blindsight

Blind brain `sees' rapid movement
21 Sep 96

                                A BLIND man who can "see" rapid motion has helped refine ideas about
                                the visual system, and possibly about consciousness as well. There
                                may be "separate consciousnesses for different attributes", said Semir
                                Zeki, a neurobiologist at University College London, speaking at the BA
                                last week.

                                Zeki's patient, a man in his thirties identified only as GY, was blinded
                                in an accident when he was seven. Later he reported being able to
                                detect moving objects such as cars. Zeki confirmed this: in the
                                laboratory, he found that GY was able to pick out fast-moving objects
                                and identify the direction they were travelling.

                                But this should not be. Scientists have believed for decades that
                                visual information passes first through the primary visual cortex in
                                the brain, known as V1, before being parcelled out to specialised
                                centres for the perception of colour or motion. Zeki himself was one
                                of the first to identify these functionally specialised areas for vision.
                                But GY's primary visual cortex does not function at all, says Zeki, so
                                the messages could not have been passed on.

                                Two types of brain scan confirmed that when GY was "seeing" motion,
                                there was activity in the motion centre, called V5, but none in the
                                primary visual cortex, V1. The primary visual cortex was being
                                bypassed, says Zeki. Messages heading straight to V5 "can lead to
                                conscious vision", he concludes. "There is substantial autonomy."

                                Zeki was also puzzled by another finding. Although GY could detect an
                                object that moved 7° across the field of vision in less than a second,
                                he could not see anything that moved more slowly. Slower motion, it
                                appears, is routed through the primary visual cortex and then on to
                                V5.

                                Zeki has studied patients whose primary visual cortex is intact but
                                who have no motion centre. These people cannot see motion at all:
                                TV screens, moving trains or even people's lips moving when they
                                speak all appear stationary.

                                The primary visual cortex is mature at birth, but the specialised
                                centres are thought to develop through experience during childhood.
                                The fact that GY had seven years with a functioning V1 may explain
                                why his motion centre was able to develop. "I think there are more
                                blind people who are able to see motion than we know," says Zeki.

                                Alison Motluk
                                From New Scientist magazine, vol 151 issue 2048, 21/09/1996, page
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