So neuroscientists use an approach called “dimensionality reduction” to make such visualization possible—they take data from thousands of neurons and, by applying clever techniques from linear algebra, describe their activities using just a few variables. This is just what psychologists did in the 1990s to define their five major domains of human personality: openness, agreeableness, conscientiousness, extroversion, and neuroticism. Just by knowing how an individual scored on those five traits, they found, they could effectively predict how that person would answer hundreds of questions on a personality test.
But the variables extracted from neural data can’t be expressed in a single word like “openness.” They are more like motifs, patterns
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