Similarities and differences between imagery and perception in early and late visual cortex
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Mental visual imagery has been suggested to require the same brain areas as visual perception. The extent of this overlap has long been investigated but visual cortex has not consistently been found to be involved during imagery. This review investigates the extent to which visual imagery and perception are similar. Behavioral, patient, and neuroimaging studies are reviewed. Specifically, recent decoding studies are reviewed. Decoding not only allows for investigation of overlapping brain activity between imagery and perception, but also reveals overlap in specific representational content related to both processes. Patient and behavioral studies indicate that many similar principles govern imagery and perception. Imaging studies show that different parts of visual cortex are activated during imagery. These activations follow the retinotopic and functional organization also observed during perception. Decoding studies show instances where a classifier trained on perceptual data could classify imagery data, and the other way around. This indicates that representational patterns for imagery and perception are highly alike in several visual areas. However, data also suggest a difference between early and later visual areas. Activity in higher order, functionally organized areas was observed during most imagery tasks. Conversely, activity in early areas depended on task demands. Imagery tasks resulted in V1 activity mostly only when stimulus details such as shape, orientation and retinotopy were important. Differences in specific task demands can strongly influence the extent of overlap between imagery and perception. Previous results should be viewed in light of the differences between task demands and future studies should take into account these differences.