Thoughts and actions are intimately linked, and the mere thought of an action is much like actually performing it. The brain prepares for an action by generating a motor simulation of it, praticising its execution of the movements by going through the motions invisibly. Seeing a manipulable object such as a tool, for example, automatically triggers a simulation of using it – a mental image of reaching out and grasping it with the hand that is nearest to the handle.
Motor simulations and movements are known to influence thought processes. Magnetic stimulation of the motor cortex influences the processing of words related to arm and leg action, whereas polonged movements in one direction slow the comprehension of sentences related to movements in the other. Psychologist Jessica Witt of the Action-Modulated Perception Laboratory at Purdue University and her colleagues now provide further evidence of this link – in a study published online in the journal Psychological Science, they show that motor simulations may enhance the recognition of tools.
Witt and her colleagues examined the effect of a motor interference task on the ability to name tools correctly. In one experiment, they recruited 63 college students, showed them photographs of tools and animals and asked to name the item in each photo as quickly as possible. The objects in the photos were oriented so that the handles of the tools and the heads of the animals faced either left or right. While the participants performed this task, they squeezed a small foam ball with their left or right hand, an activity which engages the same part of the brain required for simulating tool use, and which should therefore interfere with the simulated actions of grasping the observed tools.
The participants were faster at naming the tools when the handle faced away from the hand being used to squeeze the ball than when it was oriented toward that hand. They took, on average, 1.145 seconds to name a tool when its handle faced the hand used to squeeze the ball, and 1.12 seconds to name tools when the handle faced away from it. That is, it took them about 25 thousandths of a second longer to identify the tools whose handles faced the occupied hand. This is a small difference that would otherwise be unnoticeable, but statistically, it is significant. The effect was not seen for the photographs of animals, all of which were identified in just under a second.
In a second experiment, the researchers examined the effects of motor interference on the accuracy of tool identification. 92 different participants were shown the same set of photographs, and asked to identify the object in each. Again, they were asked to squeeze the ball in either their left or right hand during the task. Again, a similar effect was observed: they named the tools in the photos more accurately when the handle was oriented toward the unocuppied hand than when it faced the hand being used to squeeze the ball. And as in the first experiment, the orientation of the animals in the photos had no effect on how accurately the participants named them.
According to these results, then, the simple action of squeezing the ball not only slowed down the participants’ naming of tools, but also slightly reduced their accuracy in naming them correctly. This occured, the authors say, because squeezing the ball involves the same motor circuits needed for generating the simulation, so it interferes with the brain’s ability to generate the mental image of reaching out and grasping the tool. This in turn slows identification of the tools, because their functionality is an integral component of our conceptualization of them. There is other evidence that parallel motor simulations can interfere with movements, and with each other: when reaching for a pencil, people have a larger grip aperture if a hammer is also present than if the pencil is by itself.
But this is not to say that motor imagery is neccessary for recognition of tools. We know that this is not the case from stroke patients who suffer from apraxia, an impairment of skilled movements that includes the inability to use tools. In some apraxic patients, both tool use and recognition are impaired, while in others tool recogniton remains intact. Nevertheless, it appears that motor simulation plays a role in the conceptual processing of objects that can be acted upon, and can enhance their recognition.
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Witt, J.K., et al. (2010). A Functional Role for Motor Simulation in Identifying Tools. Psychological Sci. [Abstract]
Castiello, U. (2005). The neuroscience of grasping. Nat. Rev. Neurosci. 6: 726-736. [PDF]
Tucker, M. & Ellis, R. (1998). On the relations between seen objects and components of potential actions. J. Exp. Psychol. 24: 830-846. [PDF]