[SOUND] For the most part, in this course I've kept the discussion of each of these factors, age of acquisition, proficiency, and control, separately. Considering how we can think of each of these constructs in a different way. But the truth is more complex than that. These three factors interact. We can think about, for example, how age of acquisition and proficiency interact. That is, someone who learns a language earlier in life, in general, ends up with better proficiency over time if they continue using that language. We can also consider how proficiency and control interact. When someone speaks a language less proficiently it requires more control. The idea again is that the more proficient language will be the preponderant one, the one that's more automatically activated. And so, the second language or the less proficient language, will require more effort. Finally, we can think about how all three of these factors interact. So we can consider the difference between learning a language early in life, for example, in monolinguals and the speech perception studies that we discussed earlier in the course. In monolinguals what happens is that those speech categories, the difference between ba and pa, the buh and the puh is acquired quite early. By the time a monolingual is five years of age, the superior temporal gyrus can process this information quite easily, and so speech recognition becomes quite automatic. In an early sequential bilingual who acquires a second language around school age, what we see is initially the use of these same areas for the second language. But a transition which involves control, because now control areas are active for speech perception, but at the same time, involves better proficiency. So proficiency, control, and age of acquisition interact and work as a tandem to help develop these abilities. So we can ask, what kinds of models might we use to describe the bilingual brain? There are two approaches that I'll describe. One is a computational approach, the idea that we can create a computational model, a computer model that might help us to explain bilingualism. And the second is a biological approach, that we can understand how biological systems operate, how the brain operates to understand how bilingualists process language.
