I_h activation kinetics (τ_fast) comparison in WT versus Hcn-deficient SGNs (P1–3). (A) Representative currents in WT, Hcn1^−/−, Hcn2^−/−, and Hcn1,2^−/− in response to −144-mV voltage steps. I_h activation kinetics was measured by fitting a double exponential equation (black line; Eq. 2). (B) Summary of mean fast activation time constants. Number of samples and genotype are indicated below. Error bars equal +1 SD. ***, P < 0.001.