Key Points

• Learning is an experience-dependent generation of enduring internal representations and/or modifications in such representations. Memory is the retention of these experience-dependent changes over time.

• There are multiple forms of learning and memory, and multiple but distinct regions of the brain serve as loci for learning and memory.

• Declarative memory (explicit memory) encompasses the memory for facts and concepts as well as the memory for events. The medial temporal lobe and diencephalon are critical for declarative memories.

• Nondeclarative memory (implicit memory) operates at an unconscious level. It encompasses the memory for skills and habits, priming, examples of associative learning such as classical conditioning and operant conditioning, and examples of nonassociative learning such as sensitization and habituation. Anatomic loci for nondeclarative memories are diverse.

• At least short-term memory involves changes in existing neural circuits. These changes may involve multiple cellular mechanisms within single neurons.

• Second-messenger systems appear to play a role in mediating cellular changes.

• Changes in the properties of membrane channels are commonly correlated with learning and memory.

• Long-term memory requires new protein synthesis and growth, whereas short-term memory does not.

An important goal of neurobiology is to explain the anatomic, biophysical, and molecular processes occurring in the nervous system that underlie learning and memory. Specifically, what parts of the nervous system are critical for learning? How is information about a learned event acquired and encoded in neuronal terms? How is the information stored, and, once stored, how is it retrieved? Most neuroscientists believe that the answers to these questions lie in understanding how the properties of individual nerve cells and their synaptic connections change when learning occurs. Extensive behavioral, anatomic, physiologic, and molecular analyses over the past decade have revealed what appear to be some general principles of learning and memory. A list of these principles might include the following: (1) there are multiple forms of learning and memory; (2) multiple but distinct regions of the brain serve as loci for learning and memory; (3) at least short-term forms of memory involve changes in existing neural circuits; (4) these changes may involve multiple cellular mechanisms within single neurons; (5) second-messenger systems appear to play a role in mediating cellular changes; (6) changes in the properties of membrane channels are commonly correlated with learning and memory; and (7) long-term memory requires new protein synthesis and growth, whereas short-term memory does not.

WHAT ARE LEARNING AND MEMORY?

Although we all have an intuitive understanding of learning and memory, formulating a rigorous definition can pose some difficulty. One traditional definition of learning is a change in behavior as a result of experience. This definition is less than ideal, however, because some examples of learning do not involve overt behavioral changes. For example, learning may represent a change in an internal state that is behaviorally silent and therefore represents a process that is a potential for a change in behavior, rather than an immediate change in behavior. In addition, any definition of learning should distinguish learning from maturational changes and from changes in behavior produced by injury or fatigue. A fairly general definition of learning has been provided by Dudai (1989): Learning is an experience-dependent generation of enduring internal representations and/or modification in such representations, whereas experience excludes events related to maturation, injury, and fatigue. Memory can be defined as the retention of these experience-dependent changes over time. The temporal domains of memory vary considerably, from short-term forms lasting minutes, such as the memory of a telephone number, to long-term forms lasting days, weeks, or lifetimes, such as the memory of a childhood experience. In some cases, a short-term memory can be stabilized into an enduring long-term form. This process is referred to as consolidation. Finally, we need to define retrieval, the process that allows memory to be accessed. Retrieval is the use of memory in neuronal and behavioral operations (Dudai, 1989).

Amnesia is a disorder of memory which has two broad forms: retrograde and anterograde. Retrograde amnesia refers to the loss of memories that were acquired before the amnesia and is usually temporally graded. For example, a patient with head trauma will generally have the greatest loss of memory for events immediately preceding the trauma, whereas more remote memories will be preserved. This temporal gradation is likely due to the interruption of the consolidation process (see earlier). Anterograde amnesia refers to an inability to form new memories.

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