Comodulation masking release in electric hearing
R.H. Pierzycki and B.U. Seeber, MRC Institute of Hearing Research, Nottingham, UK 
 
In normal hearing, detection thresholds of a masked tone can be reduced when amplitude 
modulations of the masker are correlated across frequency channels (Hall et al, 1984). This effect 
appears to rely mostly on across-channel comparison of temporal envelopes and has been termed 
comodulation masking release (CMR). Since electric hearing with current cochlear implants (CIs) is 
based on envelope information, we test whether CMR can be available in electric hearing, and thus 
whether co-modulation of envelopes on multiple electrodes might be beneficial for CI listening. 
    The flanking band CMR paradigm was adapted to investigate CMR with CI participants 
listening to modulated pulse trains applied to separate electrodes of their CI. CMR stimuli consisted 
of a sinusoidally amplitude-modulated (SAM) on-frequency band of noise (OFB) presented either 
alone or with a tone added, and two SAM flanking bands (FBs) of noise. Envelopes of these stimuli 
were extracted and used to modulate constant rate pulse trains presented in staggered order on 
chosen target and ‘flanking’ electrodes. The envelopes on flanking electrodes were either in or out 
of phase, i.e. co- or anti-modulated, with the envelope on target electrode. Three rates of SAM: 8, 
14, and 20 Hz, were used. Tone thresholds were determined for stimulation: 1) on the target 
electrode alone or 2) on the target electrode and flanking electrodes with co- or anti-modulated 
pulse trains. 
    Electric stimulation on flanking electrodes in addition to stimulation on the target electrode 
generally leads to increased thresholds for both co- and anti-modulated conditions which suggest 
larger masking due to current spread. There is no difference between the co- and anti-modulated 
thresholds at SAM of 20 Hz but for some participants the co-modulated thresholds are lower than 
those in the anti-modulated condition at lower rates. While this may be indicative of CMR, the fact 
that in the presence of FBs thresholds are higher than for the OFB alone suggests a release from 
current spread rather than CMR. Lower thresholds with co- compared to anti-modulation 
nevertheless suggest that some subjects may benefit from listening into masker’s dips at low 
modulation rates. 

Acknowledgements 
Work supported by the Intramural Programme of the Medical Research Council (UK). Advanced 
Bionics Corporation provided the direct stimulation equipment. 

References 
Hall J.W., Haggard M.P. & Fernandes M.A.  1984.  Detection in noise by spectro-temporal pattern 
analysis.  J Acoust Soc Am, 76, 50-56.