In the previous post, we looked more closely at automatic speech with some video examples. Just returning to one of these examples have a look at the clip below.
In the three structure model, automatic speech represents the final common pathway through which speech is actuated. The final common pathway is the electrical activity occurring in the nerves (mostly cranial nerves) that actuate the muscles of speech. It is assumed that this activity is entirely unconscious and is in no way capable of being conscious activity.
If you watch the clip repeatedly and very carefully it becomes clear that unsurprisingly the tongue plays a central role in speech production. Some of the actions that are seen are
1. Mouth opening
2. Lips pursing to articulate sounds
3. Tongue depressing
4. Tongue moving up to touch hard palate
5. Tongue touching soft palate and closing nasopharyngeal air space
If we assemble some of these actions to reproduce the actions at the beginning of the video it would read something like as follows
1. Open Mouth by lowering jaw
2. Depress tongue
3. Push lips together
4. Lift tongue to touch hard palate
5. Move tongue rapidly down
6. Move tongue to soft palate to close off nasopharynx
Whilst this is going on there are two other things happening
1. The force of air expulsion from the lungs is being controlled by the muscles of respiration
2. The vocal cords are being relaxed and contracted whilst the lung air expulsion is vibrating the vocal cords
The combination of all of this results in speech. From the multiple steps above it seems entirely unlikely that conscious or at least potentially conscious activity (i.e. transient unconscious activity) is not playing a central role in the transition from one step to another of this process.
In other words it is highly unlikely that the brain sends a signal to the muscles associated with speech and then sits passively by while all of the actions above take place. It is most likely that the conscious/transient unconscious activity intervenes in each transition.
So for instance you would be able to speak very slowly. In so doing you would exert a clear and conscious control over actions. You could lower your jaw and be absolutely certain that you are consciously in control of this action. You could purse your lips and be similarly confident that you are in conscious control of this action. You could touch the roof of the mouth with your tongue and again be confident that you are in conscious control of this action. By definition such actions are not absolute unconscious activity but conscious/transient conscious activity.
This means that the place for absolute unconscious activity takes a more peripheral role in speech. It becomes much less relevant and probably applies to the last few milliseconds of the signals moving from primary cortex to effector nerves.
If this is correct it would mean that this is recipricocity between streams of conscious speech commands and brief bursts of ‘reflexive’ automatic speech. This much is clear from the above thought experiment but it is also relevant to some of the theoretical knowledge about corticospinal pathways and also their relation to the Cerebellum which is closely involved in the monitoring of motor activities.
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