Activity in motor-sensory projections reveals distributed coding in somatosensation

Cortical feed-back projections to primary sensory areas terminate most heavily in layer (L) 1(1,2), where they make synapses with tuft dendrites of pyramidal neurons. L1 input is thought to provide ‘contextual’ information(3), but the signals transmitted by L1 feedback remain uncharacterized. In the rodent somatosensory system, the spatially diffuse(4) vibrissal motor cortex (vM1)→ vibrissal somatosensory cortex (barrel cortex, vS1) feedback projection may allow whisker touch to be interpreted in the context of whisker position to compute object location(5,6). When mice palpate objects with their whiskers to localize object features(7,8), whisker touch excites vibrissal somatosensory cortex (barrel cortex, vS1)(9) and later vibrissal motor cortex (vM1) in a somatotopic manner(10,11,12,13). Here we used axonal calcium imaging to track activity in vM1→ vS1 afferents in L1 of barrel cortex, while mice performed whisker-dependent object localization. Spatially intermingled individual axons represented whisker movements, touch, and other behavioral features. In a subpopulation of axons, activity depended on object location and persisted for seconds after touch. Neurons in the barrel cortex thus have information to integrate movements and touches of multiple whiskers over time, key components of object identification and navigation by active touch.