PULSE RATE AND PULSE AMPLITUDE MODULATION SENSITIVITY IN THE INFERIOR COLLICULUS OF COCHLEAR IMPLANT RATS

Cochlear implant (CI) strategies discard temporal fine structure, which degrades pitch perception and leads to impaired music appreciation, tonal language understanding or foreground-background discrimiantion. Research varying pulse rates as pitch cue struggle with the potential confound of loudness effects: higher pulse rates increase not just pitch but also stimulus intensity. We investigated how neural populations disambiguate these cues by characterizing responses of multiunits in the inferior colliculus (IC) of CI-implanted rats to pulse trains which were modulated in pulse frequency (FM) as well as pulse amplitude (AM), with varying modulation depth.
Stimuli were centered at baseline pulse rates (100, 300, 450, 900 pps). We first identified units sensitive to both modulations, then mapped joint sensitivity near threshold modulation depths (eliciting ~15% response modulation).
Of 408 responsive multi-units, a substantial proportion were jointly sensitive (61.8% at 100 pps, 67.6% at 300 pps, 58.8% at 450 pps, 21.3% at 900 pps). A key finding was the diversity of FM encoding: while neural responses always covaried positively with AM, they could be either in-phase or in counterphase with FM (i.e., higher rates could increase or decrease firing). This anti-correlated FM response varied with baseline rate (e.g., 7.2% at 300 pps vs. 76.3% at 450 pps).
The diverse, heterogeneous responses across the IC population provide a robust neural substrate for disambiguating AM (loudness) from FM (pitch) changes. This suggests central auditory processing may naturally mitigate loudness confounds in pulse-rate pitch perception, informing future CI coding strategies.