The myth of whisker-mediated tactile motion direction sensitivity in rodents

It is well-established that neurons in the whisker area of somatosensory cortex and thalamus exhibit directional selectivity. Direction preference is also observed in the response of peripheral and trigeminal neurons. Although the directional selectivity in the periphery and brainstem originates in the uneven arborization of nerve terminals around the follicle, direction-dependent differences in the temporal profile of synaptic excitation and inhibition in barrels and non-linear dendritic processes also are believed to contribute to the directional tuning in the cortex. The directional selectivity decreases along the ascending whisker-to-barrel pathway. The functional and behavioural correlate of directional selectivity in the whisker-to-barrel system is not understood and it is not clear whether rats perceive the direction of whisker deflection. The aim of this project is to quantify the accuracy with which rats/mice perceive the direction of individual whisker deflections. Several lines of research provide evidence against an angular selectivity readout such that leads to a sensation of direction. First, neurons with multi-whisker receptive fields in cortex and thalamus do not necessarily exhibit the same angular preference to different whiskers in their receptive field. Second, in the visual system, orientation selectivity arises from specific convergence of directionally non-tuned thalamic inputs in layer IV of striate cortex and gives rise to selectivity to more complex features along the cortical visual hierarchy. On the contrary, in the whisker-mediated touch system, directional selectivity exists in the peripheral sensory afferents innervating vibrissae follicles and gets weaker along the ascending whisker-to-barrel pathway.