A research published in the journal Nature has shed light on exactly why tongue-twisters like 'she sells sea shells on the sea shore' are so hard to say.
It found the brain exercises split-second, symphony-like control to coordinate the tongue, jaw, tongue and larynx to articulate the words we speak.
In the same way an orchestra relies on a conductor to coordinate the orchestra's plucks, beats or blows to make music, speaking demands well-timed instructions from the brain to orchestrate these various parts.
So, like a conductor's gestures being picked up by the wrong player, when the brain sends messages to muscles near each other that make different sounds, confusion can ensue.
The research by a team from the University of California, San Francisco has potentially important implications for the treatment of speech disorders.
It also sheds light on an ability that is unique to humans among living creatures but poorly understood.
'Speaking is so fundamental to who we are as humans – nearly all of us learn to speak,' said senior author Edward Chang, a neurosurgeon at UCSF.
'It's probably the most complex motor activity we do.'
The complexity comes from the fact that spoken words require the coordinated efforts of numerous 'articulators' in the vocal tract – the lips, tongue, jaw and larynx.
However, until now, scientists have not understood how the movements of these distinct articulators are precisely coordinated in the brain.
To shed light on how speech articulation works, Dr Chang and his colleagues recorded electrical activity directly from the brains of three people undergoing brain surgery at UCSF.
By implanting an electrode array under the skull of the outer surface of the patients' brains, they were able to record neural activity related to the enunciation of various commonly-spoken English syllables.
They used this information to determine the spatial organisation of the 'speech sensorimotor cortex,' which controls the lips, tongue, jaw, larynx as a person speaks.
This gave them a map of which parts of the brain control which parts of the vocal tract.
They then applied a sophisticated new method called 'state-space' analysis to observe the complex spatial and temporal patterns of neural activity in the speech sensorimotor cortex that play out as someone speaks.
This revealed a surprising sophistication in how the brain's speech sensorimotor cortex works.
They found that this cortical area has a hierarchical and cyclical structure that exerts a split-second, symphony-like control over the tongue, jaw, larynx and lips.
For example, the neural patterns involved in pronouncing consonants, they found, were quite different from those for vowels, even where they used exactly the same parts of the vocal tract, Dr Chang told Nature.com.
That difference could help to explain why slips of the tongue happen in predictable ways. In so-called spoonerisms, we often mix up two consonants, or two vowels, but hardly ever do people mistakenly swap consonants for vowels.
Findings also showed the brain seems to coordinate its articulation of words not by what they sound like, as was previously believed, but by which muscles it needs to move.
The UCSF data distinguished front-of-the-tongue consonants (like 'sa'), back-of-the-tongue consonants (like 'ga'), and lip consonants ('pa'); vowels split into those that require rounded lips ('ooh') or not ('ee').
'This implies that tongue twisters are hard because the representations in the brain greatly overlap,' Dr Chang told Nature.
'Sss' and 'Shh', for example, are both recognised by the brain as front-of-the-tongue sounds, so it more easily confuses these than sounds made by different parts of the tongue.
And that's why 'she sells sea shells by the sea shore' is more difficult to say than 'he sells sea snails by the green door'.
(source:scientific research publishing )