In two recent studies published in The Journal of Neuroscience and National Science Review, researchers from Dr. SUN Yangang’s lab and Dr. XU Ninglong’s lab at the Institute of Neuroscience, Center for Excellence in Brain Science and Intelligence Technology of the Chinese Academy of Sciences, characterized the coding mechanism of multimodal somatosensations in the primary somatosensory cortex (S1), revealed the neural mechanism of itch perceptual coding in S1 in free-moving animals, and has paved the way for a deeper understanding of the processing and integration of multimodal somatosensory information in the cortex.  

Somatosensation is crucial for many physiological processes. It includes many different submodalities. Cortical processing of somatosensory information is one of the most fundamental aspects in cognitive neuroscience, but neural mechanisms underlying the coding of multiple somatosensations remain largely unknown. Itch represents another submodality of somatosensation, serving as an important protective mechanism. However, cortical representation of itch perception remains unclear. 

Previous studies have found that primary sensory afferents exhibited complicated response patterns to different somatosensory stimuli, and multiple somatosensory stimuli could activate the same neuronal population in the spinal cord, which raises the question of how the nervous system decodes multiple submodalities of somatosensation. Thus, it remains to be determined whether and how S1 encodes itch perception. 

To find out how S1 encodes different somatosensations, researchers adopted the in vivo two-photon calcium imaging in S1 while delivering pruritic, touch, or thermal stimuli in mice. They found that S1 neurons reliably responded to touch and thermal stimuli. With a newly-established optogenetics-based paradigm that allows precisely-controlled pruritic stimulation (opto-itch), they found that S1 encodes the spatiotemporal and intensity aspects of itch sensation. Moreover, they found that touch, thermal, and pruritic stimuli activated largely overlapping neuronal populations in S1. Population decoding analysis revealed that the local neuronal population in S1 encoded sufficient information to distinguish different somatosensory submodalities. 

To examine the perceptual coding capability of S1 in a natural itch perceptual task, researchers took advantage of a novel miniature two-photon microscope to perform calcium imaging in free-moving mice, while applying chemical pruritic stimuli. They found that the itch-induced scratching behavior could be well predicted by the activity of a fraction of S1 neurons, suggesting that a subpopulation of S1 neurons encoded itch perception. With the newly-established opto-itch paradigm, they found that a small fraction of S1 neurons exhibited ignition-like pattern at the detection threshold of itch perception. 

These studies demonstrated the representation scheme of different somatosensations in S1, offering a new perspective for a deeper understanding of the processing and integration of multimodal somatosensory information in the cortex. They also revealed the coding mechanism of itch perception in S1, paving the way for studying cortical representation of itch perception at the single-neuron level in free-moving animals. Furthermore, the newly-established opto-itch paradigm provides a methodological basis for further investigations of itch.