From Valerie Gibbons
Wallingford, Oxfordshire
I read with interest the article on Parkinson’s disease (“Seeing is
unfreezing”, 15 February, p 38). I frequently walk with my husband along a
concrete farm road (neither of us suffers from Parkinson’s disease). About every
15 feet is an inch-wide, earth-filled gap in the concrete running across the
road at right angles. We sometimes tease each other for altering our strides to
step over the “crack” rather than on it, apparently unconsciously because
engrossed in conversation.
We have always assumed this to be a response triggered by the memory of the
childish game of not stepping on the cracks in the pavement for fear that the
bears would get you. The article, however, suggests that our brains are being
fooled into treating the crack as an object to be stepped over not walked on. It
sometimes seems to require a conscious effort not to alter one’s stride to step
over the cracks. This admission is somewhat less embarrassing if it is because
the brain is responding to the visual stimulus of the line on the ground as
opposed to trying to play a childish game.
The phenomenon does not seem to operate on the small, square paving stones
around the house, where the cracks are less than a stride apart and run both
ways. Perhaps the brain perceives these cracks as a flat pattern that can be
walked on not stepped over.
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