processed, as opposed to sites that consolidate the memory and store it. The processed information is then transmitted to the hippocampus, where it is consolidated over several hours into a more permanent form that is stored in, and can be retrieved from, the association cortices.

Declarative and Procedural Memory. A remarkable finding from studies of surgical patients who had bilateral resections of the medial temporal lobe is that there are two fundamentally different memory systems in the brain. Declarative memory refers to memory of events and facts and the ability to consciously access them. Patients with bilateral medial temporal lobectomies lose their ability to form any new declarative memories. However, they retain their ability to learn and remember new skills and procedures. This type of memory is called procedural memory and involves several different regions of the brain, depending on the type of procedure. In contrast to declarative memory, structures in the medial temporal lobe are not involved in procedural memory. Learning and remembering new motor skills and habits requires the stria-tum, motor areas of the cortex, and the cerebellum. Emotional associations require the amygdala. Conditioned reflexes require the cerebellum.

An early demonstration of the dichotomy between declarative and procedural memory came from studies by Dr. Brenda Milner on a patient of Dr. Wilder Penfield in the mid-1950s. This patient (H.M.) had received a bilateral medial temporal lobectomy to treat severe epilepsy and, since that time, has been unable to form any new declarative memories. This deficit is called anterograde amnesia. Dr. Milner was quite surprised to learn that H.M. could learn a relatively difficult mirror-drawing task, in which (like anyone else) he got better with repeated trials and retained the skill over time. However, he could not remember ever having done the task before.

Short-Term Memory. Declarative memory can be divided into that which can be recalled for only a brief period (seconds to minutes), and that which can be recalled for weeks to years. Newly acquired learning experiences can be readily recalled for only a few minutes or more using short-term memory. An example of short-term memory is looking up a telephone number, repeating it mentally until you finish dialing the number, then promptly forgetting it as you focus your attention on starting the conversation. Short-term memory is a product of working memory,- the decision to process information further for permanent storage is based on judgment as to its importance or on whether it is associated with a significant event or emotional state. An active process involving the hippocampus must be employed to make a memory more permanent.

Long-Term Memory. The conversion of short-term to long-term memory is facilitated by repetition, by adding more than one sensory modality to learn the new experience (e.g., writing down a newly acquired fact at the same time one hears it spoken) and, even more effective, by tying the experience (through the limbic system) to a strong, meaningful emotional context. The role of the hippocampus in consolidating the memory is reinforced by its participation in generating the emotional state with which the new experience is associated. As determined by studying patients such as H.M., the most important regions of the medial temporal lobe for long-term declarative memory formation are the hippocampus and parahippocampal cortex.

Once a long-term memory is formed, the hippocampus is not required for subsequent retrieval of the memory. Thus, H.M. showed no evidence of a loss of memories laid down prior to surgery,- this type of memory loss is known as retrograde amnesia. Nor was there loss of intellectual capacity, mathematical skills, or other cognitive functions. An extreme example of H.M.'s memory loss is that Dr. Mil-ner, who worked with him for years, had to introduce herself to her patient every time they met, even though he could readily remember people and events that had occurred before his surgery.

Cholinergic Innervation. The primacy of the hippocampus and its connections with the base of the fore-brain for memory formation implicates acetylcholine as a major transmitter in cognitive function and learning and memory. The basal forebrain region contains prominent populations of cholinergic neurons that project to the hippocampus and to all regions of the cerebral cortex

Septal Nucleus Basal Forebrain

the basal forebrain nuclei innervate all regions of the cerebral cortex. Cholinergic neurons in the brainstem's pedunculopontine nucleus provide a major input to the thalamus and also innervate the brainstem and spinal cord. Cholinergic interneurons are found in the basal ganglia. Not shown are peripherally projecting neurons, the somatic motor neurons, and autonomic preganglionic neurons, which also are cholinergic.

the basal forebrain nuclei innervate all regions of the cerebral cortex. Cholinergic neurons in the brainstem's pedunculopontine nucleus provide a major input to the thalamus and also innervate the brainstem and spinal cord. Cholinergic interneurons are found in the basal ganglia. Not shown are peripherally projecting neurons, the somatic motor neurons, and autonomic preganglionic neurons, which also are cholinergic.

(Fig. 7.14). These cholinergic neurons are known gener-ically as basal forebrain nuclei and include the septal nuclei, the nucleus basalis, and the nucleus accumbens. Another major cholinergic projection derives from a region of the brainstem reticular formation known as the pe-dunculopontine nucleus, which projects to the thalamus, spinal cord, and other regions of the brainstem. Roughly 90% of brainstem inputs to all nuclei of the thalamus are cholinergic.

Cortical cholinergic connections are thought to control selective attention, a function congruent with the cholinergic brainstem projections through the ascending reticular activating system. Loss of cholinergic function is associated with dementia, an impairment of memory, abstract thinking, and judgment (see Clinical Focus Box 7.2). Other cholinergic neurons include motor neurons and autonomic preganglionic neurons, as well as a major interneuronal pool in the striatum.

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