Steven Pinker -- How the Mind Works - Hampshire High Italian ...

The Mind's Eye | 289patients named only the buildings that would be visible on the right—neglecting the left half of imaginary space! Then the patients were askedto mentally walk across the square and stand on the cathedral steps facingthe piazza and describe what was in it. They mentioned the buildingsthat they had left out the first time, and left out the buildings that theyhad mentioned. Each mental image depicted the scene from one vantagepoint, and the patients' lopsided window of attention examined theimage exactly as it examined real visual inputs.These discoveries implicate the visual brain as the seat of imagery,and recently there has been a positive identification. The psychologistStephen Kosslyn and his colleagues used Positron Emission Tomography(PET scanning) to see which parts of the brain are most active whenpeople have mental images. Each subject lay with his head in a ring ofdetectors, closed his eyes, and answered questions about uppercase lettersof the alphabet, such as whether B has any curves. The occipitallobe or visual cortex, the first gray matter that processes visual input, litup. The visual cortex is topographically mapped—it forms a picture, ifyou will. In some runs, the subjects visualized large letters, in others,small letters. Pondering large letters activated the parts of the cortex representingthe periphery of the visual field; pondering small letters activatedthe parts representing the fovea. Images really do seem to be laidacross the cortical surface.Could the activation be just a spillover of activity from other parts of thebrain, where the real computation is being done? The psychologist MarthaFarah showed that it isn't. She tested a woman's ability to form mental imagesbefore and after surgery that removed her visual cortex in one hemisphere.After the surgery, her mental images shrank to half their normal width. Mentalimages live in the visual cortex; indeed, parts of images take up parts ofcortex, just as parts of scenes take up parts of pictures.Still, an image is not an instant replay. It lacks that pungency andtang, though not because it has been bleached or watered down: imaginingred is not like seeing pink. And curiously, in the PET studies themental image sometimes caused more activation of the visual cortex thana real display, not less. Visual images, though they share brain areas withperception, are somehow different, and perhaps that is not surprising.Donald Symons notes that reactivating a visual experience may well havebenefits, but it also has costs: the risk of confusing imagination with reality.Within moments of awakening from a dream, our memory for its plotis wiped out, presumably to avoid contaminating autobiographical mem-