The video above from Kyle Manley’s channel illustrates a range of auditory illusions. The reader may recognise the initial Shepard tones from a previous post. Although the sounds are rather ominous, working through the video the most remarkable segment contains the words ‘high’ and ‘low’. When different words are presented on the screen the spoken words take on the appearance of the written words. I was able to hear these words convincingly for the first few words although it was a little less convincing for later words. I will refer to the effect eponymously as the Manley Words.
These illusions are quite impressive and have some similarities to the McGurk Effect examined in a previous post. In both cases, if you are expecting something from a visual cue then this can influence your perception of sounds. To hear the same sound as pine, magpie and bagpipe as simple examples suggests that these effects are quite powerful.
Ambigrams are symbols that manifest a different meaning when viewed from another perspective. The sounds in the videos above are effectively auditory ambigrams and the perspective is contextualised by the presented visual information. If we can characterise the McGurk Effect and the Manley Words as auditory ambigrams contextualised by visual stimuli then we can look at auditory hallucinations from another perspective.
There will be no explanation for all cases of auditory hallucinations as they are symptoms with a heterogenous collection of causes. In some cases however, these causes relate to hearing impairment. In such cases we can hypothesise that
p(H) = c (B/A)
p(H) is the probability of an auditory hallucination
A is the clarity of a stimulus
B is the strength of the contexualising stimulus
c is a constant
Unfortunately this equation is only useful in demonstrating relationships. The difficulty is that B is difficult to define. In the case above the meme is this:- what we are hearing is actually the visually presented word. The clarity of the stimulus is dependent on the person’s hearing, the loudness and proximity of the auditory stimulus and also the number of similar words that the stimulus must be distinguished from.
There is still some difficulty in distinguishing the above from an illusion. If one word is mistaken for another then it is more likely to be an illusion. Background sounds such as the wind or the sea are more difficult to interpret. There can be a case for referring to them as hallucinations although this will dependent on the context (strictly speaking hallucinations occur in the absence of a stimulus – however visual hallucinations occur on a background of a visual stimulus – it is just that they are considered significantly different).
Looking at the formula above it reads that the likelihood of an auditory hallucination is proportional to the strength of the contextualising stimulus (e.g. a strong belief about the world) divided by the clarity of the stimulus (e.g. someone saying Hello very loudly and clearly is a clear stimulus). If a person is driven by a strong belief then this can cause them to misinterpret what it is that they are hearing. In the example above if we look for the word bagpipe or the word pine that is what we will hear particularly where the stimulus is ambiguous.
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