Spatiotemporal encoding of sound level: models for normal encoding and recruitment of loudness

This study explores the hypothesis that sound level is encoded in the spatiotemporal response patterns of auditory nerve (AN) fibers. The temporal properties of AN fiber responses depend upon sound level due to nonlinearities in the auditory periphery. In particular, the compressive nonlinearity of the inner ear introduces systematic changes in the timing of the responses of AN fibers as a function of level. Changes in single fiber responses that depend upon both sound level and characteristic frequency (CF) result in systematic changes in the spatiotemporal response patterns across populations of AN fibers. This study investigates the changes in the spatiotemporal response patterns as a function of level using a computational model for responses of low-frequency AN fibers. A mechanism that could extract information encoded in this form is coincidence detection across AN fibers of different CFs. This study shows that this mechanism could play a role in encoding of sound level for simple and complex stimuli. The model demonstrates that this encoding scheme would be influenced by auditory pathology that affects the peripheral compressive nonlinearity in a way that is consistent with the phenomenon of recruitment of loudness, which often accompanies sensorineural hearing loss.