From previous studies, we have seen that bilinguals tend to outperform monolinguals in some tasks requiring subjects to switch between two stimuli; these are known as Cognitive Control tasks. The bilinguals that have taken part in many of these studies are people who speak two languages under the same modality for instance, two spoken languages (unimodal bilinguals). But what happens when a person knows two languages in different modalities, like in the case of bimodal bilinguals, who know a spoken language and a sign language? On this study, there were three groups of participants: unimodal bilinguals, bimodal bilinguals, and people who only knew one language (monolinguals).  Interestingly, all three groups performed equally well in tasks involving ignoring one stimulus in order to respond to another, but the only difference was that unimodal bilinguals were faster than both bimodal bilinguals and even monolinguals. As the authors point out,  bimodal bilinguals can, in fact, produce words simultaneously (one signed, one spoken), whereas unimodal bilinguals have to suppress one language when speaking the other one, because it is impossible to say two words at the same time. Therefore, the faster responses for the unimodal bilinguals could be accounted for by the fact that they are better ‘trained’ at suppressing one language when speaking the other one.

This is a summary of the following published article:

The source of enhanced cognitive control in bilinguals: evidence from bimodal bilinguals. by Emmorey, K., Luk, G., Pyers, J. & Bialystok, E. (2008). Psychological Science 19: 1201-1206.