When your hands are tied, your eyes are tied up too

Our eyes and hands operate in wonderful balletic synchrony. When we reach for an object, our eyes jump first, grabbing our intended target visually. Something similar also happens when we watch another person reaching. Our eyes jump ahead to their intended target, as if we were making the same grasping movement ourselves.

In an intriguing new study, Ettore Ambrosini and his team tested whether these anticipatory, vicarious eye movements still occur if our hands are tied up, literally. The researchers reasoned that watching another person's reaching movement triggers the same motor programme in our own brain and it's this programme that guides our anticipatory eye movements. But if our hands are tied, they predicted, the motor programme will stall and the eye movements won't occur so much.

Fifteen participants had their eye movements recorded whilst they watched short videos of a man reaching for one of two tomatoes. Sometimes the man clenched his fist and merely touched one of the tomatoes. This fist hand-shape doesn't provide much predictive information about the kind of movement that's being planned and so participants weren't expected to show much anticipatory gaze behaviour.

In other videos, the man either made a precise, preparatory grasping shape with his fingers, as if he were going to pick up the small tomato, which is what he then did; or he made a whole-hand grasp shape, as if he were reaching for the larger tomato, which is what he went on to do. These two hand-shapes provide clues as to the reaching movement that's underway and were expected to trigger more vicarious, anticipatory eye movements in the participants.

So what actually happened? The man's hand-shapes had just the effect that the researchers predicted. When he formed a precision-shape with his fingers, or a whole-hand grabbing shape, the participants tended to glance ahead towards his intended target, more often and sooner than they did when the man formed a fist. Crucially - and this is the intriguing result - this proactive, vicarious looking behaviour was significantly diminished when the participants had their hands tied behind their backs compared with when their hands were loose in front of them. Having their hands tied seemed to somehow tie up their eyes too.

"... having tied or somehow constrained hands does not allow one to take full advantage of specific motor cues, if any, to grab [with the eyes] the target of the observed action," the researchers said. "This suggests that actions of others are processed most efficiently when we are specifically able to perform the same actions."
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Ambrosini, E., Sinigaglia, C., and Costantini, M. (2011). Tie my hands, tie my eyes. Journal of Experimental Psychology: Human Perception and Performance DOI: 10.1037/a0026570

Further reading: Armchair experts have their limits.

Post written by Christian Jarrett for the BPS Research Digest.