That’s great that you find this interesting. It’s for my MA dissertation and the results are not published. But the experiement was successful and the results yielded were accordant with my hypotheses, which was a rather theoretical one.
Basically, one of the prevalent views in linguistics is that human is born with a set of “grammar”. People from different language backgrounds speak different languages because the set of universal rules are reordered by language environments where people live. Note that the rules are reordered not replaced. Some rules that are ranked lower in one language are ranked higher in another. For instance, one rule ® says: syllables prefer to end in vowels. Languages where R ranks higher will manifest the principle, such as Japanese and Mandarin, where syllables cannot end in consonants other than nasals like /n/, e.g., ta, man, etc. However, in languages where R ranks lower, R is covered/masked. For instance, in English, syllables can end in most of the consonants, e.g., bed, cat, etc. However, the rules that are ranked lower can be possibly manifested when listeners acquire another language. We call this “the emergence”.
This experiment on Mandarin tone actually provides support for “the emergence”. The universal grammar rules that: adjacent linguistic units (e.g., tones) prefer to be dissimilar (D). D is ranked lower in Mandarin, but because English does not have tones so Enligsh speakers abide by this rule whenever in encounter. It is found that English-speaking Mandarin learners with a lower proficiency level show more of this principle when producing Mandarin tones than those with a higher proficiency level. Especially, evidence can hardly be seen in advanced learners, whose tone production is similar to that of Mandarin native speakers.
The answer gets a bit long… I hope it makes sense!