Unconscious Processes

Unconscious Inference

One influential and persistent notion in the study of perception is the idea that it represents or depends upon unconscious inference. This phrase is conventionally attributed to the 19th-century physiologist Hermann von Helmholtz, who proposed that our percepts are not the direct products of sensory stimulation alone. Helmholtz argued that sensory stimulation is so impoverished and unconstrained that it cannot account for the richness and apparent precision of our conscious experience. Instead, the phenomenal character of visual awareness must arise from a series of unconscious reasoning processes applied to sensory input. These processes consist of a series of inferences based on premises about knowledge of the world acquired through experience. Although Helmholtz provided the first explicit conceptual formulation, the underlying ideas date back to some of the earliest accounts of vision. For example, Greek philosophers such as Euclid believed that the apparent size of an object reflected the visual angle it subtended at the eye. But if we hold our thumb up to the night sky, we do not perceive our thumb to be the same size as the moon even if they subtend the same visual angle at the eye. Five centuries after Euclid, the Greek philosopher Ptolemy therefore proposed that apparent size depended both on the visual angle an object subtended at the eye and its perceived distance. How big something appears therefore depends both on sensory stimulation and on inferences about how far away it appears to be. Perception therefore depends on the combination [Page 1020]of sensory input with acquired knowledge in an inferential process that does not reach awareness. Helmholtz believed that unconscious inferences underlying perception were similar in character to those that underlie our ability to perform general or scientific reasoning. The notion that our ability to engage in rational argument and unconscious perceptual inference relies on common cognitive machinery associated with general reasoning or intellect has subsequently come under sustained attack. In particular, the mechanisms underlying sensory perception appear to be more encapsulated, domain-specific, and inference-like rather than general. Such a narrower account of unconscious inference can explain how visual illusions (failures of inference) can persist even when the observer has knowledge of the nature of the illusion. Nevertheless, the notion that perception reflects an ability of the perceiver to combine information provided by the environment with knowledge acquired through experience remains a powerfully articulated central influence on contemporary discussion of perception.

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