The sight of our own shadows can affect our sense of touch. That’s according to Francesco Pavani and Giovanni Galfano who have shown that seeing the shadow of a part of your body automatically directs your tactile attention to that body part.
Forty-two participants sat with their hands held in front of them. Two lamps above allowed shadows of their hands to be cast just beyond their real hands.
Throughout, the participants’ task was to use a foot pedal (toe or heel) as quickly and accurately as possible to indicate where small lights appeared or vibrations were felt. The lights and vibrations, which occurred at the participants’ index fingers or thumbs, were possible thanks to special gloves that the participants were wearing. Lights could also appear at the location of the shadows.
Sometimes, a couple of seconds before a light or vibration was delivered, a shadow of one or other of the participants’ hands was cast beyond their real hands. These shadows weren’t predictive – a hand shadow appearing didn’t mean a light or vibration was more likely at the corresponding real hand. The key finding is the researchers found the participants were quicker and more accurate at responding to a vibration applied to one of their hands, if a shadow of that hand had been seen a couple of seconds earlier. This shows that seeing a shadow of their hand automatically enhanced participants’ tactile attention to that hand.
The shadows didn’t affect visual attention in the same way. For example, seeing the shadow of their right hand didn’t lead participants to respond more quickly to the appearance of lights at their right hand, nor to lights appearing at the location of the shadow.
Moreover, the tactile-enhancing effect of the hand shadows was lost if the participants wore odd-shaped gloves that disfigured the shape of the shadows. Fake hand shadows with the right shape and location, but which obviously didn’t move in synchrony with the participants’ real hands, also failed to have the same enhancing effect on tactile attention. This shows a shadow has to be recognisable as cast by our own body part for it to exert its effect on our tactile attention.
“Body-shadows attract attention to the body location they refer to, rather than the portion of visual space the occupy”, the researchers said.
Pavani, F. & Galfano, G. (2007). Self-attributed body shadows modulate tactile attention. Cognition, 104, 73-88.