Tickling machine: Watch a machine discover the most ticklish spot on feet

The TickleFoot device stimulates the most ticklish spots of the human foot – for women it was found to be the centre of the arch, while for men it is slightly closer to the toes

By Matthew Sparkes

A machine designed to create the optimal stimuli for tickling feet could serve as a stress reliever by inducing uncontrolled laughter, say researchers. The device is battery-operated and can be fitted to any normal shoe.

To figure out where the foot is most ticklish, Don Samitha Elvitigala at the University of Auckland in New Zealand and his colleagues used magnet-driven brushes to stimulate various spots on the sole of the foot. The team conducted a study with 13 participants, seven women and six men, who rated the level of ticklishness they felt on a seven-point scale as a single brush was moved to different locations.

The average value given by women was 5.57, higher than the average of 3.83 from men, and there were also slight gender differences regarding the most ticklish spots on the foot; for women the highest score was given for the centre of the arch, while for men the most ticklish place was slightly closer to the toes.

Researchers used this data to 3D-print a flexible insole that can be inserted into any normal shoe. Called the TickleFoot, it has three tickling actuators that hit the highest-scoring spots of the sole from across all experiments. The device, which is powered by lithium-ion batteries that last for 60 minutes of tickling, can be turned on and off remotely.

Scientists have previously suggested that tickling might have an evolutionary role in social interaction and bonding, and Elvitigala believes that the TickleFoot may be useful as a way to provide remote social interaction between couples or as a stress-relieving device.

But Marlies Oostland at University College London doubts that tickling could help those who are distressed. She has studied how rats respond to tickling and found they need to be in a positive state of mind for it to induce laughter.

But she believes that tickling research in people can yield new knowledge about the operation of the brain, in particular about predictive coding. This is a process by which the brain spots surprises by creating a model of a person’s environment and using it to predict sensory input, then comparing that with the actual experience.

“I think it’s incredibly useful to do research like this,” she says. “The brain uses predictive coding, and one of the naturalistic ways to study that is to study things like tickling, because this is one of the very unusual behaviours where we actively seek out surprising events. For every other behaviour we try to minimise surprise, because when surprise happens it’s probably a predator trying to kill you.”

Journal reference: ACM Transactions on Computer-Human Interaction, DOI: 10.1145/3490496