Co-modulation Masking Release in Cochlear Implant Listening 
Robert H. Pierzycki, Bernhard U. Seeber

MRC Institute of Hearing Research 

In normal hearing, detection of a signal in an on-frequency noise band (OFB) improves after adding flanking bands (FBs) 
of noise at frequencies remote from the OFB. This release from masking requires common envelope modulations in the 
OFB and FBs and is called co-modulation masking release (CMR). As current cochlear implant (CI) processors transmit 
the sound envelope, co-modulation might also be beneficial for signal detection in CI listening. 

The FB CMR paradigm was adapted for CIs by modulating electric pulse trains on chosen OFB and FB electrodes with 
envelopes extracted from narrow-band OFB and FB noises. The noise bands were sinusoidally amplitude-modulated at 
fm=20 or 8 Hz. In signal intervals, pulse trains on the OFB electrode were modulated with the envelope of a tone added to 
the OFB noise. Two conditions were tested: 1) a FB at a basal and an apical electrode, each 4 electrodes away from a 
mid-array OFB electrode or 2) one FB placed 9 electrodes away from a basal OFB electrode. In a further condition the 
same noises were used as OFB and FBs to assess the impact of correlation. CMR was defined as a positive difference 
between thresholds for: 1) OFB only vs. OFB with co-modulated FBs (OFB-CM=CMR 1), or 2) OFB with anti- vs. comodulated FBs (AntiM-CM=CMR 2). 

The addition of co-modulated FBs did not lead to a reduction of tone thresholds in any CI subject (N=5), i.e. CMR 1 was 
absent. For some subjects for fm=8 Hz, CM thresholds were lower than the AntiM thresholds, indicating CMR 2. However, 
this was unlikely a true CMR effect since both CM and AntiM thresholds were larger than the thresholds for the OFB 
alone. For large OFB and FB electrode separations, CM thresholds tended to decrease, attributed to reduced current 
spread. In none of the test conditions did CMR reach significance. The failure to observe CMR may be due to insufficient 
spectral and temporal cues in CI listening which precludes signal/masker segregation or grouping of the OFB with FBs.