ABSTRACT
Until relatively recently, the evidence that the brain is changed by experience was based solely on indirect behavioral assessments of changes: Training can produce long-lasting changes in behavior; therefore, the brain must, it would seem, be changed by experience. There is now abundant evidence that change does occur. One of the recent major advances in neurobiology is an ever-expanding list of long-term changes in brain structure and function elicited by training procedures and neural manipulation. Early demonstrations that animals raised in enriched or impoverished conditions differ in brain weight (Rosenzweig & Bennett, 1978) were followed by anatomical evidence of precise remodeling of synaptic terminations following brain damage (Lynch & Wells, 1978; Steward, 1982), as well as evidence of changes in dendritic branching patterns produced by differential experiences (Floeter & Greenough, 1979; Juraska, Greenough, Elliot, Mack, & Berkowitz, 1980). Neurophysiological changes induced by stimulation include altered cell firing rates during conditioning (Oleson, Ashe, & Weinberger, 1975; McCormick, Clark, Lavond, & Thompson, 1982), as well as long-lasting changes produced by electrical stimulation of certain brain areas, such as long-term potentiation (Lynch, Browning, & Bennett, 1979; Goddard, 1980), and kindling (Racine, 1978; McNamara, Byrne, Dashieff, & Fitz, 1980). Finally, biochemical studies have revealed changes in neurotransmitter receptor sensitivity under a variety of experimental and endogenous conditions (Reisine, 1981) and long-term induction of transmitter-related enzyme activity and synthesis following stress or brain damage (Thoenen, 1975; Costa & Guidotti, 1978; Acheson, Zigmond, & Stricker, 1980).
