David Green offered an explanation for this phenomenon with his "inhibitory control model." Proposed in 1998, this model references a bilingual's constant need to suppress one language while using another. Because this task requires suppressing a source of distraction, this kind of control is then applied to other tasks. This assertion was bolstered by a study of unimodal bilinguals (bilinguals who communicated with two spoken languages) and bimodal bilinguals (bilinguals who used one spoken language and sign language). Because bimodal bilinguals can express themselves in both languages at the same time, they may require less inhibition. This idea was supported by the results of the study; only unimodal bilinguals were found to have an advantage, as measured by the flanker task (a cognitive task that measures attentional focus and inhibition). Bimodal bilinguals also switch languages less frequently, because they are more likely to use both languages at once than to completely switch from one to the other. For this reason, the researchers of this study hypothesized that it may be the switching between languages that gives unimodal bilinguals the advantage. Prior and Gollan conducted a study investigating this idea, and found that bilinguals who switched languages often had an advantage in task shifting over bilinguals who did not frequently switch languages. However, this study did not control for similarity between the languages (languages that are more similar might require more attention to keep straight). When Verreyt, Woumans, Vandelanotte, Szmalec, and Duyck ran a similar study but with all participants having the same languages, they replicated the results of Prior and Gallan. Additionally, because their study looked at tasks measuring inhibition even though language switching should directly affect switching tasks, they argued that the effects of language-switching carry over multiple facets of executive control.