Five children in India have helped to answer a question posed in 1688 by Irish philosopher William Molyneux: can a blind person who then gains their vision recognise by sight an object they previously knew only by touch?

The children – aged between 8 and 17 – had been blind since birth but had then undergone surgery to restore their sight. Within 48 hours of the operation, Richard Held and colleagues at the Massachusetts Institute of Technology asked the children to feel a toy block without looking at it.

They were then presented with two similar but distinctive looking blocks – one of which was the block they had just touched – and asked to identify it from its appearance alone.

Their average success rate was just 58 per cent – barely better than chance – suggesting that mapping touch to sight must be learned. (Other tests showed conclusively that the two blocks used were easily distinguishable both by sight alone and by feel alone.)


Natural improvement

However, with no further training the children improved their performance significantly in just five days. “Experience with the natural world was most likely responsible for the improvement,” says Held.

But why is there a lack of hard-wired direct mapping between the two senses? “As a child grows, their sensory apparatus undergoes physical changes, as do the internal representations of the external world. In the face of such variability, a learnable mapping between modalities would offer significant advantages over a hard-wired one,” Held suggests.

Loes van Dam at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, is impressed by the speed with which the children’s performance improved. “The fact that this learning needs so little time suggests that the necessary hardware and wiring was already in place in these children before the operation” despite never having been used, she says.

In 1688, when Molyneux first pondered this problem, it was just a thought experiment. It has proved difficult to investigate even since the advent of surgery to reverse some forms of blindness, because sight restoration operations typically occur in children who are too young to offer unambiguous answers.

Journal reference: Nature Neuroscience, DOI: 10.1038/nn.2795