## 1 Introduction
In the real world, the listener exposed mostly to the sounds, which are the combination of many frequencies rather than a single tone. Therefore, in later years researchers went beyond the classical duplex theory and studied the utility of ongoing ITD cues in the envelopes of low-frequency, high-frequency and complex sounds presented via headphones [8; 14]. Other groups of studies focused on processing information in different layers of the hearing system, from the external ears to the auditory cortex, by investigating the combination or integration of ITD and ILD cues by the means of electro-encephalography (EEG) and/or psychophysical experiments. [PERSON] [15] used a single tone and a complex sound to study the availability of two separate regions in the brain to process ITD and ILD information. [PERSON] et al. [16] used independently manipulated stimulations to produce sounds with either only one or both ITD and ILD cues by using head-related transfer function (HRTF) of the participants. The value of only one of the cues was applied by keeping the other cue at zero for ITD-only and ILD-only conditions. Their magneto-encephalographic (MEG) and psychoacoustical results, however, reveal the existence of neurons sensitive to both ITD and ILD cue information in the auditory cortex. Physically in the real world, ITD and ILD change together with the azimuthal position of a sound source. However, in dichotic hearing conditions, where the sound to the ears is transmitted via stereophonic earphones, the ILD and ITD cues can be changed independently of each other. It is even possible to compensate an ITD-induced sound image shift towards one ear by changing the ILD in favor of the other ear [17]. There are two approaches available regarding the sound images based on the ITD and ILD cues. The first one suggests that the cues may not be completely combined and the subjects sometimes perceive two images when ITDs and ILDs conflict [18]. The second scenario is that the two pieces of information carried by these two cues should either share a common pathway or undergo a combining process in the brain. The commonality in the coding of ITD and ILD cues is discussed by [PERSON] et al. [19]. The results show a combination of the cues by algebraically adding the d'values resulted from the judgments of the ITD-only and ILD-only stimulus types. They argue that the ITD and ILD cues are sharing a common neural representation. In order to understand the significance of cues in different frequencies, [PERSON] and [PERSON] [8] used a virtual auditory space approach to quantify the relative strength (weight) of ITD and ILD cues in low-pass, high-pass, and wideband noise bursts. In their stimulation sounds, the interaural time and level differences were manipulated by delaying or attenuating the stimuli sound to one ear. Their results revealed that, while the ITD is weighted weakly for high-pass stimuli the ILD is weighted strongly for high-pass stimuli even when substantial biases were introduced. In some studies, the performance of the participants in judgments differs depending on the direction and the cue used in the stimulation sounds. To test the existence of bias in lateralization, [PERSON] and [PERSON] [20] studied the relative sound localization (the target sound originated from the left or right of a reference sound) with rather greater values of the time and level differences when a multi-source noise background is present besides stimulation sounds. Their results showed that the subjects were more biased in the band-pass noise condition. They claimed this type of bias which shifts the decision criteria in the direction of the hemifield in which the stimulation is presented as the \"response bias\" [20; 21]. To test the effect of direction in localization, however, [PERSON] and [PERSON] [22] studied EEG responses for the low-pass filtered stimulations (1500 ms-250 ms pair of noise sounds with a frequency less than 1 kHz) in which the ITD change was either away from (outward) or toward the midline (inward).