Experienced order in space is always structurally identical with the functional order in the distribution of underlying brain processes. ~ Wolfgang Köhler
Historically, empiricists held to a one-to-one correspondence between stimulus and sensation, which is termed the constancy hypothesis. Gestalt warped that simple schema, declaring that perception supplements sensation. Brain activity patterns and patterns of conscious experience generally conform with sensation, but the map is not the territory.
According to rationalism, reason furnishes certain elements, without which experience is not possible. ~ Scottish philosopher William Fleming in 1857
According to Gestalt, perceptual fields are caused by underlying brain activity patterns, albeit with a caveat. The perceptual and brain patterns are equivalent, but represent different domains – one physical, the other psychological – and so are not the same. Whence psychophysical isomorphism: iso from the Greek for similar, and morphic for shape.
To Gestaltists, psychophysical isomorphism means that organized brain activity dominates perception, not the stimuli that enter into that activity. Going top-down instead of bottom-up represents a reversal of one of psychology’s oldest traditions. The paradigmatic contrast to behaviorism is especially stark.