Prolog
Technically speaking none of these questions have been asked very frequently about the Glial Illusion Hypothesis.
So please imagine a hypothetical future where the questions have been asked frequently.
Or you can just think of this article as a question-and-answer-style exposition of the hypothesis.
Firstly, Some Background Definitions
Glial Cells
In the brain, glial cells are all the cells that are not neurons.
To a first approximation, it is neurons that do all the information processing in the brain, and glial cells only exist to "support" neurons in various ways.
However scientists are discovering that some of the functions of glial cells do include elements of information processing.
Adaptation
An adaptation is a feature or function of a living organism which somehow increases the expected long-term reproductive success of the organism. An adaptation is typically determined genetically, and is presumed to have evolved as a result of natural selection.
For the purposes of this article, the main organisms of interest are us, ie human beings, and also some of our hominid ancestors (perhaps going back two or three million years).
Illusion
An illusion is a false perception.
That is, you think that you hear or see something, but actually the thing you hear or see is not there, or it is different from what you think it is.
Illusions can be regarded as failures of the mechanisms of perception.
In practice our perceptual systems are well adapted to correctly perceive most of the things that we encounter in our daily lives.
Most known illusions arise from stimuli that have been deliberately contrived to lie outside the range of "normal" stimuli that our perceptual systems are adapted to perceive correctly.
Tempo
Tempo is how often something happens per minute (typically applied to describe the result of some deliberate human activity).
For speech it would be syllables per minute, and for music it could be notes per minute.
In musical contexts, tempo is normal expressed as beats per minute (bpm), where the beats are the underlying regular beats, which is not quite the same thing as the notes per minute (although the rate per minute will typically be similar).
Questions and Answers about the Hypothesis
What is a glial illusion?
A glial illusion is when glial cells falsely perceive something. A false perception by glial cells will result in a false response from those glial cells, which may, for example, cause dysregulation of neuronal activity, in the case where those glial cells are responsible for regulating some aspect of neuronal activity.
What is that glial cells "perceive"? I thought that it was the neurons that do all the perceiving.
Neurons process information received from outside the body and also from within the body (but outside of the brain) and the result of some of that processing is what we call "perception".
Glial cells act to support neurons. Some of the support requires the glial cells to observe the state and activity of neurons. The result of those observations constitute a special kind of "perception", ie "glial perception".
Illusions usually require some kind of contrivance of the stimulus that is being observed. How does this work with glial cells?
Good question! To create a glial illusion, it is necessary to present the glial cells with a contrived stimulus which consists of contrived patterns of neural activity. And to create those contrived patterns of neural activity, it is necessary to present a contrived stimulus to the neurons, in this case a contrived auditory stimulus in the form of music.
Does this mean that our brains falsely perceive the auditory characteristics of music?
With a glial illusion, only the glial cells have a false perception – there is no neuronal false perception, and the neuronal perception of the auditory characteristics of the music is as accurate as the neuronal perception of any other auditory stimulus.
What actually is it about the glial perception of the neural activity in response to the music that is false?
The Glial Illusion Hypothesis in its basic form is an abstract hypothesis.
It asserts that some kind of false glial perception is responsible for the effects of music, but, it doesn't go into specific details.
To make the hypothesis more concrete it is necessary to specify the missing details.
The most plausible specific hypothesis I have developed is that music is a super-stimulus for the glial perception of slow speech tempo.
According to this hypothesis, the false perception is that the tempo of speech or music is much lower than it really is.
But I'm reasonably confident that my perception of musical tempo or speech tempo is fairly accurate. So where is the false perception?
As I've already emphasized, your neuronal perception of the auditory characteristics of music (or speech), including your perception of tempo, is quite accurate. It's only the glial perception of tempo that is inaccurate.
How come I'm not aware of this false glial perception of tempo?
Your conscious perception of the tempo of speech or music is entirely derived from the neuronal perception of tempo.
Glial cells do not have direct access to neuronal perception of tempo, partly because glial cells have no way of knowing which neurons represent the perception of particular values of tempo, or even which neurons represent the perception of tempo at all.
Also your conscious perception of anything can only be derived from perceptions represented by the activity of specific groups of neurons representing those perceptions.
So you can't be consciously aware of the slow tempo that is (falsely) perceived by the glial cells.
In that case, how come music makes me "feel" something?
What we consciously feel when we listen to music is the indirect effects of the neural dysregulation caused by the false glial perception of slow speech tempo, where this neural dysregulation causes certain types of neural perception to be intensified, in particular the emotional effects of listening to music or speech.
How does the glial illusion hypothesis explain the quality of musical emotion?
It doesn't.
The glial illusion hypothesis, and in particular the hypothesis that music creates a glial illusion of slow tempo, can only explain the one-dimensional intensity of musical emotion.
To explain the multi-dimensional quality of musical emotion, or even why music should have any emotional effect at all, we have to assume the existence of something else that gives rise to the emotional quality of the emotional response to music.
So, what is the "something else" that determines the quality of musical emotion?
The something else is protomusic.
Protomusic was the prehistoric ancestor of music.
But it wasn't musical.
Protomusic consisted of a language of emotion, ie a language that was used by our ancestors to communicate emotion to each other.
This protomusical language was used to communicate "shared" emotion, that is, to members of a group, asserting that, in the current situation, there is some reason for all members of the group to feel a particular emotion, be it happiness or sadness or something else.
Is this protomusical language of emotion still used by anyone?
It is not.
So if the prehistoric protomusical language of emotion is not used anymore, how come it can determine our response to music?
As modern humans, we no longer have any motive to produce protomusical utterances, but we have retained the ability to understand them.
So what does protomusic sound like?
That is a good question.
We don't know what protomusic sounds like because no one is producing it anymore.
However, if indeed protomusic is something that we still have the ability to understand, then this might be enough to allow us to re-create it somehow.
Further research is required.
What kind of research would we have to do to re-create protomusic?
We would have to ask people to make non-musical but emotional sounds, and then record those sounds, and play them to other people, and see if the listeners agree on both the emotional quality and the non-musicality of the recorded sounds.
We would also expect that the perception of emotion in protomusic should be fairly universal, similar to the degree that the perception of the emotional quality of music is universal. That is, it should not depend significantly on the culture or the spoken language of the listeners.
What type of contrived patterns of neural activity give rise to the glial illusion of slow tempo?
Glial cells in the auditory cortex estimate the tempo of speech by measuring how often neurons go from minimally active to maximally active.
For typical speech, this gives a reasonable estimate of speech tempo, because speech varies in a manner such that, within specific cortical maps, all the neurons are inactive some of the time and active some of the time.
Music is a contrived audio stimulus which has properties such that, for many cortical maps in the auditory cortex, activity is restricted to a specific subset of neurons in each map. As a result, for each such cortical map, neurons within the subset are chronically active, and neurons outside the subset are chronically inactive, and the transitions from minimally active to maximally active do not occur.
As a result, the glial estimate of speech tempo is much lower than the true value.
How low is the illusory glial perception of speech tempo when listening to music?
At this point in the development of the hypothesis, I have no way to give an actual number in answer to this question.
For a wild guess, the estimated tempo might be 5 or 10 times lower than the correct value.
Making a further simplistic assumption, we could assume that the degree of intensification is equal to the ratio between the falsely perceived value and the true value.
So that would imply that music causes musical emotion to be 5 to 10 times more intense.
Why do glial cells need to estimate speech tempo?
The unique thing about speech perception is that the perceived meaning of speech is invariant under time-scaling.
To put that in plainer language – if you say the same sentence faster or slower, the meaning of the sentence remains exactly the same.
It is true that the absolute tempo of speech might tell the listener something about the identity or state of mind of the speaker. But the actual meaning of what is said is completely independent of the speed at which it is said.
What is so special about speech? Why aren't glial cells estimating the tempo of other things?
Speech is a dynamic phenomenon.
Other than speech, the main dynamic phenomenon that is perceived by people, or non-human animals in general, is motion of things.
The meaning of perceived motion is not invariant under time-scaling.
The meaning of a bear moving towards you slowly is very different from the meaning of a bear moving towards you quickly.
Even where the motion doesn't involve the person chasing or being chased, all moving things are subject to the laws of gravity, and gravity on the Earth's surface has a fixed constant value, which is definitely not invariant under time-scaling.
So prior to the evolution of word-based spoken language, there was no need for the human brain to optimise for the perception of dynamic stimuli that is invariant under time-scaling, because there wasn't anything to perceive where the perception is time-scaling invariant.
Why do glial cells specifically need to estimate speech tempo? Neurons are already perceiving speech tempo – isn't that sufficient?
It is true that neurons have their own estimate of speech tempo, ie as represented by the activity of certain neurons in certain cortical maps.
However, the time-scaling invariance of speech perception implies that a corresponding time-scaling adjustment needs to be made to all neural processing that is downstream of the perception of speech sounds, ie starting with auditory cortex, and ending with the activity of those neurons that represent the perception of the meaning of speech and even those neurons representing the emotional response to the perception of that meaning.
The perceived tempo is in effect a single one-dimensional value that needs to be transmitted to all neurons that are downstream of the perception of speech.
Neurons could in principle perform the task of transmitting this value everywhere that it needs to be transmitted to.
But neurons are not optimised for transmitting a single value to millions or tens of millions of receivers. Each neuron effectively performs a job of transmitting a single value specific to the "meaning" of that neuron to maybe hundreds or at most a few thousands of other neurons specifically connected to receive the output of that neuron.
Glial cells, on the other hand, have the option of going "extra-neuronal", and transmitting information in a more global and less targeted manner.
So how do glial cells transmit the estimate of speech tempo to the neurons that need to receive it?
At this point I have no specific answer that I can give to this question.
There are various means that glial cells use to transmit information to each other and to neurons, including changes in concentrations of various ions and transmission of certain neurotransmitters (or "gliotransmitters") into the extracellular medium.
One of more of these means of transmission might be involved in the transmission and reception of the value of the estimated speech tempo.
What features of music actually cause the glial illusion?
The patterns of neural activity that cause the glial illusion are patterns within certain cortical maps where some neurons stay mostly active and other neurons stay mostly inactive.
The aspects of music that cause these patterns of activity necessarily relate to the actual perceptual values directly represented by activity in each of those cortical maps.
That is, cortical maps responding to the rhythmic aspects of speech will respond to the rhythm of music with these patterns, and cortical maps responding to the pitch aspects of speech will respond to the melody (and harmony) of music with these patterns of activity.
Very specifically, these patterns of activity will occur if certain perceptual values occur, and other perceptual values don't occur.
For example, with regard to musical scales, the values that occur are the pitch values of the notes in the scale, and the values that don't occur are the pitch values in between the pitch values of the notes in the scale.
The critical feature of musical scales is that if a tune contains some number N of notes, and there are, for example, only 7 notes in the scale, then all but N-7 of the notes are necessarily exact repetitions of pitch values that have previously occurred.
Extrapolating this observation, we can tentatively assume that all "illusory" musical features of music correspond to the exact repetition of something.
The thing being exactly repeated causes the perception of the values that occur, and the exactness of the repetition prevents alternative values from occurring at all.
(A full analysis of how all the musical features of music might relate to this hypothesis would require a major exposition which would be a bit too long to include within this FAQ. I hope to deal with this matter in more detail in a later article.)
Which is more fundamental, melody or rhythm?
Neither.
The fundamental thing underlying music is the glial illusion caused by certain activity patterns in cortical maps.
Both melody and rhythm cause these activity patterns to occur, but in different cortical maps.
(There may also be some cortical maps that respond to combined aspects of melody and rhythm, so that melody and rhythm are not completely decoupled from each other, and are therefore not completely interchangeable between different musical items, ie you can't just mix the melody from item A with the rhythm from item B and get a perfect result.)
What about dance?
The glial illusion is driven by the glial perception of contrived patterns of neural activity in cortical maps representing the perception of the auditory features of speech or music.
But speech also has gestural components, implying that the perception of speech includes visual aspects.
So the glial illusion could include the glial perception of activity patterns of neurons representing these visual aspects of speech perception.
As in the auditory case, to achieve contrived patterns of neural activity that trigger the illusion, it is necessary to contrive the stimulus that those neurons are responding to, and in the visual case that contrived stimulus is dance.
Probably protomusic also had gestural components, although we may have no easy way to determine what they are. (But by analogy with the auditory case, we might be able to re-discover the form of protomusical gesture by creating "non-musical" dance performances, which would constitute what would in effect be protodance.)
Is music an adaptation?
No.
According to the hypothesis, music is based on a glial illusion. An illusion is a false perception, which by definition is not adaptive.
Is music "auditory cheesecake"?
Cheesecake is not a glial illusion – at least there is no reason to suppose that our love of cheesecake has anything to do with the glial mis-perception of neural activity.
Whether or not the love of cheesecake can be considered to be illusory, in the normal sense of a neuronal illusion, is something that could be argued one way or the other. After all, cheesecake actually is food – it is not "illusory food".
However I would say that, other than suggesting that music is not an actual adaptation, the metaphor of "auditory cheesecake" is not particularly useful.
Is music a form of communciation?
If music was a form of communication, then it would be an adaptation. But it is not an adaptation, therefore it is not a form of communication.
But I went to a concert, and the band powerfully communicated their feelings about political issues to the audience.
Music often feels likes it's communicating something. But this is just part of the illusion.
In practical situations, people do not use music to communicate. No one ever introduces music or singing into a conversation in order to say something.
And in those cases where a band plays songs that are, for example, political or issue-based, what mostly happens is that people enjoy the emotional feelings of music that expresses emotions that they already agree with.
This is a musical version of "preaching to the converted", and there isn't really any communication going on.
I would say that hardly anyone ever goes to a concert, and comes home, and says "I thought that climate change was a hoax, but now after listening to the band sing their songs I realise that it's a real thing that we all have to do something about".