Proefschrift_vd_Beek

Recorded responses are larger when the recording contact is close to the responding neural fibers. The presence of a significant shift in the SOE illustrates that, for both scanning and selectivity measures, the outcome is to some extent influenced by current spread from the fibres towards the recording contact, leading to a skewing of the SOE curve towards the recording electrode. Consequently, it was evident that apical recordings (i.e. with the recording electrode apical to the fixed probe or masker) shift the recorded flank of the SOE curve apical-wards, and basal recordings basal-wards. As a consequence, the data presented here (which have mainly been recorded from an apical position) show a shift of the SOE in the apical direction, resulting in a smaller SOE width for EA-B (Figure 2) and an increase in SOE width for the EB-A condition (Table 4). In contrast, the middle electrode (EM) did not show such a clear shift. However, this may be because most of the EM-A curves did not reach the 60% criterion resulting in arbitrary, fixed values, reducing the possibility of showing the effect of the recording locations. Our findings are in contrast to data presented by Cohen et al, who reported little overall difference between selectivity recordings made apically or basally (Cohen et al, 2003, 2004). And although Hughes and Stille (2010) describe that for recording positions equidistant from the probe, amplitudes were generally larger when recorded from the apical side, only in a small minority of cases different recording positions resulted in a significant shift in selectivity measures. There is also a lack of consistency in other previous studies. eCAP measurements have sometimes been obtained from apical recordings (Busby et al, 2008), sometimes from basal recordings (Lai et al, 2009), and sometimes not clearly specified (Cohen, 2009). This lack in consistency in the use of recording contacts makes interpretation of conclusions about SOE measurements difficult. For the selectivity measures, asymmetry along the array was seen, with wider SOE functions at the apical contact than at the basal contact (Table 2). This is generally in agreement with other studies (Eisen & Franck, 2005). Moreover, in line with previous research, an asymmetry in the middle of the array was evident (Cohen et al, 2003; Cohen, 2009). However, asymmetry in the middle part does not imply an asymmetry in the neural excitation (Cohen, 2009). This can be explained as follows: the forward masking paradigm measures overlap of excitation produced by different contacts. This overlap consists of neurons excited by the contact of interest (the probe) and neurons excited by the masker. If the width of the pattern of excited nerve fibres is not constant along the array, but wider in the apex than in the base, then the overlap of a probe in the middle with an apical masking contact would be larger than with a basal masking contact. The resulting SOE curve would thus become asymmetric towards the apex, as is seen in the data of the present study. Psychophysical experiments of SOE determine thresholds of masking and are thus theoretically less influenced by the width of excitation of the masking contact. Accordingly, psychophysically obtained forward masking curves have shown no significant asymmetry (Cohen et al, 2004; Nelson et al, 2008). The differences between the SOE seen in the apical and basal parts of the cochlea may relate to the smaller distance to the modiolus or a smaller volume of the apical cochlea. Alternatively, a larger SOE apically could be due to crossturn stimulation, which is known to be more likely apically where the cochlea is more tightly coiled (Frijns et al, 2001). These factors caused by the tapered morphology of the cochlea

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