Melbourne: Scientists have found that threading a needle can be such a challenge because visual stimuli are processed differently when they occur near our hands.
It has been known for some time that there is something special about the space around your hands when it comes to vision, said Dr Stephanie Goodhew of the Australian National University`s Research School of Psychology.
"Visual stimuli are processed differently when they occur near our hands," said Goodhew, who is lead author of a new study published in the Psychonomic Bulletin & Review.
If you are trying to detect flashing targets on a computer screen, you will perform better if your hands are near the screen, Goodhew explained.
However, carrying out tasks that require detailed processing like reading is subtly less efficient when your hands are in the picture, said Goodhew.
Different theories have tried to explain the `near-hand space` - the visual field around your hands.
The "attentional theory" suggests that in this field, the brain allocates greater cognitive resources to attention. But another theory argues this does not explain why tasks like reading are not better performed when your hands are close by.
This competing theory argues that the visual system is ruled by two types of cells. P cells, which are good at processing small spatial details like reading, and M cells that are not so good at picking up details, or colour, but are very fast to respond, `ABC Science` reported.
The theory is that the near-hand space is dominated by M cells, which are good at picking up moving objects, a flashing target but not so good at reading.
Goodhew and colleagues set up an experiment to test both the attentional and M cell theories.
"We found evidence for the M cell theory," she said.
Participants in the experiment were shown a computer screen with two different random shapes on coloured backgrounds. The shapes momentarily disappeared behind a dark rectangle and when they re-emerged some had changed.
Either the shape or the background colour, or both, had been changed. Changing the background colour made the participants slower at telling if the shapes were the same or different.
But, this only happened when the participants` hands were not near the screen. As soon as their hands were put near the screen, changing the colour made no difference to performance of the task, said Goodhew.
"In near-hand space we found that they didn`t use the colour information. It didn`t affect their responses at all. Given that M cells don`t process colour, that`s consistent with a pattern of M cell processing," she said.
This finding supports the idea that M cells, rather than P cells used for reading, are involved in processing visual stimuli near the hands.
"It would explain why threading a needle can be so difficult, because the M cells are not good at seeing details near your hands," she said.