Shepherds Madness
Auditory illusions have always been of great inspiration to me. Sounds as we
perceive them in music are subjectively warped by our perception. Good composers are
aware of the brain’s limits, biases, and conditioning. Taking advantage of these is what
can make music so impactful. One auditory illusion that clearly shows the power that
sound can have on the brain is known as the shepard's tone illusion. In this assignment
I will explain the illusion and how it works.
The shepard’s tone illusion consists of notes or frequencies that seem to the
listener to ascend in pitch indefinitely. The effect can also be inverted so that the pitches
are instead perceived to get lower in pitch without end. The experience of listening to
the shepherd's tones can be extremely disorienting to some listeners. Inciting feelings of
nausea, stress, and anxiety. For me personally this is true and it is difficult for me to
listen to the shepard's tone for longer periods of time. Many filmmakers have used this
illusion to raise tension in scenes such as the movie Dunkirk.
To explain how the effect is achieved it is helpful to relate it to a visual illusion
that shares some similarities. In the barbershop pole illusion each line on the pole is
parallel and angled diagonally, therefore, when spinning the lines not only move
horizontally but also give the effect that they are ever ascending vertically as well. If one
were to follow one line as it seemingly rose to the top of the pole and disappeared, a
new stripe would take its place at the bottom. This is very similar to the way the pitches
are organized in the shepard's tone illusion. In the shepard's tone, multiple frequencies
that are all equally spaced an octave apart, all raise in pitch together. As one tone raises
up and out of our hearing range it slowly decreases in volume just as a new frequency
is introduced at the bottom of our hearing range and increases in volume as it rises in
pitch. Objectively, the pitch and volume content of the sound stays absolutely constant,
however subjectively it can seem to be rising in both pitch and volume.
This trick relies on the way our brains are conditioned to hear. Researchers
Shimizu et al., (2007) studying brain MRIs of people subjected to the illusion found that
although human auditory reception has a range of 10 octaves, particular sensitivity to
the illusion was recognized between just 4 octaves (p.119). Unsurprisingly, those 4
octaves between 500 Hz and 5 kHz are around the average pitch range of human
speech. Our auditory comprehension developed in parallel with and primarily to
understand speech, therefore, it is conditioned to hear in that range (Shimizu et al.,
2007 p. 113). The brain gives less cognitive weight to those ranges and this allows the
Shepherds' tone illusion to sneak tones away at the higher ranges of our hearing and
slip them back in at the bottom indefinitely. Further the same researchers found that
“neural responses [were] not restricted to the auditory cortex, but also involved areas of
the visual cortex” (Shimizu et al., 2007 p. 113). This may explain the disorienting
feelings of anxiety that the illusion causes.
Citations
Shimizu, Y., Umeda, M., Mano, H., Aoki, I., Higuchi, T., & Tanaka, C. (2007).
Neuronal response to Shepard's tones. an auditory fmri study using multifractal
analysis. Brain Research, 1186, 113–123.
https://doi.org/10.1016/j.brainres.2007.09.097