Summary: Touch is a vital sense, yet the brain processes active and passive touch using distinct pathways. In mice, researchers found that different regions of the thalamus—the brain’s sensory hub—respond uniquely depending on whether the animal actively explored with its whiskers or passively received a touch.
While one part of the thalamus was engaged in both types of touch, the posterior medial thalamus primarily responded to passive stimuli, especially when those touches were unexpected. This separation likely helps animals interpret and react appropriately to environmental cues, including potential threats.
Key Facts:
- Distinct Pathways: Active and passive touch are processed in different thalamic regions.
- POm Sensitivity: The posterior medial thalamus is especially reactive to passive and unexpected touches.
- Behavioral Insight: Separating touch types may help animals distinguish between self-directed actions and external threats.
Source: PLOS
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 8th 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.”
About this tactile perception and neuroscience research news
Author: Claire Turner
Source: PLOS
Contact: Claire Turner – PLOS
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Active and passive touch are differentially represented in the mouse somatosensory thalamus” by Anton Sumser et al. PLOS Biology
Abstract
Active and passive touch are differentially represented in the mouse somatosensory thalamus
Active and passive sensing strategies are integral to an animal’s behavioral repertoire. Nevertheless, there is a lack of information regarding the neuronal circuitry that underpins these strategies, particularly at the thalamus level.
We evaluated how active versus passive whisker deflections are represented in single neurons of the ventral posteromedial thalamus (VPM) and the posterior medial thalamus (POm) in awake mice. These are the first- and higher-order thalamic nuclei of the whisker system, respectively.
VPM neurons robustly responded to both active and passive whisker deflections, while POm neurons showed a preference for passive deflections and responded poorly to active touches.
This response disparity could not be explained by stimulus kinematics and only in part by the animal’s voluntary whisking state. In contrast, cortical activity significantly influenced POm’s responses to passive touch.
Inhibition of the barrel cortex strongly attenuated whisker responses in POm and simultaneously increased the whisking phase coding. This suggests that POm receives touch information from the cortex which strongly adapts and is gated by rare events.
Together, these findings suggest two thalamic relay streams, where VPM robustly relays both active and passive deflection, while POm’s sensitivity requires top-down cortical involvement to signal salient events such as unexpected deflections, originating in the environment.
