Friday, November 06, 2015

Critical period for visual pathway formation? - another dogma bites the dust.

India, which may have the largest number of blind children in the world, with estimates ranging from 360,000 to nearly 1.2 million, is providing a vast laboratory that has overturned one of the central dogmas of brain development - that development of visual (and other) pathways must take place within a critical time window, after which formation of proper connections becomes much more difficult or impossible. Until recently, children over 8 years old with congenital cataracts were not considered appropriate subjects for lens replacement surgery. In Science Magazine Rhitu Chatterjee describes a project begun in 2004, Led by neuroscientist Pawan Sinha, that has restored sight to much older children. The story of one 18-year old is followed, who over the 18 months following lens replacement begin to see with clarity that permitted him to bike through a crowded marketplace.

Of the nearly 500 children and young adults that have undergone cataract operation, about half became research subjects. One fascinating result that emerged is that visual experience isn't critical for certain visual function, the brain seems to be prewired, for example, to be fooled by some visual illusions that were thought to be a product of learning. One is the Ponzo illusion, which typically involves lines converging on the horizon (like train tracks) and two short parallel lines cutting across them. Although the horizontal lines are identical, the one nearer the horizon looks longer. If the Ponzo illusion were the result of visual learning, newly sighted kids wouldn't fall for it. But in fact, children who had just had their vision restored were just as susceptible to the Ponzo illusion as were control subjects with normal vision. The kids also fell for the Müller-Lyer illusion, a pair of lines with arrowheads on both ends; one set of arrowheads points outward, the other inward toward the line. The line with the inward arrowheads seems longer. These results lead Sinha to suggest that the illusion is being driven by very simple factors in the image that the brain is probably innately programmed to respond to.

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