Our remarkable sense of touch provides us the feedback that is crucial for successfully manipulating a wide range of objects.
The unconscious synergy between touch and the precision grip is particularly astonishing.
During precision manipulation, humans constantly control their grip force to maintain a safety margin of approximately 25 percent above the minimum force required to prevent held objects from slipping.
The ability to accurately control this safety margin heavily relies on tactile feedback founded on sensed deformations of our fingertips.
Previous studies have demonstrated that, by using this feedback, humans even manage to maintain this safety margin independently of the weight or friction of a lifted object, and when the weight of a held object is perturbed.
However, it is still unknown whether the sense of touch can help us to maintain this safety margin when the friction of a statically held object is perturbed.
As previous methods could not deliver these friction perturbations, we demonstrated the viability of a new friction perturbation method that we employed to fill this knowledge gap.
Here we show that humans in fact do not adapt their grip force in response to an abrupt increase of friction, but do increase their grip force in response to an abrupt decrease of friction.
The asymmetry of these grip adaptations is consistent with current hypotheses on the limitations of our sense of friction.
Our results support the existence of the hypothesized inability of our sense of touch to directly sense an increase of friction.
These findings can help to enhance the haptic interaction between humans and machines, and may inspire the design of an artificial sense of touch that can greatly improve the manipulation dexterity of robotic grippers.