Illusion

Illusions and Music Psychology

The scale illusion, first reported by Diana Deutsch, informs about the relative importance of different criteria, according to which musical events are grouped together perceptually. It involves the repeated, simultaneous presentation of an ascending and a descending scale, the notes of each of which emanate alternately from the left and right ear of a pair of headphones (see Figure 1, top). Most listeners perceive this stimulus as shown in Figure 1 (bottom): all the higher notes appear to come from one side, and all the lower notes from the other (which is which appears to depend on laterality). This is because the listeners' preference for grouping notes according to their pitch proximity overrides (and even corrects) the conflicting groupings arising from the similarities in their points of origin. Other illusions reveal interactions between the different dimensions of music perception: experiments indicate that melodies are judged to be faster when they are played at a higher pitch and/or with a brighter timbre, and that melodies of constant volume that increase in pitch are heard to get louder (and vice versa). The auditory tau and kappa effects, which lead one to hear tones that are close to each other in time to be closer in pitch than they really are (and vice versa, respectively), suggest a similar interdependence of pitch and rhythm perception.

Figure 1(Top): The scale illusion: presentation (dotted lines connect the notes of the two scales). (Bottom): The scale illusion as commonly perceived.

It has been hypothesized that these phenomena reflect not only an accustomization to the frequent correspondence between such parameters in Western music but also the legacy of an evolved tendency to make time-saving generalizations about the movement of sound-producing entities in a natural environment (animals tend to make more noise when they run faster, for example).

Some psychological illusions have a direct impact on the way in which music is performed. The filled duration illusion, for example, shows that if the beats of a musical meter are subdivided by events that occur between them, the tempo is perceived to be slower. To compensate for this, it appears that performers tend to increase their speed when such rhythmic subdivisions are first [Page 576]introduced in a piece (as might be the case when the first of a set of variations follows the initial presentation of the theme).

Illusions and Speech Processing

Illusions known to occur in speech processing also appear to apply to music perception. When extraneous sounds are presented in the midst of melodies, one perceptually relocates them to the boundaries between phrases (the same is true with the syntactical units of a sentence). When one note of a melody is replaced by a crackle, one assumes that it must be masked by it and so supplies the missing note from the context (as one would a missing word).

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