(Click for big) The primary visual cortex is normally understood as being a direct map from the retinal image onto the brain. Apparently we were wrong. From Nature Neuroscience: Perceived size matters Using retinotopic mapping to delineate primary visual cortex, Murray and colleagues examined whether the size of activation patterns in V1 differed when subjects looked at either the front or back spheres. Remarkably, when the sphere that subjects were looking at was perceived to be bigger (due to the contextual cues), activity in V1 spread over a larger area than when it was perceived to be smaller, even though the size of the retinal image produced by the spheres was identical. Activity at the earliest stages of cortical processing does not therefore simply reflect the pattern of light falling on the retina. Somehow the complex three-dimensional cues present in the scene can be integrated to take into account perceived depth in the representation present in V1. There has been work suggesting as much before, but this provides clear evidence. The article goes on: This work is not the first to show that V1 activity can be strongly linked to conscious perception rather than to physical (retinal) stimulation. It is also clear that neural processing in V1 reflects not just feed-forward signals but also feedback influences from higher areas. However, this work not only provides a particularly clear and compelling example of these properties but also, for the first time, clearly links the spatial extent of what we perceive (rather than, for example, contrast or direction of motion) to the spatial extent of activity in V1. More fundamentally, these findings force us to re-evaluate the notion of a ‘hard-wired’ retinotopy in V1. The finding that V1 contains a topographic map of the retinal projection of the visual field has been […]