A new study to be published in the journal Psychological Science challenges the long-held notion that when we see a picture of a left or right hand, our brain engages our “motor imagination” and uses muscle control processes to flip over mental images of our hands to decide which is in the picture. The researchers say their different understanding of how the brain solves the problem of “hand laterality” helps us understand why amputees with phantom itches or clenching in missing limbs get relief by looking at the mirror image of the other limb.
Together with colleagues from the University of California Santa Barbara, Dr Shivakumar Viswanathan, a computer scientist who now studies how the brain solves action problems, suggests that the brain does not undergo mental gymnastics when trying to deal with “left or right hand?” problems, but does something else called “binding” that brings together visual and proprioceptive information to solve what is really a low-level sensory problem.
Viswanathan told the press last week:
“For decades, the theory was that you use your motor imagination.”
Because of the response times, psychologists thought we flipped mental images of each hand to find one matching the picture, and this high-level cognitive feat involved using the same brain processes that command muscles to move.
But Viswanathan and colleagues suggest that a lower-level brain process first “binds” the seen hand to the correct felt hand and the feeling of moving only comes in afterwards, at the cognitive level, when the brain tries to align the felt hand with the seen hand.
To arrive at their alternative understanding, Viswanathan and colleagues invited participants to take part in two experiments.
In the first experiment, the participants could not see their hands, which they held palm down. They were put in two groups and shown shapes of hands at different angles. A colored dot showed whether the hand shape was palm up or palm down.
In one group, the participants were shown the shape first, followed by the dot, and in the other group, they were shown the dot first, and then the shape.
In both groups, the participants put the shape and dot together mentally and then showed which hand they thought it was by pushing a button with that hand.
But when the participants saw the dot and the hand shape appear at the same time, the ones in the first group felt their right hand move when they saw a left hand, and their left hand move when they saw a right hand. The participants in the other group, however, always felt movement in the correct hand.
This experiment established that an earlier sensory process made the decision, and this was followed by the perception of the seen hand, which was different in the two groups (the researchers established this behavioral difference from the differences in response times).
In the second experiment the participants were told which hand was being shown and had to judge whether the hand was palm up or palm down. They had to show their answer using only one hand.
This time the participants only felt a movement in their hand when it matched the appropriate palm-down hand shape, but not at other times.
Viswanathan said that even though the experiments did not involve right or left judgements, it was the automatic “binding” of the seen and felt hands that came first, followed by the illusory movement.
The researchers say their findings should help us understand why amputees, when they feel a “phantom” itch or clenching sensation in the absent limb, sometimes get relief when they look at the other limb in the mirror. Seeing the opposite limb evokes the low-level sensory process to mentally relieve the discomfort, by “binding” vision and feeling, they suggest.
Written by Catharine Paddock PhD