Fitting based on Objective Measures: Shift method (or Smoorenburg) with ECAP profile.

Since this method was developed for Nucleus implants the terminology in this section is Nucleus focused.

The shift and tilt method is based on a number of ideas and principles
First, Smoorenburg, like Plant et al, (2002), assumed that live mode fitting would give better results than arbitrary single-electrode bursts (Smoorenburg et al., 2002). However, Smoorenburg argues to apply this method not only to the C-levels but also to the T-levels.
Second, Smoorenburg assumed (as Brown et al., 2000) that although the correlation between T and C levels and ECAP thresholds is weak, the profile shapes have some correlation.
In an initial study, Smoorenburg et al. (2002) tested a fitting method that is based on these two assumptions. In this fitting method the following happens:
  1. The ECAP threshold profile is measured.
  2. The T-levels are set to the ECAP profile but dropped down until sub-threshold.
  3. The C-levels are set just above T-level.
  4. The processor is switched on in live mode.
  5. Both T and C levels are increased until a soft auditory sensation is heard.
  6. At this moment the T-level profile is fixed and only the C-level profile is increased further until the live sound is comfortably loud. This requires some sound input in the form of the audiologist speaking or a CD playing etc.
This fitting method resulted in some very interesting findings:
  • A) The T-levels of the maps set in this way were on average much lower (by around 25 CL) than conventional T-levels. This effect is mainly caused by temporal integration.
  • B) In a group of 7 experienced users, there was no group mean significant change in performance, although 2 individuals performed better with their old map.
This is a clear indication that the exact position of the T-levels is not very important for the quality of the map, although this has been questioned in other studies.
A second study used a prospective balanced crossover design (AB/BA) and showed no significant difference between this mapping method and conventional mapping. A third study applied this mapping method to young children and the outcome was again the same: no difference in performance (most children preferred the ECAP based map) but a large difference in required fitting time (Battmer et al., 2004).
Advantage: Fast. Takes temporal integration into account.
Disadvantage: Effect of very low T-levels may be suboptimal speech scores at very low levels, although data does not support this (Smoorenburg et al., 2002). May not be accurate enough for some recipients.
 
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