Figure 9.

Idealized forces generated by airflow acting on the whisker. A, Idealized neurons A and B prefer forces in the x and y directions, respectively. B, In all panels, the whisker is imagined to come out of the page. F_b and F_v are the forces exerted by the airflow on the whisker, as they would be measured at the whisker base. These forces represent the combined effect of distributed airflow along the length of the whisker and the way that the whisker's intrinsic mechanical properties filter those forces. F_b indicates force due to bending (a quasistatic force acting solely in the x direction), and F_v indicates force due to vibration (a dynamic term, containing components in both x and y directions). The magnitudes of F_b and F_v both increase with airspeed, but only F_v changes its direction of action. C, A possible two-neuron readout scheme for airspeed and direction. If neurons A (x direction tuned) and B (y direction tuned) exhibit the same response patterns for a given force stimulus, then airflow direction can be determined by comparing which neuron fires more, and speed can be inferred from the ratio of the firing rates. Specifically, with A and B now representing firing rates FR_A and FR_B: if A > B and A/B is small, then airspeed is high in the x direction. If A > B and A/B is large, then airspeed is low in the x direction. If B > A and B/A is small, then airspeed is high in the y direction. If B > A and B/A is large, then airspeed is low in the y direction. If the neurons do not have the same response profile for a given force input, then this scheme still holds, provided the mapping to firing rate is known. The absolute value of the log ensures that the ratio is symmetric with respect to neuron identity (A/B is equivalent to B/A). D, Relationships between the whisker's mechanical response, the physical parameters of airflow, and the responses of primary sensory neurons in the Vg.