(As with the old military adage, “When the terrain differs from the map, trust the terrain.”) Although little is known of the neuronal mechanisms by which probability influences this process (but see Girshick, Landy and Simoncelli, 2011), there are Sorafenib mouse well known psychopathologies and drug-induced alterations of sensory processing in which the imaginal component dominates regardless of its likelihood or the quality of stimulation, and perceptual experience becomes hallucination. By this view, visual hallucinations are a pathological product of the same top-down system for pictorial recall that serves perceptual inference—a view supported by the finding of activity patterns in visual

cortex that are correlated with visual hallucinations in cases of severe psychosis (Oertel et al., 2007). Moreover, evidence indicates that sensory cortex is less sensitive to exogenous stimulation during hallucinations (Kompus et al., 2011), suggesting that the imaginal component is given a competitive advantage. A particularly striking pathological case of overreaching imaginal influences on perception is Charles Bonnet Syndrome (CBS)—a bizarre disorder characterized by richly detailed visual imagery in individuals who have recently lost sight from pathology to the retina (e.g., macular GPCR Compound Library high throughput degeneration) or optic nerve (Gold and Rabins, 1989). The images perceived are

commonly elicited by associative cues. For example, upon hearing an account of the revolutionary war, one patient with CBS reported a vivid percept of a winking sailor: “He had on a cap, a blue cap with a polished black beak and he had a pipe in his mouth” (Krulwich, 2008). Similar imagery-dominated perceptual experiences have been reported

for normal human subjects artificially deprived of vision for extended periods (Merabet et al., 2004). In all of these cases in which stimulus properties and probabilities, or myriad pathological and pharmacological states, influence the perceptual distinction between stimulus and imagery, we can assume that Tyrosine-protein kinase BLK there are patterns of neuronal signaling correlated with that distinction. Likely candidates are those brain regions found to be differentially engaged in the neuropsychological and fMRI studies of explicit imagery cited above. Much additional work is needed, however, to identify the specific mechanisms and neuronal events that underlie these effects. This review has focused on vision because it is the sensory system for which there exists the greatest understanding of perceptual experience as well as relevant neuronal organization and function. There are nonetheless good reasons to believe that the same principles for associative recall and perception pertain to all senses. Moreover, these principles apply well to interactions between sensory modalities. Perceptual phenomena reflecting such interactions can be robust and dramatic.