Graeme Clark Collection
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ItemPsychophysics of a prototype peri-modiolar cochlear implant electrode array(ELSEVIER SCIENCE BV, 2001-05)Psychophysical measurements were performed in three hearing-impaired adult subjects implanted with a CI22 cochlear prosthesis (Cochlear Ltd.) fitted with a developmental peri-modiolar electrode array. The array was manufactured with a curvature approximating that of the inner wall of the scala tympani but, after straightening and insertion, lay on average about half way between the inner and outer walls of the scala. All subjects were tested with bipolar stimulation; two were also tested with monopolar, employing the most basal electrode as the return. Maximum comfortable level and threshold reduced with decreasing distance of electrode from the modiolus, whereas dynamic range increased. The linearity of the loudness growth function did not vary significantly with electrode position but the function was more non-linear for lower maximum comfortable levels. Current level discrimination, normalized with respect to dynamic range, improved with decreasing distance of electrode from the modiolus in two subjects. Pitch varied regularly with insertion depth of the stimulated electrode for bipolar stimulation in two subjects and also for monopolar stimulation in one subject. Electrode discrimination was enhanced by closeness to the modiolus. Whereas the forward masking patterns for bipolar stimulation of electrodes close to the modiolus had a sharp double-peaked structure, those for monopolar stimulation were flatter and had a single peak.
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ItemCochlear implant place psychophysics: 1. Pitch estimation with deeply inserted electrodes( 1996)Numerical estimation of pitch was performed by 8 adult subjects implanted with cochlear prostheses manufactured by Cochlear Limited. The electrode arrays had been inserted into the scala tympani to between one and one and a half turns of the cochlea. Using bipolar stimulation, the pitch estimates for each subject showed an overall reduction with insertion depth of the stimulated electrode. However, for several subjects, after decreasing regularly for the more basal electrodes, pitch estimates showed an abrupt decrease, followed in some cases by a region of low pitch. Two of the subjects, implanted with a modified electrode array, the '20 + 2' which allowed monopolar in addition to bipolar stimulation, exhibited an abrupt decrease in pitch estimate with bipolar but not with monopolar stimulation. In these two subjects, for stimulating electrodes inserted more deeply than about three quarters of a turn, bipolar stimuli produced lower pitch sensations, and presumably more apical neural excitation patterns, than monopolar stimuli.
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ItemCochlear implant place psychophysics: 2. Comparison of forward masking and pitch estimation data( 1996)Results for forward masking and numerical estimation of pitch were compared in a group of 6 adult subjects implanted with cochlear prostheses manufactured by Cochlear Limited. Data were collected for bipolar +1 stimulation in all subjects, and for stimulation in one other mode, either common ground or monopolar, for all subjects but one. The pitch data show various irregularities and in each case can be seen to be broadly consistent with the corresponding forward masking data. It is shown that a 'centre of gravity' of the forward masking distribution varies with masker electrode in a manner that is qualitatively very similar to the variation of pitch estimate. It is suggested that, while pitch estimation results are consistent with those from forward masking, the latter contain more detailed information that may be useful in understanding intersubject variations in speech comprehension.