Sound detected by a single auditory hair cell is relayed by 10-30 type I spiral ganglion neurons (SGNs), the primary sensory neurons of the auditory system, to the central nervous system. Although these neurons exhibit differences in spontaneous firing rates and activation thresholds, it is unclear whether the physiological diversity reflects their endogenous heterogeneities and whether they differentially process auditory information. In Ulrich Mueller’s lab, I demonstrated that type I SGNs consist of three molecularly distinct subtypes (named type IA, type IB and type IC), using single-cell RNA sequencing (scRNAseq). These three subtypes exhibit distinct innervation patterns and spontaneous firing rates. Moreover, these subtypes are specified postnatally in an activity-dependent manner. Deafness mutations that disrupt hair cell mechanotransduction and glutamatergic communications from hair cells to SGNs disrupt this SGN subtype specification. To further characterize the functional differences of SGN subtypes, I have generated CreERT2 mouse lines that specifically target the type IA and IC populations. Based on their physiological properties, the low spontaneous firing neurons with high activation thresholds are predicted to be dispensable for hearing sensitivity, but their large dynamic ranges are expected to play critical roles in hearing in noise. Consistently, silencing the type IC subtype significantly impaired hearing in a noisy environment, but not in quiet. Surprisingly, inhibition of the type IC population also affected frequency discrimination, suggesting previously unappreciated fundamental functional differences among SGN subtypes.