Neuroscientists from the University of Geneva (UNIGE) have observed what happens in the brains of mice whose forepaws perceive vibrations.
They discovered that neurons in the somatosensory cortex are activated in a manner similar to those in the sound-reactive auditory cortex.
Using two-photon microscopy, Daniel Huber’s team visualized the activity of hundreds of neurons in a mouse’s somatosensory cortex as vibrations of different frequencies were delivered to its forepaw.
Like in the auditory cortex, individual neurons were selectively tuned: they strongly responded to some frequencies and less so to others.
In other words, a mouse is unable to distinguish a high-frequency vibration with a low amplitude from a low-frequency vibration with a higher amplitude.
In a second step, the researchers sought to identify the origin of the somatosensory stimuli involved by performing a detailed histological analysis of Pacinian corpuscles in mouse forelimb.
Pacinian corpuscles are known to transduce high frequency vibrations in mammals and are densely expressed in the dermis of primate fingertips.
“Surprisingly, we found that the vibration responses in the mouse brain stem from Pacinian corpuscles located on the forearm bones, whereas they were totally absent in the paw’s skin,” explains Géraldine Cuenu.
Using optogenetics, the scientists confirmed the link between cortical responses and the particular configuration of mechanoreceptors in the forelimbs.
“Our discoveries probably reveal the existence of an ancient sensory channel, which could be an evolutionary precursor of hearing,” concludes Mario Prsa.
This somewhat vestigial, yet highly sensitive modality might also explain how we are able to identify subtle clues linked to upcoming natural disasters, or why construction or traffic causes nuisances even when inaudible.