Sensory limits in the rodent whisker system predict an internal forward model for sensorimotor estimation of object touch
© Gyring and Aldo Faisal; licensee BioMed Central Ltd. 2011
Published: 18 July 2011
However, the above biomechanical reasoning for object detection ignores any noise limits to the detectability of beam deformations. To estimate the size of bending that would have to be determined by such mechanoreceptors we analyzed whisker bending using the Euler-Bernoulli beam equation, with relevant parameters taken from the anatomical literature, in order to test this assumption. For a whisker deflection of ~2°, which is factor two above the animal's discrimination threshold, we find a maximum deflection within the follicle on the order of ~1μm. This is likely an overestimate of the true deflection, as we assume a rigid geometry of the follicle and the hinge points. Given the small value of this deflection, and also considering common levels of noise in neurons and detection thresholds in the most sensitive known mechanoreceptors, inner hair cells , it is unlikely that object contact is directly detected.
We propose that object contact may be reliably inferred using recursive state estimation of whisker position, akin to findings in human sensorimotor integration , by combining sensory information with motor neuron commands moving the follicles. However, during object contact, the force from the object on the whisker would render the internal, forward model biased. The recursive position estimate would thus deviate systematically from a direct position estimate of the whisker orientation from the whisking cells, indicating object contact. The use of recursive state estimation by touch cells would also have additional advantages, such as improving the system performance in the face of noisy sensors and muscle contractions, as well as sensory delays. We predict that receptors driving touch and whisking cells, the putative computations may be carried out monosynaptically using pre-synaptic inhibition and dendritic computation.
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