Eric Kandel, initially trained in psychiatry, decides in the 1960s to address the question of what physically happens in the brain when a memory forms, choosing a model organism extremely far removed from the complexity of the human brain: the sea slug Aplysia californica, whose neurons are exceptionally large — up to a millimeter in diameter, visible to the naked eye — allowing microelectrodes to be inserted easily to directly record the activity of individual cells. Studying Aplysia's defensive withdrawal reflex, Kandel and his collaborators succeed for the first time in tracing a complete neural pathway, from sensory stimulus to resulting motor behavior, and show that learning translates into measurable changes in the strength of synaptic connections between specific neurons in that circuit. The central finding distinguishes two completely different molecular mechanisms: short-term memory, which involves functional modifications of proteins already present at the synapse with no need for new synthesis, and long-term memory, which requires activation of gene expression and synthesis of new proteins that generate lasting structural and anatomical changes — new synaptic connections — in the neural network. Kandel later extended these cellular principles discovered in a relatively simple mollusk to the mammalian hippocampus, showing that the same fundamental molecular mechanisms of synaptic plasticity operate, with variations, across the evolutionary scale. The work established the dominant conceptual framework of modern neuroscience for how experience physically becomes lasting memory, with direct implications for understanding cognitive disorders and for research into psychotherapy as an intervention that literally modifies the brain's synaptic structure.