Proefschrift_vd_Beek

Parameter Manipulations in SOE (eCAP)

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are not completely matching the analysis with mixed linear models, which take missing data points into account. Results Comparison of scanning vs. selectivity For analysis of subsets of data, three additional linear mixed models were constructed with only data of the fi ve subjects tested in the subset. Besides the comparison of scanning versus selec-tivity measures, these subsets included the analysis of selectivity measures alone as well as the measurements with the recording contact at apical or basal position, and with the masker or probe fi xed. Additionally, a subset with data on artefact rejection method in scanning was analysed. Furthermore, a mixed linear model was constructed in which the patients ’ speech perception was compared with their selectivity widths measured with the apical recording position. Using a mixed linear model made it possible to take the current level and location along the array into account in the analysis with speech data. Statistics The design of this study is basically a within-patient analysis with three fact rs, which mean that at the p tient lev l the measurements are correlated. So-called linear mixed models take this correlation into account, by considering the responses from a subject to be the sum of so-called fixed effects, affecting the population ean, and random effects, associated with a sampling procedure (e.g. subject effects). The random effects often introduce correlations between cases and should be taken into account to elucidate the fixed, popu- lation affecting, effects. The SE (standard error of the mean) gen- erated by the model is used in significance analysis. Using linear mixed models enables investigation of the effects of each parameter separately as well as the interactio between different parameters. Furthermore, linear mixed models can effectively use all data, even when one or more data points are missing (Fitzmaurice et al, 2004). In the present study SPSS 16.0 was used to construct mixed lin- ear models to address the influence of the measuring technique, the electrode position, and the current level separately. For significance levels in this study t-tests are used, both in the context of descriptive statistics as well as with linear mixed models. Figure 3 shows boxplots for scanning (upper panels) and selectivity (lower panels) measures at the apical (EA-B), middle in apical and basal directions (EM-A and EM-B respectively), and basal (EB-A) electrodes, measured at three different current levels (low, medium, and high). The majority of curves (95% for selectivity and 75% for scanning) met the 0.6 criterion. However, for the curves of the middle electrode c ntact in the apical irecti this criterion was often not met (42% for scanning and 14% for selectivity). The curves that did not meet the 0.6 criterion were set as the limit of the array in the apical or basal direction (as per Abbas et al, 2004). Descriptive statistics, showing the means and differences of scanning and selectivity along th array, are summarized i Table 2. The calculated linear mixed model, containing data of 31 dif- ferent subjects (for 26 subjects, measures of both scanning and selectivity were obtained, for three subjects only scanning data, Comparison of scanning vs. selectivity Figure 3 shows boxplots for scanning (upper panels) and selectivity (lower panels) measures at the apical (EA-B), middle in apical and basal directions (EM-A and EM-B respectively), and basal (EB-A) electrodes, measured at three different current levels (low, medium, and high). Table 2. Descriptive statistics including mean widths in terms of electrode contacts for scanning (top rows) and selectivity (lower rows) for the different electrode contact locations (with standard deviations). Significance levels are shown in the bottom row. Data shown are descriptive statistics and are incorporated in the linear mixed model for further analysis of separate parameters, which are described in the Results section. EA-B EM-A EM-B EB-A Figure 2. Selectivity curves obtained in one subject, in response to stimulation at the apical electrode contact (EA, left), middle contact (EM, middle) and basal contact (EB, right). The figures show the normalized response amplitudes recorded from locations along the array, using low (dotted line), medium (dashed line), and high (solid line) current levels, together with the 0.6 criterion line. The width of the curve is then defined by the number of electrodes above the 0.6 line (indicated by arrows) for apical electrode in basal direction (EA-B), middle electrode in apical and basal direction (EM-A, EM-B), and basal electrode in apical direction (EB-A). The location of electrode of interest (dotted line over full figure height) and position where curve meets the 0.6 criterion (dotted line over half figure height) are indicated. Figure 2. Selectivity curves obtained in one subject, in response to stimulation at the apical electrode contact (EA, left), middle contact (EM, middle) and basal contact (EB, right). The figures show the normalized response amplitudes recorded from locations along the array, using low (dotted line), medium (dashed line), and high (solid line) current levels, together with the 0.6 criterion line. The width of the curve is then defined by the number of electrodes above the 0.6 line (indicated by arrows) for apical electrode in basal direction (EA-B), middle electrode in apical and basal direction (EM-A, EM-B), and basal electrode in apical direction (EB-A). The location of electrode of interest (dotted line over full figure height) and position where curve meets the 0.6 criterion (dotted line over half figure height) are indicated. RESULTS To create a comprehensive overview data are plotted in boxplots. However, it should be noted that boxplots are not completely match- ing the analysis with mixed linear models, which take missing data points into account. For analysis of subsets of data, three additional linear mixed models were constructed with only data of the five subjects tested in the subset. Besides the comparison of scanning versus selec- tivity measures, these subsets in luded the analysis of selectivity measures alone as well as the measurements with the recording contact at apical or basal position, and with the masker or probe fixed. Additionally, a subset with data on artefact rejection method in scanning was analysed. Furthermore, a mixed linear model was constructed in which the patients’ speech perception was compared with their selectivity widths measured with the apical recording position. Using a mixed linear model made it possible to take The majority of curves (95% for selectivity and 75% for scanning) met the 0.6 criterion. However, for the curves of the middle electrode contact in the apical direction this criterion was often not met (42% for Selectivity Mean 5.3 6.3 3.3 Std. Dev. 3.5 2.5 1.8 at the three current levels. A horizontal line indicates 60% of the amplitude. The horizontal solid arrows then indicate the width of the curve (in basal or apical direction) as defined above for the highest current level. the current level and location along the array into account in the analysis with speech data. Scanning Mean Std. Dev. 8.2 4.0 6.8 1.1 5.9 2.4 9.0 3.6 4.2 2.8 Significance p 0.01 p 0.1 p 0.01 p 0.01

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