Neuronal and behavioral discrimination between upward and downward pulse interval modulation in cochlea implanted gerbils

Speech coding strategies for cochlear implants commonly use amplitude modul ations of constant high rate pulses to differentially stimulate separate fr equency channels in the cochlea, Thereby, time domain information in the fi ne structure of speech sounds, especially on transients, is largely lost. I n gerbils with a single electrode cochlear implant was explored, whether up ward and downward interval modulation of pulse trains can carry discriminab le information. This question was pursued with unit recordings in primary a uditory cortex (AI) and with behavioral discrimination training in a shuttl e box. Units in Al showed multiple differences in the dynamic responses to the two directions of interval modulation and notably ON-response dominated patterns with increasing intervals and OFF-response dominated patterns wit h decreasing intervals of stimulation. In accordance with these neuronal co rrelates gerbils learned to distinguish the directions of interval modulati on within 3 days, but only with certain specifications.