Academic journal article Perception and Psychophysics

Subjectively Divided Tone Components in the Gap Transfer Illusion

Academic journal article Perception and Psychophysics

Subjectively Divided Tone Components in the Gap Transfer Illusion

Article excerpt

When a long glide with a short temporal gap in its middle crosses with a continuous short glide at the temporal midpoint of both glides, the gap is perceived in the short glide instead of in the long glide. In the present article, we tested possible explanations for this "gap transfer illusion" by obtaining points of subjective equality of the pitches and durations of the two short tones that are subjectively divided by the gap. The results of two experiments showed that neither an explanation in terms of envelope patterns nor explanations in terms of combination tones or acoustic beats could account for the perception of the short tones in the gap transfer illusion. Rather, the results were compatible with the idea that the illusory tones were formed by the perceptual integration of onsets and offsets of acoustically different sounds. Implications for the perceptual construction of auditory events are discussed.

In order to derive information about the environment, the auditory system must be able to process a mixture of sounds. The auditory system has to identify each individual sound of a mixture by segregating it into several auditory streams that correspond to sound sources in our surroundings (Bregman, 1990; Deutsch, 1983; McAdams & Bregman, 1979). The formation of auditory streams is governed by a number of principles (Bregman, 1990; Darwin & Carlyon, 1995) that describe how several auditory events-or sometimes a single event-can constitute an auditory stream (Handel, 1989). Nakajima and Sasaki (1996) proposed that the formation of each auditory event in a stream can be described through the perceptual connection of even smaller perceptual units-auditory subevents-such as sound onsets and offsets. They developed their ideas by using an auditory illusion called the gap transfer illusion (Nakajima et al., 2000). They found that when a long glide with a temporal gap in the middle crossed with a shorter, continuous glide at the temporal midpoint, the gap was perceived in the shorter pitch trajectory (Figure 1). Although physically present in the long glide, the gap was thus perceptually allocated to the shorter glide, thereby dividing it into two successive short tones.

Nakajima et al. (2000) explained the gap transfer illusion as follows (see also Remijn & Nakajima, 2005). They assumed that the auditory system deals with onsets and offsets of sounds ("sound edges") as independent entities that can be utilized to construct auditory events (see also Kubovy & Van Valkenburg [2001] on the importance of sound edges in defining "auditory objects"). The idea is that an onset and an offset that are close to each other in time and frequency can be perceptually integrated, even if they are parts of physically different sounds. In the gap transfer illusion, proximity between the onset of the short glide and the offset of the first long glide before the gap causes their perceptual integration. This results in the perception of the first short tone. In a similar vein, the onset of the second long glide after the gap is perceptually coupled with the offset of the short glide, constituting the second short tone (Figure 2). With the offset of the first long glide component and the onset of the second long glide component allocated to the two short tones, the long glide is no longer perceived as delimiting a gap and can hence be perceived as continuous.

In the present study, we tested the validity of this edge integration explanation against three alternative explanations for the gap transfer illusion. These three alternative explanations, respectively, state that the two subjectively divided tones in the gap transfer illusion could also be the result of the perception of combination tones or acoustic beats, as well as the result of processes related to auditory induction (Warren, 1999). These alternatives all concern rather low-level phenomena and they needed consideration in light of the results of an informal experiment. …

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