Course:PHYS341/2022/Project3

Auditory Illusions

By Joshua Wong

Auditory Illusion is the false perception of external auditory stimuli. Listeners perceive sounds that are not present in the stimuli due to the additive psychoacoustics effects of the actual presented stimuli. Moreover, this misrepresentation is not due to psychological or physiological cause, such as hallucinations or auditory cortex lesions.  

Background

The Perception of Sound

In physics, sound is defined as a vibration that propagates through a transmission medium such as gas, liquid or solid. Perception of sound is the reception of such waves and their interpretation by the brain. In humans, the outer ear channels air vibrations into the ear canal, which then amplifies middle ranged hearable frequencies (especially frequency range from 1000Hz to 3000Hz) in normal, healthy ears. Pressure fluctuations inflicted upon the eardrums will be transmitted and amplified through the middle ears in the form of mechanical energy. The mechanical vibrations will set the inner ear fluids into motion in the cochlea. Hair cells that line the cochlea ducts (scala media) detect the vibrations, transferring the mechanical signals into electrical signals that are then projected to the primary auditory cortex through the auditory pathways.

Illustration of Auditory Pathway

Illusions in the Psychoacoustic Perspective

Psychoacoustics is the scientific study of the human perspective of sound, how we perceived it and interpret it. It is an interdisciplinary study that includes physiology, cognitive psychology and physics. Illusions are the natural by-products of neural processing of sensory information under the conditions of limited brain resources. In order to maintain processing speed without diminishing the quantity of information input, the brain may falsely interpret our surrounding environment. The brain tries to fill in gaps in the sensory inputs by systematically distorting incoming stimuli, eventually contributing to illusions. Also, auditory illusions are not caused by physiological injuries - such as primary auditory cortex lesions, auditory neuropathy - but rather is a perceptual phenomenon that is experienced by the majority of healthy individuals.

Contrary to auditory illusions, hallucinations are false sensory perceptions that present a compelling sense of reality despite the absence of external stimuli; it may only be experienced by a small number of individuals. It is easy to confuse illusions with hallucinations, the latter being false sensory perceptions - often without exposure to actual external auditory stimuli - rather than misinterpretations of external stimuli . Hallucination is often considered as a symptom of psychotic disorders, but illusions are not. Despite auditory illusions being a universal perceptual experience, the precise experience among individuals vary. This suggests that the dissimilarity must be caused centrally and cortically, encouraging further investigation on the variability of anatomical and functional units within the human brain. 

Example of Auditory Illusions

Binaural beats and Missing Fundamental

Binaural beats (BB) consist of two sinusoids with slightly different frequencies that are superimposed upon each other.  The resulting interference causes periodic amplitude fluctuations which frequency corresponds to the frequency difference between the component waves. The frequency difference is also referred to as a beat. In other words, when two pure tones with a subtle frequency difference are separately presented to each ear, a third tone with a frequency identical to the frequency difference of the presented tones will be perceived in the brain. Beat detection is limited at around 1500Hz, and the detection probability is maximized at 500hz .

When slightly different frequencies from the left and right ear converges, they interact in the central auditory brainstem pathways and generate beats of neural activity to modulate activities in the left temporal lobe, giving rise to the illusion of binaural beats . The cortical activity presented suggests that binaural beats are perceptual phenomenon

Tritone Paradox

The Tritone Paradox is an auditory illusion is caused when a pair of Shepard’s tones separated by an interval, (either three adjacent whole tones or six semitones) are sequentially played . Causes of the Tritone Paradox also point to brain activities, especially those in the language pathway. Recent findings suggest that the variability of  how listeners perceive the tone (either ascending or descending) may be linked to their first language or speech development . The findings suggest that speech and music (tonal perception) correlates with each other in a specific way.

The Penrose Stairs is a visual parallel to Shepard’s tone.

Shepard’s Tones

Shepard’s tone or Shepard’s scale, also known as pitch circularity, consists of multiple sinusoidal pure tones with varying amplitudes, creating an auditory illusion of a tone that seems to continually ascend or descend in pitch. Shepard’s tone was first introduced by Roger Shepard in 1964 . Shepard’s tone is created by playing at least three notes in which each are one octave apart. These three tones are layered carefully where the top and bottom layers intensities changes over time while the middle layer intensity remains the same. Shepard’s tone also applies to tempo, which is also called the Risset beat or tempo circularity.

Scientist believe that primary auditory cortex is the key player of the Shepard’s scale illusion based on fMRI time series studies, where participants’ brain activity were monitored while Shepard’s tones were continually presented to them .

Implication of Auditory Illusions in Music and Film Industry

When we listen to music, listener’s attention is focused more on the context rather than the note. Every sound carries a primary piece of musical information – it resonates for a particular duration with a certain intensity and timbre at a pitch by which we first identify it as a note . Music composer realize the effect of auditory illusion; they apply such technique into their music. One example is the first song in FranzFerdlnand’s “Always Ascending” album, Always Ascending.

In film industry, Shepard’s scale or tone is frequently utilized in film production to alter audience’s auditory sensation. Christopher Nolan , a British-American film director, producer, and screenwriter of several sci-fi movies often explores the field of complex dimensional analyses. A few examples of his works are Inception (2010), Interstellar (2014) and TENET (2020). In most of his films, he combines visual and auditory effects to induce sensational illusions. Hans Zimmer , a soundtrack composer and record producer, synthesizes and has directed the music in several of Nolan’s films. In the below is an example of Zimmer’s composition with the use of Shepard’s tone, taken from the movie Interstellar (2014) .

Another application of auditory illusions is the missing fundamental. Missing fundamental is useful in producing the illusion of bass in sound systems that are not capable of such bass. This technique is used in audio system by emphasizing higher frequencies harmonics to fill in the missing fundamental .

References

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