When you pick up an object, you probably don't think too hard about the way you grip it. Your hand position and the force you use are adjusted automatically, depending on the weight and texture of the object, to make sure that it doesn't slip through your fingers. But if you know that the object is particularly delicate — a piece of fine glassware or origami, for example — you will be careful to use the gentlest grip possible without dropping it. Kuhtz-Buschbeck et al. have used functional imaging techniques to investigate how the brain performs such precise scaling of grip force.

The subjects in the study were asked to pick up an object between their forefinger and thumb while undergoing functional magnetic resonance imaging (fMRI). They had to hold it in one of three ways: using a natural, automatic grip force; holding it as gently as possible without letting it slip; or increasing the grip force to hold the object more firmly.

Unsurprisingly, holding the object with the right hand caused increased activity in sensory and motor areas on the left side of the brain. The activity increased when the subjects had to hold the object either more strongly or more gently than in the natural grip condition. Rather than increasing in proportion to the grip strength, the brain activity appeared to increase in proportion to the degree of voluntary control required. Of all three conditions, the gentle grip produced the strongest brain activation in primary and secondary sensorimotor areas, perhaps because this condition required much more precise control than either of the others to avoid dropping the object.

The specific areas most affected were the supplementary motor area and the cingulate motor areas, indicating that these regions might be particularly important for precise scaling of fingertip forces. The findings could be relevant for the evaluation of patients with damage to sensorimotor areas of the brain, who often use inappropriately strong grip forces and appear to have lost this ability to scale fingertip forces precisely.