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4
Selectivity vs. speech perception
The average monosyllabic word score at two year follow up in the 15 adult CI-recipients in this study was
62% words correct (range 20%–91%). A final linear mixed model, containing the data of the 15 persons
with selectivity data and speech perception data showed that no significant prediction could be obtained
for speech perception using the width of the selectivity curves, taking location along the array and current
level into account (
p
= 0.3).
DISCUSSION
No study has been able to verify the hypothesis that eCAP derived SOE measures are correlated with
speech understanding (Hughes & Abbas, 2006b; Hughes & Stille, 2008). In line with this, no correlation
was found in the present study. Theoretically, SOE would be expected to correlate with spectral resolution,
which is an important component of speech recognition. The principle aim of this study, therefore, was
to analyse the variables influencing SOE measures, which might indicate possible reasons for this lack of
correlation. Two fundamentally different methods of measuring SOE were compared and the effects of
several variables presumed to influence SOE were investigated.
The finding that scanning produces wider curves than selectivity measures has been previously reported by
Cohen et al (2003) and Hughes & Stille (2010), and is in line with theoretical expectations. This can be
explained as follows: in the scanning method, where recordings are made at several points along the electrode
array, the current from the stimulating contact (i.e. stimulus artefact) and the current generated by the nerve
fi bers (neural response) both spread easily through the fl uid of the cochlea and result in a potential at the
recording electrode some distance from the stimulating contact. The selectivity method, however, using a
fi xed recording contact mainly shows the spread of excitation of the stimulating pulse. Wider curves for
scanning were evident at all positions analysed with the excep-tion of the middle contact measured in the
apical direction. This exception is likely a consequence of the method and due to the fact that many curves
for both methods did not reach the 0.6 criterion. The rationale for the choice of this 0.6 criterion was
covered in the methodology section. The limitation of the method resulted in the introduction of arbitrary,
fi xed values at the ends of the array as proposed by Abbas et al (2004) and complicated comparison along
the array and across subjects. shows the spread of excitation of the stimulating pulse. Wider curves for
scanning were evident at all positions analysed with the exception of the middle contact measured in the
apical direction. This exception is likely a consequence of the method and due to the fact that many curves
for both methods did not reach the 0.6 criterion. The rationale for the choice of this 0.6 criterion was
covered in the methodology section. The limitation of the method resulted in the introduction of arbitrary,
fixed values at the ends of the array as proposed by Abbas et al (2004) and complicated comparison along
the array and across subjects.