Examining Bilingual Language Control Using the Stroop Task

The Stroop task, named after its creator John Ridley Stroop, has enjoyed over 80 years of popularity in the literature¹. This simple task has been used in hundreds of studies, with different variants created to examine diverse populations and research questions in fields such as psychology, linguistics, and neuroscience. In particular, it has allowed researchers to examine language processes such as the automaticity of written word reading², as well as associated cognitive control processes. The latter are also termed "executive control", and encompass a set of processes that include, but are not limited to, inhibition (i.e., interference suppression), attention, conflict monitoring and resolution, selection, and task switching. Looking specifically at bilinguals, the task is highly suitable for obtaining measures of language interference and control in both a bilingual's first language (L1) and second language (L2), as well as for testing the bilingual advantage hypothesis³, which is currently a topic of considerable debate.⁴

In the original task, the colors red, green, blue, brown, and purple were used in a series of three experiments. In the first experiment, participants read aloud a list of color words printed in incongruent colors (e.g., the word PURPLE printed in blue ink; correct answer "purple") and the same list printed in black ink. In the second experiment, participants said aloud the color of the words in the list (e.g., the word PURPLE printed in blue ink; correct answer "blue") and also named the same colors presented simply as colored squares. Finally, the third experiment examined whether practice of these tasks would affect the outcome, measured in all cases as the time required to read or to name the items in a given list.

The results revealed an interesting asymmetry between word-reading and color-naming: the difference between reading words in different colors and in black was a nonsignificant 2.3 seconds (or a 5.6% increase in the time required to read the color words), while the difference between naming the colors in which incongruent color words were written and naming the colors of squares was a 47.0 seconds, or a significant 74.3% increase in naming time for the words¹. In other words, colors did not interfere with word reading, but reading strongly interfered with color naming. This increase in color naming time in the presence of a conflicting written word has come to be known as the Stroop effect, and while practice can reduce its magnitude, the interference cannot be altogether eliminated.

Different theories have been proposed to account for the Stroop effect, and in an extensive review of 50 years of Stroop literature, MacLeod⁵ describes two of the more predominant ones: 1) the relative speed of processing and 2) selective attention. In the former, words are read faster than colors are named, and this difference in relative processing time causes the Stroop effect. In the latter, the controlled, resource-intensive process of color naming occurs in parallel with the automatic process of reading; the direct competition between diverging word and color information is the source of interference and the Stroop effect². The theory of selective attention is currently the more accepted view^6,7,8,9.

Resolving the word-color competition is a cognitively demanding process that requires inhibiting distracting information while turning attention to the goal of the task. In order to correctly name the color, this interference from the automatically processed written word must be suppressed, while attention must be turned to the less practiced, and therefore controlled, task of color naming. Thus, the Stroop task becomes a measure of not only inhibition, but also of attention, and through various manipulations, it allows different levels of cognitive control to be examined¹⁰. Typically, a decreased magnitude of Stroop interference is considered indicative of better inhibition and attentional resource allocation. A full theoretical review of the Stroop task is beyond the scope of the current paper, but is available from previous works^2,5. Although nonlinguistic versions of the Stroop task exist (e.g., numerical¹¹, oculomotor¹², spatial¹³), the current work is interested in linguistic cognitive control, and the remainder of the discussion will therefore be focused on the original linguistic version of the task.

The original Stroop task has undergone various modifications since 1935, and the task commonly used now includes a congruent condition where the color word is presented in the color it names (the word BLUE presented in blue ink; correct answer, "blue") and a control condition that can be made up of colored shapes as in the original task, asterisks (***), a row of symbols such as X's, %'s or #'s, or high-frequency, noncolor-related words (e.g., DOG). These additions allow the Stroop task to examine facilitation effects. Facilitation effects are brought about by the convergence of information from the written word and the visual color in the congruent condition and thereby speed up naming times⁷ (although see MacLeod⁵ for reliability issues regarding facilitation effects). They can also be used to obtain baseline reading time or color naming time measures using the control items, in addition to Stroop interference effects. Facilitation and interference are defined as the difference in response time between the congruent and control, and as the difference between the incongruent and control trials, respectively, although the Stroop effect can sometimes be calculated as the difference between the congruent and incongruent trial, as in the original task⁶. In the task, the congruent, incongruent, and neutral conditions can be presented in individual blocks (all congruent, all incongruent, or all neutral trials) or in a single block made up of all three conditions, and in a variety of languages.

In the bilingual Stroop task, color terms in the bilingual's first language (L1) or second language (L2), or both, may be presented, and participants may be instructed to name the colors in either of their two languages. This type of task allows researchers to examine both within-language interference (by having the participant name L1 words in the L1 and L2 words in the L2) and between-language interference (naming L1 words in the L2, and vice versa), giving greater insight into how bilinguals process and manage their languages. For example, between-language interference can tell us about lexical representation and the strength of connections between languages in the mental lexicon, and this interference is typically smaller than within-language interference¹⁴.

A variety of language pairs have been used for the Stroop task's bilingual adaptation, including Chinese-English^15,16, Japanese-English^15,17,18, Spanish-English^15,19,20, French-Arabic²¹, Arabic-Hebrew²², English-Greek¹⁴, English-German¹⁴, and Dutch-English⁷. As some have noted¹⁵, factors such as orthography in these languages can influence the magnitude of the within- and between-language interference. Still, despite differences in magnitude, the Stroop effect is always apparent.

It is important to note that many of the studies that incorporate two languages simultaneously²⁰ require overt naming, which is susceptible to lexical access effects²³. In tasks requiring a button press, the L1 and L2 are typically presented separately²⁴, and only include three or four colors^6,8. However, such a presentation may make the task too simple for young adults, whose cognitive control abilities are at their peak²⁵, and result in all participants performing at ceiling. Thus, a single-language presentation with a small number of possible responses may not be sufficient to elicit any possible differences between groups of young adult bilinguals. Therefore, the current protocol seeks to increase the task challenge by increasing the number of color terms and mixing both the L1 and L2 items in a single block, thus not only examining reading automaticity, attention, and cognitive control processes, but also creating a task that is suitable for testing bilingual cognitive control in the young adult population.

In this bilingual task, stimuli were selected in English and French. An example of our stimuli in used is in Figure 1. However, any two languages can be used. For this reason, the protocol below will simply use Language A (La) and Language B (Lb) to describe stimuli in each language. For full details of the experimental protocol, please refer to our previous work^26,27.