Abundant research has examined the relationship between bilingualism and working memory
(WM), a system that keeps information accessible while dealing with concurrent processes,
distractions, or attention shifts (Baddeley and Hitch, 1974; Engle et al., 1999; Conway
et al., 2002). Some studies have reported no WM differences between bilinguals and
monolinguals (Bialystok et al., 2008; Feng, 2009; Bialystok, 2010; Namazi and Thordardottir,
2010; Bonifacci et al., 2011; Engel de Abreu, 2011), leading top scholars to maintain
that this domain is impervious to bilingualism. For instance, Bialystok (2009) first
claimed that WM is indifferent to the development of a non-native language (L2). Later,
she slightly reframed her position, stating that WM is only occasionally enhanced
by the bilingual experience (e.g., Bialystok et al., 2009, 2012). Likewise, in another
study, Engel de Abreu (2011: p. 6) concluded that “bilingual experience does not seem
to convey any advantage in working memory abilities,” which aligns with recent criticism
on the very notion of bilingual benefits (Duñabeitia and Carreiras, 2015; Calvo et
al., 2016; Paap et al., 2016).
However, there is no shortage of evidence for enhanced WM in bilinguals. While full-blown
WM advantages have been only sparsely reported, several studies yielding no overall
benefits did find such effects in specific tasks or conditions. This is also true
of comparisons between bilingual groups who daily exert different levels of demand
on their WM systems (in particular, simultaneous interpreters vs. non-interpreting
bilinguals). These findings indicate that WM is not completely unaffected by the distinctive
executive demands of bilingualism. Instead, they suggest that a bilingual advantage
may indeed exist in some aspects of WM, as we argue below.
The hypothesis underlying the field is that cognitive skills developed to cope with
the demands of controlling two languages generalize to more efficient processing in
executive domains, including WM. Relevant evidence is typically garnered as follows.
First, two sociodemographically matched samples are recruited, one comprising bilinguals
and the other composed of monolinguals—alternatively, these could be interpreters
and non-interpreters. A set of tasks (including WM paradigms) are then administered
to both groups, and their respective results are compared. Crucially, WM tasks vary
widely across studies, as they involve different stimuli, procedures, and presentation
modalities.
Within that literature, some studies reported concrete advantages for bilinguals.
For instance, Bialystok et al. (2004) compared bilingual and monolingual adults (aged
30–80) in three different studies using a non-verbal Simon task. Overall, bilinguals
outperformed monolinguals when WM demands were high, and the extent of the difference
was proportional to age. Further evidence for a bilingual WM advantage was reported
by Morales et al. (2013) in two experiments with children. To this end, the authors
used a Simon-type task and a visual-spatial task. Their overall results showed that
bilinguals surpassed monolinguals in all the conditions involving high WM and executive
demands. Similarly, the bilingual children studied by Blom et al. (2014) showed better
performance in visuospatial (Dot Matrix/Odd-One-Out) and verbal (Forward Digit Recall/Backward
Digit Recall) WM tests when vocabulary was controlled for, especially in tasks that
involved processing and not just storage.
Moreover, studies often cited as disconfirmatory evidence have actually reported enhanced
performance by some bilingual groups under specific conditions. Feng (2009) presented
various WM tasks to monolinguals and bilinguals from two age groups: children and
adults. Despite null results in most conditions, a general bilingual advantage was
observed in a spatial WM task (recalling the position of randomly ordered items).
A similar result was reported by Bialystok et al. (2008), who evaluated bilingual
and monolingual younger and older adults. In this case, participants completed different
WM, lexical retrieval, and executive control tasks. While the adult groups showed
no significant WM advantages, this effect did emerge for younger bilinguals in a Corsi
Block task. Also, Namazi and Thordardottir (2010) compared the performance of young
bilingual and monolingual children through assessments of verbal short-term memory,
verbal WM, visual WM, and visual controlled attention. Although both language groups
performed similarly in most tasks, bilinguals showed positive correlations between
visual WM and attentional control skills. Finally, Bonifacci et al. (2011) tested
bilingual and monolingual children with a choice reaction-time task, an anticipation
task, a go/no-go task, and two WM tasks (numbers and symbols). In this case, only
bilingual infants were faster in a visual anticipation task calling on WM resources.
In sum, even those studies which failed to find overall WM advantages did report such
an effect under certain circumstances.
In this sense, most studies have explored the issue using words or digits as stimuli
(e.g., Bialystok, 2010; Engel de Abreu, 2011). Given that bilinguals generally have
more difficulty than monolinguals in word processing (Bialystok et al., 2009), tasks
with high verbal requirements may not be well suited to test the bilingual WM advantage
hypothesis. Indeed, as seen above, WM tasks employing (non-verbal) visual stimuli
have yielded consistent advantages for bilinguals.
Two views may account for this pattern. On the one hand, the bilingual experience
may selectively enhance a visually-specialized subcomponent within WM. This possibility
is compatible with Baddeley's model (Baddeley and Hitch, 1974; Baddeley, 2000), which
posits that WM comprises a visuospatial sketchpad, separate from the so-called phonological
loop. Moreover, it aligns with meta-analytic data indicating that the development
of specific components of WM may be differentially associated with L2 proficiency
(Linck et al., 2014). On the other hand, it may be that an undivided WM interacts
with several systems in long-term memory. Those systems which are inherently weakened
by bilingualism—in particular, verbal processing (Bialystok, 2009)—would carry over
their processing disadvantages to any task which taps into them, including WM.
Note that executive skills needed to direct visual attention to location and space
may be honed by increased language processing demands. In fact, attentional control
mechanisms are essential to process visual (Chun and Wolfe, 2001) and verbal (Bialystok
and Cummins, 1991) information. Moreover, the attentional control processes of WM
may account for individual differences in the bilingual literature (Linck et al.,
2014). In this respect, modality-specific bilingual advantages in WM may be related
to increased attentional skills. Recent evidence supports this conjecture. Tse and
Altarriba (2014) assessed bilingual children with varied proficiency levels through
the Simon task (Simon/Simon switching) and an operation-span WM task. More proficient
bilinguals showed better conflict resolution and WM capacity when the tasks demanded
more attentional control.
Finally, if the proposed effects stem from increased control demands during bilingual
processing, they should be greater in bilinguals who daily face particularly stringent
processing conditions, such as simultaneous interpreters (García, 2014). Relationships
between WM and interlingual processing skills have been reported in studies which
did not consider interpreters. For example, Kroll et al. (2002) compared word naming
and translation performance between native English speakers with different levels
of L2 competence. In addition to the main finding of the study (better performance
for the more fluent group), a positive correlation was found between the participants'
WM and their translation performance. Such a result fits well with meta-analytic evidence
that WM is robustly associated with L2 processing/proficiency outcomes (Linck et al.,
2014). In light of these findings, it is also worth considering comparisons between
professional interpreters (whose language processing is repeatedly subject to high
WM demands) and non-interpreter bilinguals—an empirical corpus that previous discussions
have mostly neglected.
Bajo et al. (2000) assessed lexico-semantic, comprehension, and WM abilities in professional
interpreters, interpreting students, non-interpreter bilinguals, and monolinguals.
The interpreters showed increased WM spans for digits and words, in addition to faster
categorization, reading, and lexical access skills. Interpreters also showed increased
abilities in other studies tapping WM storage through visual span tasks (Christoffels
et al., 2006; Yudes et al., 2011). For instance, Christoffels et al. (2006) compared
language and WM skills among professional interpreters, bilingual university students,
and highly proficient L2 teachers. The interpreters outperformed both other groups
in WM measures, including word span and reading span—for a fuller discussion, see
García (2014).
Moreover, those advantages have been repeatedly observed in tasks involving verbal
stimuli. Thus, while WM enhancements led by bilingualism proper (as opposed to monolingualism)
may be more pervasive in (non-verbal) visual tasks, those guided by differential processing
skills between bilingual groups could possibly manifest in other domains. Indeed,
the meta-analysis by Linck et al. (2014) revealed that positive correlations between
L2 proficiency and WM may be more pronounced for verbal than non-verbal measures of
the latter domain.
In sum, specific aspects of WM may actually be enhanced by the bilingual experience.
Discrepant results seem to reflect methodological differences among the studies, especially
in terms of task- and stimulus-related variables. Specifically, failure to observe
WM differences between bilinguals and monolinguals in most previous studies may be
explained by the use of verbal stimuli, given that bilingualism seems detrimental
to vocabulary skills. Future studies should evaluate which particular components within
WM functioning are sensitive to the effects of bilingualism. For instance, it would
be useful to assess whether bilingualism enhances the attentional components of WM
in a stimulus- and modality-independent fashion.
To conclude, WM is a complex domain both in its internal configuration and in its
connections to other cognitive systems. Bilingualism may not enhance WM function at
large, but it may improve certain aspects of it. Whether such selective advantages
correspond to improvements in mechanisms within WM remains to be empirically determined.
However, extant evidence suffices to raise a word of caution: failure to observe an
effect in certain aspects of a function should not be automatically taken as evidence
for a null effect in all of its components. Further research on the distinctive aspects
of bilingualism might benefit from this general premise.
Author contributions
Overall idea: NC, AG. Literature review: NC, AI, AG. Manuscript elaboration: NC, AI,
AG.
Conflict of interest statement
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.