Place specificity measured in forward and interleaved masking in cochlear implants

Abstract

Interleaved masking in cochlear implants is analogous to acoustic simultaneous masking and is relevant to speech processing strategies that interleave pulses on concurrently activated electrodes. In this study, spatial decay of masking as the distance between masker and probe increases was compared between forward and interleaved masking in the same group of cochlear implant users. Spatial masking patterns and the measures of place specificity were similar between forward and interleaved masking. Unlike acoustic hearing where broader tuning curves are obtained in simultaneous masking, the type of masking experiment did not influence the measure of place specificity in cochlear implants.

Evaluating multipulse integration as a neural-health correlate in human cochlear-implant users: Relationship to spatial selectivity

“…If the masker and probe electrodes stimulate overlapping neural populations, the masker that is presented forward in time will cause some neurons in the population normally stimulated by the probe to enter a refractory period, thus elevating the threshold for detecting the probe stimulus. Forward masking generally diminishes as the spatial distance between the masker and probe increases (e.g., Azadpour et al, 2013). The masker used in the experiment was 300 ms long, followed by a 10 ms gap, and a 20 ms probe.…”

Procedural Factors That Affect Psychophysical Measures of Spatial Selectivity in Cochlear Implant Users

“…At high stimulation rates, where the masker and probe pulses are very close in time, the masker can increase the membrane potential and lower the minimum current needed for the probe to elicit an action potential. This phenomenon may be responsible for the instances of small or no masking reported in Azadpour et al. (2013) , where four out of nine CI subjects exhibited flat PTCs in interleaved masking at rates above 4,000 pps.…”

“…This may particularly affect the results of experiments obtained using Method B, which is designed to use the probe threshold as an estimate of the amount of excitation at the probe place and to do this for different masker positions. In some previous studies ( Azadpour, AlJasser, & McKay, 2013 ; Fielden et al., 2013 ), the different maskers have been chosen so as to have the same loudness as each other. If one or more maskers lie in regions of poor neural survival, their levels will be increased to match the equal-loudness criterion; however, such an increase would reflect the neural health at the masker , and not the probe place.…”

“…The majority of experiments on spatial selectivity in CI users has used forward-masking paradigms. An alternative is to present the probe pulses interleaved with the masker pulses ( Azadpour et al., 2013 ; Kwon & van den Honert, 2009 ). A potential limitation of this approach is suggested by evidence from both psychophysical and physiological experiments that the response to, or detection of, a single pulse can be enhanced by the presence of a preceding pulse ( Bierer & Middlebrooks, 2004 ; Nelson & Donaldson, 2001 ; Stypulkowski & van den Honert, 1984 ).…”

“…There is, however, a potential confusion effect by using forward masking, which relates to the user’s ability to detect gaps between two stimuli. Many forward-masking studies with CI users have used the same pulse rate for the masker and probe, combined with short, or sometimes zero, silent gaps between the masker and probe ( Azadpour et al., 2013 ; Cohen, Busby, & Clark, 1996 ; Cohen, Richardson, Saunders, & Cowan, 2003 ; Dingemanse et al., 2006 ; Fielden et al., 2013 ; Nelson et al., 2008 ). In such circumstances, it is likely that the probe will not be heard as a separate sound, but, instead, as a mere continuation of the masker.…”