T» 1 P value

Ipsilateral clicks-

contralateral white noise 0.783 0.004** Ipsilateral clicks-

contralateral IkHz tone 0.735 0.016*

Ipsilateral IkHz tone-

contralateral white noise 0.628 0.029* Ipsilateral 1kHz tone -

contralateral IkHz tone 0.630 0.028*

The correlation coefficients are all positive indicating that in these subjects, those with higher suppression levels in the 2kHz region were better at detecting tones, in what was effectively binaural 1 kHz NBN.

8.3.8.5 Age

The mean age was 23.3 years with a standard deviation o f 2.8 years. The age o f the subjects was not found to be correlated with any o f the bandwidths measured during contralateral NBN.

8.4 D iscussion

The aim o f this experiment was to acoustically stimulate the efferent system in specific frequency bands to see what effect this had on the auditory filter measured at 1kHz. The filters tested during contralateral NBN at 500Hz and 1kHz were found to be slightly, but not significantly wider than the filter tested with no contralateral stimulation. In

** statistically significant, p<0.01 * statistically significant, p<0.05

contrast, when tested during 2kHz NBN, the filter was found to be (non-significantly) narrower. Comparing with the results from the previous chapter where white noise was the contralateral stimulus, NBN had a much smaller effect on the filter shape. This is in agreement with past work on the effect o f contralateral noise on the TOAE amplitude. (Norman and Thornton 1993) found that NBNs were much less effective suppressers than white noise. Presumably, this was because fewer medial efferent fibres were activated by the NBN.

Although changes to the filter shape were small in this study, we examined whether, on an individual basis, efferent activity level was related to the filter shape during efferent input at various frequencies. The frequency specific nature o f action o f the efferent system on OAEs has been described previously (Chery Croze et al. 1993; Mott et al. 1989; Veuillet et al. 1991). Thus, NBN contralateral stimulation should produce maximal efferent effect on the auditory filter in the corresponding frequency range.

However, the filter shapes during contralateral NBN were not in general related to the strength o f efferent activity measured by OAE suppression or to the non-suppressed OAE level. The change in width o f the filter between the ‘with’ and ‘without’ noise situations was also not related.

A few parameters did show some correspondence between the OAE and the filter shape tests. Since so many correlation coefficients were calculated, one has to treat the odd significant correlation with caution. Therefore, only results that had numerous or particularly strong correlations will be discussed further. The BWsoonbn was found to be negatively related to the TOAE amplitude. This was particularly strong for the relationship between the TOAE amplitude in the 2kHz region and the upper slope o f the filter. It is not clear why this may be the case, since the efferents are presumably being activated in the 500Hz region for the filter shape test.

The DP level, at 1kHz, 2kHz and averaged across the frequency range, was correlated negatively with ABWinbn- This result implied that the greater the activity o f the cochlear amplifier in this region, the more the auditory filter was resistant to change when presented with contralateral 1kHz NBN. This seems counter intuitive since the efferent system generally is most effective at suppressing BM motion and auditory nerve activity

at the best frequency and therefore point o f greatest activity (Murugasu and Russell 1996; Williams and Brown 1995).

For the auditory filter measured during contralateral 2kHz NBN, the symmetry index was found to be negatively correlated to the suppression of click or tone evoked TOAEs by contralateral tones. The relationship was only significant in the 2kHz region and when windowed temporally. The direction o f the relationship indicates that in subjects with higher levels o f efferent function in the 2kHz region (when the efferent system is stimulated by a 1kHz tone), the lower slope o f the filter is shallower in comparison to the upper slope i.e. the filter is skewed towards the lower frequencies. This follows logically if one considers that the action o f the efferent system stimulated by 2kHz NBN may be to damp the motion o f the BM in the 2kHz region specifically. This frequency specific action could feasibly therefore sharpen one side o f the filter. However, this result is not backed up by a significant relationship between p(upper) and the OAE suppression results.

Another interesting result to come from this work is the relationship between the suppression o f TOAEs and the masked threshold o f the 1kHz tone during contralateral NBN stimulation. Significant correlation coefficients were only found for the threshold o f the detection o f a 1kHz tone in noise during contralateral 1kHz NBN. Since the ipsilateral masker was also narrow band, the masking noise in both ears was approximately the same. There were significant relationships for the suppression o f clicks evoked OAEs by white noise and for the suppression o f 1kHz tone evoked OAEs by 1kHz tones. However, when examining the suppression more carefully, it was found that the relationship only held for the suppression in the 2kHz region, but that in this region the relationship was statistically significant for all four o f the OAE suppression tests. The direction o f the result implied that the subjects in whom the contralateral noise produced greater efferent activity in the 2kHz region were able to hear the tone better in the masking noise. Thus, in these subjects, the efferent system may act more effectively to dampen the BM motion, and therefore the neural response, to the ipsilateral masker at higher frequencies than 1kHz whilst leaving the signal frequency itself less damped. Previous work has suggested an anti-masking role for the efferent system (Kawase et al. 1993; Kawase and Liberman 1993; Liberman 1988). Micheyl and Collet (1996) carried out a similar comparison to the present study, and compared the suppression o f TOAEs with the detection o f tones in dichotic (broadband) noise. A

relationship was found, although only for the detection o f the 2kHz tone, which they interpreted as suggesting that the OCB is involved in the detection o f tones in noise. No such relationship was found for the detection o f a tone at 1kHz. It is possible that the detailed analysis o f the OAE suppression results carried out here helped us to reveal a relationship at 1kHz, which was hidden to them. In addition, we used signals that were more specific in frequency than Micheyl and Collet (1996). From the previous chapter, we also found no relationship with broadband signals. Stimulating the efferent system over a wide range o f frequencies may add variability to an already sensitive measure, especially if, as is likely, the efferent system is not equally active over the length of the BM.

The same possible explanations for the general lack o f a relationship between the OAE and the psychoacoustic tests apply as in the previous chapter. Obviously, the results may indicate that the efferent system is not involved in frequency selectivity. However, one must consider other factors. As before, the stimulus levels used mean that acoustic cross over or activation o f the acoustic reflex were unlikely to be interfering with the results. It is possible that a change in shape o f the filter did occur due to efferent activation, but that the change was so small that other test errors swamped the result. Another consideration is that the efferent system may cause a shift in the best frequency o f the filter. If this was the case, the filter modelling procedure would not be able to accurately estimate the shape o f the filter and therefore a relationship to the OAE results would be lost.

Finally, some o f the past studies on humans have found a link between efferent activity and detection o f tones in noise (Micheyl and Collet 1996) or between OAE magnitude and psychoacoustic tuning (Micheyl and Collet 1994), at 2kHz but not at 1kHz. In order to test whether there was some anomaly between the efferent activity at the two frequencies, the relationship between efferent function and psychoacoustic tuning was measured at 2kHz and is described in the next chapter.

8.5 Conclusion

This study did not show significant changes to the shape of the 1kHz auditory filter by the addition o f contralateral NBN at 500Hz, 1kHz or 2kHz. There was some evidence however, for an involvement o f the efferent system (particularly around 2kHz) in the detection o f 1kHz tones in binaural 1kHz NBN. The results indicated that greater efferent activity was related to better detection-in-noise performance.

Assessing the results as a whole, there was little evidence for a link between efferent activity, as measured by OAE suppression, and psychoacoustic tuning when the efferent system was stimulated in these specific frequency bands.