How tool use is encoded in the brain

How is tool use encoded in the brain?

Most movements involving tools involve the complex manipulation of objects in space, and it is possible that they could represented in the brain in this way -- i.e. as objects in space. On the other hand, the purpose of tools is to extend the range of motions available to the body, so it is also possible that tool use could be encode as an extension of the body representation onto the tool.

Some cunning work by Umilta et al. at the University of Parma shows the second option is the case. The brain represents tools by incorporating them into representations about the body.

To test this, Umilta et al. trained monkeys to reach for a piece of food using either just their hand, or regular (squeeze to pinch) and reverse (squeeze to release) pliers. After sufficient training, the researchers implanted electrode in two areas of the monkey's brain -- F1 and F5 -- to see how their brain represented these motions.

F1 is an area in the monkey's brain that is sort of like the primary motor cortex in humans. F5 is an area in the monkey's brain that is sort of like the premotor cortex. Both areas are involved in the initiation and coordination of motor acts. The areas of the brain are depicted in the diagram below (from Supplementary Fig. 6):

By doing the recordings with the monkeys reaching with just their hands, the researchers were able to identify those neurons in these areas that were active when the monkey grabs for food. The researchers compared these recordings to the activity in the same neurons when the monkeys used either type of pliers.

Below is an experimental video of the researchers doing the testing. The red lines indicate the direction of the animals gaze. I think the food is colored yellow. (From Supplementary Movie 1)

The researchers found when they performed this experiment that the same neurons that are active when the monkey grabbed for food with its hand are involved when the monkey grabbed for food using the tools. This was true for both the regular and reverse pliers -- suggesting that the nature of the motion involved is not the reason for the activity. The regular and reverse pliers require different types of motion, but similar types of activity were observed in F1 and F5 for both tasks.

(It is a tad more complicated than that. It turns out that the activity was always similar for both pliers in F5. In F1 on the other hand, the cells divided into two types. There were cells that coded for the particular motion type -- regular vs. reverse -- that they called F1m cells. There were also cells that are similar to F5 cells, coding the goal rather than the type of movement called F1g cells. This is not really surprising actually because F5 probably has more abstract information about movement that F1. F5 is higher in the movement-initiating brain hierarchy.)

These results suggest two important conclusions:

• The similar activity in these regions during grasping and tool use suggests that in a sense the representation of the tool has become part of the representation of the body. This change in the abstract person has important implications to how we understand learning to use tools. It appears that you really do view the tool as an extended appendage.
• It also appears that the goal of the task is represented in these areas much more than the muscular components. This is suggested by the common representation when using either the regular or reverse pliers. This is also not that surprising. F5 is a region in the monkey brain that we knew had neurons that encode goals rather than steps. For example, neurons called mirror neurons have been identified there. These neurons are active either when the monkey itself or another monkey it can see performs a similar task. These neurons have been speculated to form the basis of a system for imitation (and possibly even the basis for a theory of mind, but that is is a little speculative).

The authors summarize the significance of their research thusly:

The use of pliers requires the capacity to separate a proximal goal (grasp the pliers) from a distal goal (grasp an object), a distinction that is not present in natural actions in which the two goals coincide. Which transformations had occurred in the motor system once the monkey has learned to use pliers? Our results show that the end effect of training has been the transfer of the temporal discharge pattern that controls hand grasping to the tool use, as if the tool were the hand of the monkey and its tips were the monkey's fingers. This transfer occurs not only when the mechanics of pliers mimics that of the hand (normal pliers), but also when the mechanics is its exact opposite. Also in this case the distal goal, i.e., grasp the object by opening the hand, is the pivotal element around which movements are organized.

This incorporation of the tool in the motor act representation is somehow reminiscent of the finding of Iriki et al., who showed that, with practice, a rake became part of the acting monkey body schema. The present finding shows that, in addition to being incorporated into the body schema, the tool, after learning, is coded in the motor system as if it were an artificial hand able to interact with the external objects, as the natural hand is able to do.

How can this embodiment take place? The most plausible explanation is that this occurs because both F5 and F1 contain neurons that code the goal of the motor act. Some previous evidence suggested that the goal of motor acts, rather than the movements, is also coded in the cortical motor areas that controls reaching movements. The present data indicate that a goal-coding mechanism is also at the basis of the much more complex motor organization as that of grasping and that it underlies tool embodiment in primate behavior. (Emphasis mine. Citations removed.)

Fascinating stuff. I take away from this paper that the motor system in the brain is very hierarchical in encoding abstractions. These abstractions are activated, and that activation filters down to various subsystems that actually execute the motor act. But the degree to which goals and sets are implemented in the brain really astonishes me.

(Hat-tip: Science)

Umilta, M.A., Escola, L., Intskirveli, I., Grammont, F., Rochat, M., Caruana, F., Jezzini, A., Gallese, V., Rizzolatti, G. (2008). When pliers become fingers in the monkey motor system. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0705985105