Sensory processing does not appear to be a simple bottom-up process: the biasing effects of expectations on perception has long been known, and have been explained as resulting from sensory predictions being generated in higher level cortical areas and sent to lower-level, sensory processing areas. However, there is little direct evidence for this perception prediction process.
Tomorrow at Sensorimotor Journal Club, Adrian Guggisberg will present an article on a fMRI study that investigates this issue: in it, the authors find direct evidence for the existence of a perception prediction in frontal cortex, as well as the top-down connectivity needed to route this prediction to the lower-level sensory processing areas:
Summerfield, C., Egner, T., Greene, M., Koechlin, E., Mangels, J., & Hirsch, J. (2006). Predictive codes for forthcoming perception in the frontal cortex. Science, 314(5803), 1311-1314. (link to pdf of article)
Incoming sensory information is often ambiguous, and the brain has to make decisions during perception. "Predictive coding" proposes that the brain resolves perceptual ambiguity by anticipating the forthcoming sensory environment, generating a template against which to match observed sensory evidence. We observed a neural representation of predicted perception in the medial frontal cortex, while human subjects decided whether visual objects were faces or not. Moreover, perceptual decisions about faces were associated with an increase in top-down connectivity from the frontal cortex to face-sensitive visual areas, consistent with the matching of predicted and observed evidence for the presence of faces.