Representations of Time-Varying Cochlear Implant Stimulation in Auditory Cortex of Awake Marmosets (Callithrix jacchus)

Electrical stimulation of the auditory periphery organ by cochlear implant (CI) generates highly synchronized inputs to the auditory system. It has long been thought such inputs would lead to highly synchronized neural firing along the ascending auditory pathway. However, neurophysiological studies with hearing animals have shown that the central auditory system progressively converts temporal representations of time-varying sounds to firing rate-based representations. It is not clear whether this coding principle also applies to highly synchronized CI inputs. Higher-frequency modulations in CI stimulation have been found to evoke largely transient responses with little sustained firing in previous studies of the primary auditory cortex (A1) in anesthetized animals. Here, we show that, in addition to neurons displaying synchronized firing to CI stimuli, a large population of A1 neurons in awake marmosets (Callithrix jacchus) responded to rapid time-varying CI stimulation with discharges that were not synchronized to CI stimuli, yet reflected changing repetition frequency by increased firing rate. Marmosets of both sexes were included in this study. By comparing directly each neuron's responses to time-varying acoustic and CI signals, we found that individual A1 neurons encode both modalities with similar firing patterns (stimulus-synchronized or nonsynchronized). These findings suggest that A1 neurons use the same basic coding schemes to represent time-varying acoustic or CI stimulation and provide new insights into mechanisms underlying how the brain processes natural sounds via a CI device.