When you want to judge the texture of an object, you actively use your fingers to touch and feel the given surface. The very act of moving your fingers already provides sensory stimulation (e.g.: the skin in between your fingers stretches). How does the brain make sure this sensory stimulus does not drown the information from the salient touch ?
Often, our brain uses corollary discharges (CD), copies of the movement command from the motor regions of the brain, to suppress the sensory consequences of the movement. This recent paper from Yu et al., highlights a thalamocortical mechanism that suppresses movement related activity in the somatosensory cortex of mice.
The authors performed in vivo whole cell recording of neurons in layer 4 of the vibrissal somatosensory cortex, also called barrel cortex, while the animal performs a behavioural task. The animals were trained to lick when a pole touches a particular whisker, in order to get a water reward. When the pole appears in a different position, away from the whisker, the animals were trained to withhold licking. The animal was actively moving its whiskers throughout the experiment to sense if the pole is nearby; this is termed “whisking”.
Sensory information from the whisker follicle reaches the ventral posteromedial nucleus (VPM) of the thalamus and from there it is transmitted to the layer 4(L4) neurons of the barrel cortex. The group observed that both touch and whisking triggered responses in the thalamus. But, the L4 principal neurons were responding selectively to touch. They looked at the reversal potential of the touch/whisking response and found that reversal potential of whisking was more negative. This suggested more inhibitory contribution during whisking.
They then recorded from L4 PV interneurons, which inhibits the L4 excitatory cells, and found that they responded to both touch and whisking like the thalamus. Optogenetically silencing the thalamus VPM neurons silenced the L4 cells and this revealed that they are primarily driven by the VPM neurons.
Typically CDs update the sensory systems to the impending movement. To check if the thalamus was driven by CD, they severed the infraorbital nerve, which carries all the sensory information from the whiskers. This did not affect the motor pathway and the animal was able to move its whiskers. But, the activity of both L4 excitatory cells and PV interneurons was silenced. This shows that VPM thalamocortical loop and not CD, suppresses the movement related activity in L4 excitatory cells, via inhibitory L4 PV interneurons.The authors have not reported if the perception of touch is altered when the thalamocortical loop is manipulated and are planning to further explore the topic.