When crossing an unfamiliar room in the dark, many animals can naturally sense their surroundings. For instance, a mouse efficiently navigates in the dark by grazing its whiskers against walls and obstacles. Fan Wang, a professor of brain and cognitive sciences at MIT, discovered how neurons in a mouse's brainstem use signals from the animal's touch-sensitive whiskers to estimate an object's distance from its face. The findings, published in the journal Neuron, unveil critical circuitry the brain utilizes to represent the space surrounding the body.
Mapping Space
The circuit identified by the team is part of the brain's system for creating an egocentric map of space—understanding where things are relative to one's own body. Neuroscientists are aware that the brain uses specialized circuits for this spatial understanding, distinct from its system for mapping space using external landmarks. Wang's team explored how the brain maps the space closest to the body, known as peripersonal space, which is vital for movement and interaction with the environment.
Wang notes that mice are ideal for studying how the brain perceives object distance in peripersonal space, as their whiskers resemble built-in measuring devices. These whiskers, which vary in length, sway back and forth while the animals explore, with mechanical sensations relayed to the brain by sensory neurons at their base. Neurons fire more when a whisker bends closer to the face, conveying proximity information.
Wang's team aimed to determine if the brain uses these signals to create a more precise internal representation of distance beyond just “near” or “far.” Graduate student Wenxi Xiao and Research Scientist Kyle Severson monitored neural activity in a small sensory-processing region in the brainstem where tactile signals from whiskers first arrive. They observed what happened as mice walked on a treadmill while brushing their whiskers against a wall at varying distances.
The Whisker Rule
Some neurons mirrored the behavior of the sensory neurons they received information from, firing more as the wall approached the face, thus forming a proximity-based distance code. Other cells were tuned to specific distances, firing only when the wall was within a certain range. Wang explains, “Each of these neurons represents a specific distance, and together they span the full range reached by the longest whisker, like tick marks on a ruler.”
The team sought to understand how the brain converts proximity signals from different whiskers into an accurate map code for object distances. Wang emphasized that simply listening to individual whisker neurons is insufficient; a brain circuit is necessary to build a unified distance map. Through computational modeling and manipulating neural signaling, the team demonstrated how distances can be calculated by comparing inputs from different sensory neurons.
Their findings indicate that each brainstem neuron comprising the map code receives direct excitatory inputs from proximity-sensitive whisker neurons as well as inhibitory inputs from neurons driven by proximity-dependent touch signals. “Essentially, the inhibitory pathway allows the brainstem to compare two inputs by subtraction,” Wang explains. “If one input signals ‘this is how far it is’ and the other signals ‘this is how far I estimate it to be,’ subtracting one from the other yields an intermediate value.”
Wang notes that despite their significance, the brain's body-centered spatial representations have received little attention from neuroscientists, who know much more about how we understand locations in space relative to landmarks (allocentric maps). She is eager to investigate how the egocentric map code her team discovered integrates with other brain systems to guide movement, social interactions, and other behaviors, and hopes the findings will stimulate further exploration by other groups.
The study was funded by grants from the National Institutes of Health.
Blogger's Review: This research delves into how mice perceive their spatial environment through whiskers, providing vital insights into the neural mechanisms behind navigation in complex settings for both animals and humans. Future studies could not only reveal more about spatial cognition in the brain but also influence the development of robotic sensory systems.