Why do we remember some of our experiences but forget most of them? One important reason for this was suggested by the philosopher Francis Bacon in 1620, when he noted that “memory is assisted by anything that makes an impression on a powerful passion.” My major research contributions focused on two issues related to Bacon's observation: (1) drug enhancement of memory consolidation, and (2) physiological regulation of memory consolidation involving stress hormone release and amygdala activation. Both issues help explain how the emotional significance of events regulates our memory.

Drug Enhancement of Memory Consolidation

When I was a graduate student at the University of California, Berkeley, in the mid-1950s, I initiated experiments investigating the effects of central nervous system (CNS) stimulant drugs on learning in rats. I chose this project for two reasons. First, I thought that understanding the drugs’ effects on learning would provide some insights into the neurobiology of memory. Second, I discovered a 1917 paper published by Karl Lashley (the dominant physiological psychologist/behavioral neuroscientist of the last century) reporting that the stimulant drug strychnine administered to rats, each day before training in a maze, enhanced learning. I repeated Lashley's experiment and replicated his findings. Although this was an exciting finding for me, it offered only the conclusion that the drug enhanced learning performance. As the drug was administered before training, it may have influenced learning simply by influencing the animals’ performance in some way, rather than by influencing the neural processes underlying learning. Thus, it was clear to me that some other approach to the problem was required to understand how the drug acted to enhance learning.

I was aware of a finding, reported a few years earlier, that electroconvulsive shock – delivered to rats each day within a few minutes after they received training – impaired their learning. Those findings provided novel experimental evidence supporting the hypothesis (proposed by Mueller and Pilzecker in 1900) that newly acquired information is initially fragile and becomes consolidated over time after learning.

The consolidation hypothesis suggested that it might be possible to enhance memory by administering stimulant drugs after learning. I investigated this possibility and obtained the first evidence that administration of stimulant drugs shortly after training enhanced learning performance.