## 5 Discussion
This directional bias in the SNR data, may be discussed in connection to some recent studies examining a population rate-based opponent-channel model in ITD [22; 42; 43] and ILD coding [44; 45; 46]. According to the findings of these studies, a rapid increase in ITD (i.e., outward lateralization shift), results in a stronger brain response than does a decrease in ITD (i.e., inward lateralization shift). The opponent-channel model seems to provide an explanation for these findings. The bias observed in favor of inward lateralization shift in the present study seems, however, to be in contradiction with what would be predicted by the opponent-channel model. In a recent study by [PERSON] et al. [47] on hemispheric ITD and ILD cues coding, the stimuli were presented to both the left and right hemifields. The most strong N1 responses (N1 and P2 are components of the human scalp-recorded event-related potentials), evoked by ILD cue, were recorded from the right auditory cortex, while the strongest ITD evoked responses were recorded from the contralateral hemisphere. The main effect of shift magnitude for N1 and P2 latencies suggested a common neural weighting in the lateralization of the cues. Moreover, larger responses for inward shifts in the right hemifield compared to the outward shifts in the ipsilateral primary auditory cortex were recorded. From the other side, the inward shifts in the left hemifield resulted in stronger P2 responses in the ipsilateral primary auditory cortex. Dominance of the responses to inward shifts reported in this study is parallel to the inward bias of SNR in the present study, and the authors concluded that there must be an alternative processing scheme apart from those predicted by the opponent-channel model. In contrast to the opponent-channel model, a modified topographic model, in which the ITD-selective units tuned to locations near the midline are more numerous and tightly tuned than those to periphery [48; 49], would predict a stronger neural response [22], thus a better detection performance [20] for inward stimulations compared to outward stimulations. Both models predict, however, that performance should be better for stimuli around midline than in the periphery. The SNR results of the present study, which indicates a bias for inward changes in addition to the putative midline advantage, are in harmony with the prediction of the midified topographic model. Considering the results of some recent neuroimaging studies supporting the opponent-channel hypothesis of sound lateralization in humans [22; 42; 50], the results of the present study may not lead to a firm discussion. However, as suggested by [PERSON] et al. [51], lateralization coding could be achieved simultaneously by complementary functioning of an opponent two-channel mechanism at low frequencies and a topographic mechanism at higher frequencies. This may be an explanation for the inward bias observed in the present study in which the stimulus employed is a complex sound with a power spectrum dominated by high frequency harmonic components. The discrimination bias, which is revealed in the experimental results and fitted the proposed diversity combining model appropriately, indicates that the participants' performance in identifying the direction of a change in the azimuth of an auditory source is lower for a change toward the periphery than for a change toward the center. Even though the exact reason underlying the observed bias for inward azimuthal changes is unknown, it can be considered as a result of an attentional priority formed throughout life. This priority may be in the focused attention and cognition mechanisms which are active during the tests, or in an unintentional and thus pre-attentive change detection mechanism similar to the one described by [PERSON] [52] and [PERSON] et al. [53]. The limitations regarding the generality of the experiments in the current study could be summarized as follows.