The "highly sensitive person" describes a cluster of traits — deep processing of information, strong empathy, and heightened responsiveness to subtle environmental stimuli — that overlaps substantially with introversion but isn't identical to it. This isn't simply a self-reported personality label; brain-imaging research has found that people who score high on sensitivity measures show measurably more neural activity than others when asked to notice small differences between similar images, supporting the idea that sensitive people are processing their environment in a genuinely more thorough way, not just reporting a subjective impression of being sensitive.
The practical value of this framework is separating "processes information deeply" from "has low motivation" or "is easily overwhelmed" — traits that get bundled together in casual descriptions of sensitive people, but which the underlying research treats as a specific, measurable difference in how thoroughly incoming information gets processed, with real costs (easier overstimulation) and real benefits (richer, more thorough processing) rather than a simple deficiency.