Humans tend to classify things that are more similar and closer to each other into groups, as shown in the figure below. [1]
In figure (a), we tend to group the two black dots that are closer together. In figure (b), we tend to treat the dots of the same color in the same column as a group.
The grouping effect plays an important role in the evolution of mankind. It helps us quickly understand the conditions of our surroundings.
In the previous experiment about Octave Illusion, it was discovered that when the pitch interval between high and low tones becomes small, the original illusion disappears. This phenomenon can be related to the grouping effect. The pitch interval becomes so close that our brain treats them as the same group. Therefore, there is no longer an effect of different perceptions between ears.
Grouping effects appear in music frequently. Many musicians have tried similar examples, such as the melody in the picture below. [2]
In auditory, we treat the last three Dos of the four 16th notes in each beat as a group. The first high note of each beat will be split to reveal the melody: Fa, So, La, So, La, Ti, Do.
Another good example in Diana Deutsch's book "Musical Illusions and Phantom Words" is "Galloping Rhythm" [3]. When the pitch interval becomes larger, we naturally distinguish the notes as two unrelated melody lines depending on their pitches. When the pitch interval becomes close, we integrate all the notes into the same stream. This is what I observed in doing the extension of "Octave Illusion" by changing the pitch interval. I suddenly thought of the grouping effect.
Galloping Rhythm
Scale Illusion
This is another example in Diana Deutsch's book "Musical Illusions and Phantom Words". It can be said to be the combination of octave illusion and grouping effect. [4] It is recommended to listen with headphones.
In the figure above [5], the upper row shows the actual pattern played by the left and right channels, and the lower row presents the melody we may perceive. Try to compare the difference between listening only to one of the two channels and listening to both. We on one hand treat the notes with closer pitches as continuous, on the other hand our right ear tends to perceive the high tones and the left ear tends to perceive the low tones (for most people). This phenomenon is similar to the results of the Octave Illusion experiment.
[1] Deutsch, Diana. Musical Illusions and Phantom Words (p. 48). Oxford University Press. (2019)
[2] G.Ph.Telemann. Sonata in C Major. TWV 41:C2
[3] Same as [1] (p. 50).
[4] Same as [1] (p. 33).
[5] Same as [1] (p. 34).