Sign theory is a theory of perception developed by Hermann von Helmholtz of how the brain processes and propagates information (i.e. a theory of perception). The main contribution of the theory is that stimuli present themselves as signals and those signals are modified as they propagate forward. The mind makes a series of “unconscious inferences”, or mental adjustments, to construct a “picture” of the experience. Reasonably, we can lay this as a significant step towards representationalism. A key aspect of sign theory is that the rising learned signals are not exact copies of their initial sensations, but instead a sign of these sensations (or representation of these sensations).
Resources: Stanford Encyclopedia of Philosophy