Study reveals how the brain processes active and passive touch differently

Though the sense of touch underlies how we and most other animals interact with the world around us, much remains unknown about how this sense is processed in the brain. Researchers from Heidelberg University and Ludwig Maximillan University Munich in Germany measured the neuronal activity differences between active touch and passive touch in mice. As reported April 8^th in the open-access journal, PLOS Biology, the researchers find that active and passive touch are processed by different pathways in the brain.

Active touch can be thought of as grabbing something with your hand, while passive touch would be something brushing against you. Mice, as well as many other animals, use their whiskers to sense the world around them, much like how we use our fingertips. To actively touch something, mice move their whiskers around, "whisking" the object they are investigating. They also passively sense touch through their whiskers when an object touches them.

The researchers measured whisker movement and neuronal activity in the mice, focusing on the thalamus, the part of the brain that processes all senses except for smell. The mice either actively interacted with an object or the researchers delivered a puff of air to passively stimulate the whiskers.

The baseline activity of the thalamus was generally higher before the mice engaged in active touching, and one part of the thalamus responded to both active and passive touch. However, another part called the posterior medial thalamus primarily responded to passive touch. This region also had the highest activity when long intervals had passed between puffs of air, which the researchers propose may be connected to the mouse's surprise at the touch.

Touch is important for many essential activities from finding food to social interaction. Passive touch, in particular, can also help animals sense danger, such as a predator lurking nearby. Processing these types of touch in different regions of the brain may allow animals to respond quickly and appropriately in their natural environments.

The authors add, "Being touched and touching something can lead to the same primary sensation but mean entirely different things. We explored how active and passive touch are represented differently in the brain and find that the higher-order thalamus (POm) strongly differentiates these situations."