The sense of touch is important but often taken for granted in daily life because it seems simple and automatic. New research suggests that the apparent simplicity of tactile sensation comes from a clever two-stage brain circuit. By manipulating this circuit with light-driven optical genetic tools, researchers made laboratory mice literally “lose touch” with their surroundings as their feet became unable to sense rough or smooth surfaces.
The study published in Neuron by a team at the RIKEN Brain Science Institute in Japan, shows that the perception of touch relies on two signals, one from the skin to the brain and another within the brain itself. This second signal relays the first signal from a lower-level brain area to a higher one and then boomerangs it back to the lower level. The higher brain area is required for touch perception and its inactivation renders mice unable to use sensations in their footpads to discriminate different floor textures.
The research team led by Dr. Masanori Murayama observed the brains of mice after touching their paws and saw immediate activity in the sensory cortex — the brain area that receives signals from the skin. Unexpectedly, they recorded a second slower source of activity tens of milliseconds after the first. Murayama explains, “we investigated the source of this second activation and found that high level motor cortex receives information from the sensory cortex and sends it back to the sensory cortex. This means that, for tactile perception, the flow of information from the skin to brain requires communication not only from the periphery to the brain but also reverberation between two brain areas.”
While it was previously thought that one signal from the skin to the brain was sufficient to produce touch sensation, this study reveals that without the second signal, mice cannot feel or use the incoming sensory information, suggesting that they may not even perceive differences in texture. To investigate this idea, the researchers trained mice to distinguish two different floor textures, rough or smooth, by associating one of them with a food reward. When they prevented the second signal by shutting off the responsible neurons with light-activated optical genetic technology, the mice could not distinguish differences in floor texture.
“Our results show that a reverberant neural circuit from sensory cortex to higher motor cortex is required for the perception of touch”, said lead researcher Satoshi Manita. Murayama speculates that this two-stage circuit design may be a safety mechanism to ensure continuous accurate perception even when distracted by other senses, such as when feeling a steering wheel while concentrating on the road. “This form of perception, an external signal and its internal rebound, may extend to other senses,” concludes Murayama, “and we may find that communication between brain areas refines perception for more accurate and integrated behavior.”
About this optogenetics research
Source: Adam Phillips – RIKEN Image Source: Image is credited to RIKEN Original Research:Abstract for “A Top-Down Cortical Circuit for Accurate Sensory Perception” by Manita S, Suzuki T, Homma C, Matsumoto T, Odagawa M, Yamada K, Ota K, Matsubara C, Inutsuka A, Sato M, Ohkura M, Yamanaka A, Yanagawa Y, Nakai J, Hayashi Y, Larkum ME and Murayama M in Neuron. Published online May 21 2015 doi:10.1016/j.neuron.2015.05.006
A Top-Down Cortical Circuit for Accurate Sensory Perception
Highlights •Somatosensory (S1) and secondary motor (M2) cortices form a top-down circuit •Sensory stimulation induces sequential S1 to M2 and M2 to S1 input patterns •M2 evokes a dendritic spike and persistent firing in S1 layer 5 (L5) neurons •Optogenetic inhibition of M2 to S1 axons degrades accurate sensory perception
Summary A fundamental issue in cortical processing of sensory information is whether top-down control circuits from higher brain areas to primary sensory areas not only modulate but actively engage in perception. Here, we report the identification of a neural circuit for top-down control in the mouse somatosensory system. The circuit consisted of a long-range reciprocal projection between M2 secondary motor cortex and S1 primary somatosensory cortex. In vivo physiological recordings revealed that sensory stimulation induced sequential S1 to M2 followed by M2 to S1 neural activity. The top-down projection from M2 to S1 initiated dendritic spikes and persistent firing of S1 layer 5 (L5) neurons. Optogenetic inhibition of M2 input to S1 decreased L5 firing and the accurate perception of tactile surfaces. These findings demonstrate that recurrent input to sensory areas is essential for accurate perception and provide a physiological model for one type of top-down control circuit.
“A Top-Down Cortical Circuit for Accurate Sensory Perception” by Manita S, Suzuki T, Homma C, Matsumoto T, Odagawa M, Yamada K, Ota K, Matsubara C, Inutsuka A, Sato M, Ohkura M, Yamanaka A, Yanagawa Y, Nakai J, Hayashi Y, Larkum ME and Murayama M in Neuron. Published online May 21 2015 doi:10.1016/j.neuron.2015.05.006