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

Speech coding strategies for cochlear implants commonly use amplitude modulations of constant high rate pulses to differentially stimulate separate frequency channels in the cochlea. Thereby, time domain information in the fine structure of speech sounds, especially on transients, is largely lost. In gerbils with a single electrode cochlear implant was explored, whether upward and downward interval modulation of pulse trains can carry discriminable information. This question was pursued with unit recordings in primary auditory cortex (AI) and with behavioral discrimination training in a shuttle box. Units in AI 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 with decreasing intervals of stimulation. In accordance with these neuronal correlates gerbils learned to distinguish the directions of interval modulation within 3 days, but only with certain specifications.