Current steering has been proposed to increase place pitch resolution in cochlear implant (CI) users. Many studies have shown that a current steering effect can be achieved when simultaneous stimulation is used (Koch, Downing, Osberger, Litvak and Greco, 2007, Saoji and Litvak, 2010, Wu and Luo, 2013). Some literature has shown that a current steering effect can also be achieved when sequential stimulation is used (McDermott and McKay, 1994, Kwon and van den Honert, 2006, Swanson, 2008). Literature proposes different features that could underlie place pitch and consequently possibly also current steering effects (McDermott and McKay, 1994, Kwon and van den Honert, 2006, Swanson, 2008, Frijns, Kalkman, Vanpoucke, Bongers and Briaire, 2009, Macherey and Carlyon, 2012, Venter, 2015).
The present study confirmed that a current steering effect can be achieved when sequential stimulation is used by using multi-dimensional scaling and statistical analysis in addition to the convention of using cumulative d' values to analyse pitch ranking results of current steering experiments. It was however observed that a current steering effect could only be achieved in listeners who were at least able to pitch rank the pitch of the two individual stimulating electrodes correctly according to expectation.
The effect of different stimulation parameters on the pitch ranking ability of CI users during current steering experiments was investigated. Results showed that some parameters only had an effect on the pitch ranking performance of some listeners, while other stimulation parameters affected the results of all the listeners. Wider stimulation pulse widths, for example, led to improved pitch ranking results for some listeners. Most listeners benefited from wider electrode separation distances. Statistical analysis showed that there was a significant improvement in the pitch ranking performance of the listeners during experiments where the stimulation rate was the same as the rate indicated on the clinical MAP of the listener.
Person-specific current distribution models were used to predict the cochlear position of different stimuli because of different features that could underlie place pitch, for each of the experiments for four of the listeners who participated in this study. The model predictions were related to the measured pitch ranking results using correlation and mutual information analysis. The results indicated that the current centroid at electrode level, the position of the peak current at the auditory nerve level (because of either an individual stimulating electrode or because of summed currents) and the centroid of neural activation could underlie place pitch. All these features except the position of the peak of the current distribution at the auditory nerve level because of each individual stimulating electrode could underlie current steering effects. Results showed that the centroid of the current distribution at the auditory nerve level probably does not underlie place pitch.
Knowledge about the impact that different stimulation parameters have on the ability to achieve a current steering effect could result in more efficient implementation of current steering effects. Proper knowledge of which features underlie place pitch and current steering effects could be used to create models that can be used to predict the results of place pitch experiments.
Afrikaans: Stroomstuur is voorgestel om plek-toonhoogteresolusie in gebruikers van kogleêre inplantings (KI) te verbeter. Baie studies het getoon dat ’n stroomstuur-effek verkry kan word as gelyktydige stimulasie gebruik word (Koch et al., 2007, Saoji and Litvak, 2010, Wu and Luo, 2013). Van die literatuur het getoon dat ’n stroomstuur-effek ook bereik kan word as opeenvolgende stimulasie gebruik word (McDermott and McKay, 1994, Kwon and van den Honert, 2006, Swanson, 2008). Die literatuur verwys na verskillende verskynsels wat moontlik die grondslag is van plek-toonhoogte en gevolglik moontlik ook van stroomstuur-effekte (McDermott and McKay, 1994, Kwon and van den Honert, 2006, Swanson, 2008, Frijns et al., 2009, Macherey and Carlyon, 2012, Venter, 2015).
Die huidige studie het bevestig dat ’n stroomstuur-effek bereik kan word wanneer opeenvolgende stimulasie gebruik word deur die aanwending van multi-dimensionele skalering en statistiese analise bykomend tot die konvensie om kummulatiewe d'-waardes te gebruik om toonhoogteplasingresultate van stroomstuur-eksperimente te analiseer. Daar is nietemin waargeneem dat ’n stroomstuur-effek net bereik kon word in luisteraars wat minstens in staat was om die toonhoogte van die twee individuele stimulasie-elektrodes korrek te plaas volgens verwagting. Die effek van verskillende stimulasieparameters op die toonhoogteplasing-vermoë van KI-gebruikers tydens stroomstuur-eksperimente is ondersoek. Die resultate het getoon dat van die parameters net ’n uitwerking op die toonhoogteplasingprestasie van sommige luisteraars gehad het, terwyl ander stimulasieparameters die resultate van al die luisteraars beïnvloed het. Wyer stimulasie-pulswydtes het byvoorbeeld gelei tot verbeterde toonhoogteplasingresultate vir sommige luisteraars. Die meeste luisteraars het baat gevind by wyer elektrode-skeidingafstande. Statistiese analise het ’n beduidende verbetering in the toonhoogteplasingprestasie van die luisteraars getoon tydens eksperimente waar die stimulasietempo dieselfde was as die tempo wat op die kliniese MAP van die luisteraar aangedui is.
Persoon-spesifieke stroomverspreidingmodelle is gebruik om die kogleêre posisies van verskillende stimuli te voorspel omdat verskillende eienskappe die onderbou vir plek-toonhoogte mag vorm, vir elk van die eksperimente vir die vier luisteraars wat aan die studie deelgeneem het. Die modelvoorspellings is in verband gebring met die gemete toonhoogteplasing-resultate deur die gebruik van korrelasie en wedersydse informasie-analise. Die resultate het getoon dat die stroomsentroïed op elektrodevlak, die posisie van die piekstroom op die gehoorsenuweevlak (omrede óf individuele stimulasie-elektrodes óf totale stroom) en die sentroïed van neurale aktivering die grondslag van plek-toonhoogte mag wees. Al hierdie kenmerke, behalwe die posisie van die piek van die stroomverspreiding op die gehoorsenuweevlak danksy elke individuele stimulasie-elektrode, kan die grondslag van stroomstuur-effekte wees. Die resultate het getoon dat die sentroïed van die stroomverspreiding op die gehoorsenuweevlak waarskynlik nie die grondslag van plek-toonhoogte is nie.
Kennis van die impak van verskillende stimulasieparameters op die vermoë om ’n stroomstuur-effek te bereik kan meer doeltreffende implementering van stroomstuur-effekte tot gevolg hê. Voldoende kennis oor watter kenmerke die grondslag vorm van plek-toonhoogte en stroomstuur-effekte kan gebruik word om modelle te skep wat gebruik kan word om die uitkomste van plek-toonhoogte-eksperimente te voorspel.
