Summary: Touch is an important sense, but the brain processes active and passive touch in different ways. In mice, researchers found that different parts of the thalamus (the sensory center of the brain) respond uniquely depending on whether the animal is actively exploring with its whiskers or passively receiving touch.
Although part of the thalamus was involved in both types of touch, the posteromedial thalamus responded primarily to passive stimuli, especially when these touches were unexpected. This separation likely helps animals interpret and respond appropriately to environmental cues, including potential threats.
Important facts:
- Different routes: Active and passive connectivity is processed in different thalamic areas.
- POM Sensitivity: The posterior medial thalamus primarily responds to passive and unexpected touch.
- Behavioral perspective: By distinguishing between different types of touch, animals can distinguish between self-directed actions and external threats.
Source: PLOS
Although the sense of touch is fundamental to how we and most animals interact with the world around us, much is unknown about how this sense is processed in the brain.
Researchers from the University of Heidelberg and the Ludwig Maximilian University of Munich (Germany) measured the difference in neural activity between active and passive communication in mice.
As reported April 8 in the open access journal PLOS Biology, researchers discovered that active and passive connections occur through different pathways in the brain.
Active touch can be thought of as holding something with your hand, while passive touch is like brushing against something. Rats, like many other animals, use their whiskers to understand the world around them, much like we use our fingers.
To actively touch something, mice move their whiskers and “knock” on the object they are examining. When something touches them, they passively sense the contact through their whiskers.
The researchers measured the movement and neural activity of the mice’s whiskers, specifically the thalamus, the part of the brain that processes all senses except smell. The mice actively interacted with an object, or the researchers used a puff of air to passively stimulate their whiskers.
Basal thalamic activity in rats was found to be generally elevated just before active physical contact, suggesting that the brain may be preparing for self-initiated touch. Interestingly, while one region of the thalamus responded to both active and passive contact, a more specialized area the posterior medial thalamus was primarily responsive to passive touch. This distinction points to a functional division within the thalamus, where different subregions process different types of tactile input, enabling animals to interpret and respond to touch with greater precision.
The posterior medial thalamus showed particularly heightened activity during moments when there was a long pause between breaths. Researchers believe this response may be linked to the mouse’s startle reflex an instinctive reaction to unexpected or potentially threatening stimuli. The correlation between breathing patterns and thalamic activity highlights how sensory processing in the brain is deeply intertwined with physiological states, and how subtle bodily cues can influence how touch is perceived and acted upon.
Touch, in itself, remains a vital sense for animals, including humans, as it supports essential behaviors like foraging, exploration, and environmental awareness. It also serves as a powerful medium for emotional communication and social bonding. The brain’s nuanced interpretation of touch whether active or passive not only aids in navigating the physical world but also enriches social interactions, helping organisms form connections and adapt to their surroundings.
Passive touch, in particular, can also help animals recognize danger, such as whether a predator is lurking nearby.
Touch is not a one-dimensional sense; it plays a dynamic role in how animals interpret and respond to the world around them. By processing different types of touch in distinct areas of the brain, animals may be better equipped to respond swiftly and appropriately to various environmental cues. This neural specialization allows for context-specific reactions whether it’s pulling away from a harmful stimulus or engaging in social bonding through gentle contact ultimately enhancing an animal’s ability to survive and thrive in its natural surroundings.
Importantly, not all touch is perceived in the same way. As the authors of the study emphasized, “Touching and being touched can evoke the same basic sensation, but mean completely different things.” The difference between active and passive touch is critical: actively reaching out to explore an object is processed differently from passively being touched by that same object. This distinction helps the brain assign meaning and relevance to tactile experiences, allowing animals to discriminate between initiating contact and simply receiving it an ability that may influence behavior, decision-making, and social dynamics.
To investigate this, the researchers studied how the brain distinguishes between these two modes of contact. Their findings revealed that a specific area of the brain known as the higher-order thalamus particularly the medial part of the posterior nucleus (MOP) plays a key role in differentiating active from passive touch. This brain region appears to encode contextual information about tactile experiences, offering insight into how complex sensory processing supports adaptive behavior. By uncovering these mechanisms, the study contributes to a deeper understanding of the neural basis of touch and its role in both perception and action.
Abstract
Active and passive connectivity are differentially represented in the mouse somatosensory thalamus.
Active and passive sensory strategies are an integral part of the behavioral repertoire of animals. However, there is a lack of information about the neural circuits underlying these strategies, particularly at the level of the thalamus.
We examined the representation of active and passive whisker deviations in individual neurons of the ventral posteromedial thalamus (VPM) and posterior medial thalamus (POm) in awake rats.
VPM neurons responded robustly to both active and passive whisper deflections, whereas POm neurons showed a preference for passive deflections and responded poorly to active contact.
These response differences cannot be explained by stimulus kinematics, but only partly by the voluntary movements of the animals. Instead, cortical activity significantly influenced the POm response to passive touch.
Inhibition of the barrel cortex significantly reduced the whisper response in the POm and simultaneously increased whisper phase coding. This suggests that the POm receives tactile information from the cortex, which is highly adaptive and organized through irregular events.
Together, these results suggest that there are two thalamic relay streams, with VPM strongly relaying both active and passive deviance, whereas POm sensitivity requires top-down cortical involvement to signal salient events, such as unexpected deviance, that arise in the environment.
