The scale illusion detection task: Objective assessment of binaural fusion in normal-hearing listeners
The normal auditory system can fuse sounds from both ears into a single sound object (binaural fusion). This ability can be assessed subjectively by asking whether listeners perceive one or two sounds or by the scale illusion percept described by Deutsch (1975). The aim of the current study is to develop an objective task to measure binaural fusion. Twelve normal-hearing participants had to detect one deviant note within a stream composed of a repeating melody while simultaneously being presented with another stream of randomized notes. The experiment included 3 conditions. First, in a monaural condition both streams were presented to the same ear. Then, in a binaural condition every second note from each of the two streams was presented to the other ear. Finally, in a binaural control condition, the timbre of all the notes presented to one ear was altered severely, to prevent binaural fusion. The expected result was a better detection of deviant notes for listeners that are able to fuse streams across the two ears. Each condition had 24 repetitions. In the binaural and monaural conditions, average performance was about 80% correct, while the control condition showed a significantly lower performance of about 50%. Thus, this type of experiment can be used to test objectively if fusion takes place. It lays the foundation for further studies with bilateral and bimodal cochlear implant listeners.
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