The subjects' brain activity was measured when they were asked to perform in French. The patterns for each group were very different and for the group that learned French later there was a short, though measurable, lag time. The lag time was noted even in those subjects that would have been considered completely fluent with no accentI've often wondered about this. I first started learning Italian when I was 3 or 4, and I also spent kindergarten in French Immersion. Then to further confuse me linguistically, I lived in Sardinia for two years. Although you couldn't really call me fluent in French any more, what I do say comes naturally, there is no stopping to translate words before saying them. And I wouldn't have any trouble communicating in Quebec if I had to. I've always thought I had somehow managed to internalize all the French that my teachers were saying to me, even though I didn't understand it at the time. I did go back into immersion for three years in junior high and I found the language came quite naturally; I never had much trouble picking up vocabulary.
In contrast, I have been trying to learn Turkish, and it is a STRUGGLE. It has absolutely no connection to Latin, as French and Italian do. I was shocked at how different my Turkish experience has been!
