The Proust Effect describes the way a sensory cue—classically a smell or taste, but also a sound or texture—can involuntarily trigger a vivid, often emotionally charged autobiographical memory. The phenomenon is named after Marcel Proust's In Search of Lost Time, in which the taste of a madeleine dipped in tea unlocks a flood of childhood recollection.
Smell-triggered memory is particularly direct because olfactory information bypasses the thalamus and connects rapidly to the amygdala and hippocampus—regions involved in emotion and memory. This is why someone with low voluntary olfactory imagery can still experience powerful Proustian recall when an actual scent arrives. The Proust effect is involuntary and triggered by perception, while voluntary mental imagery is generated internally without external cue. Both can be vivid; only the latter is what Imagination Index measures.