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Transfer to response inhibition

The transfer was further assessed with regard to response inhibition using the Simon task which requires inhibition of spatially compatible (pre-potent) responses. The strength of response inhibition was measured by subtracting the mean response times on spatially compatible trials from spatially incompatible trials (on which the pre-potent spatial response needs to be inhibited). Trials with incorrect responses or with outlier response times longer than 1.5 interquartile ranges above the 75% percentile of the entire RT distribution (> 572 ms, 4.6% of all trials) were removed prior to the analysis. The resulting Simon effects at pre-and post-test are illustrated separately for the three experimental groups in Fig. 3. The average Simon effects at pre-and post-test demonstrated moderate test-retest reliability, ICC(C,2) = 0.68 (N = 74).

a general practice effect for response inhibition. However, there was no main effect of group, F(2,71) = 0.65; p = 0.53; η2G = 0.01, and no interaction between group and test, F(2,71) = 0.27; p = 0.76; η2G < 0.01, suggesting that the present two trainings with dual n-back tasks did not reduce the stimulus-response incompatibility effect, as compared to a passive control group. This interpretation is confirmed also by the Bayes factors indicating that it is more likely that the decrease of the Simon effect did not differ between the two training groups and the control group (BF10 = 0.34 and BF10 = 0.33 for the inhibitory and standard n-back training groups, respectively). Hence, in particular, the inhibitory n-back training does not seem to have strengthened the inhibition of pre-potent responses in general.

Fig. 3 Average response time differences between spatially compatible and incompatible trials in the Simon task at pre-and post-test. Error bars depict ± one standard error of the mean

Fig. 4 an Average serial recall performance (proportion of digits recalled in the correct serial position) on trials with task-irrelevant speech and noise at pre-and post-test, and b the difference scores in recall performance between speech

and noise trials referring to the degree of auditory distraction (ISE score). Error bars depict ± 1 standard errors of the mean

Transfer to auditory distraction

standard n-back training group (p = 0.01) and the control group (p = 0.05), whereas there was no difference between the standard n-back training and the control group (p = 0.25). Likewise, Bayes factors indicate that a differential decrease of the ISE from pre-test to post-test in the inhibitory n-back training group, compared to the two other groups, is much more likely than the null hypothesis (BF10 = 12.56), whereas there is no evidence for a difference in the change of ISE scores between the standard n-back training and the control group (BF10 = 0.50).

There was also a significant main effect of sound, F(1,71) = 229.30; p < 0.001; η2G = 0.24, confrming the over- all irrelevant speech efect. In addition, there was a main efect of test, F(1,71) = 28.04; p < 0.001; η2G = 0.05, indi- cating a general improvement in the serial recall task (from M = 0.62; SD = 0.13 at pre-test to M = 0.68; SD = 0.12 at post-test). The ANOVA revealed no other signifcant efects,F < 1.45; p > 0.24.

Far transfer to task‑switching and fluid intelligence

Transfer of working memory training was assessed also with regard to tasks that require cognitive set-shifting and general problem-solving abilities.

For the task-switching data, response times during task-switching were analyzed only for trials with correct responses. In addition, outlier response times longer than 1.5 interquartile ranges above the 75% percentile of the entire RT distribution (> 2230 ms, 5.6% of all trials) were removed prior to the analysis. The response-time costs due to switching from one task set (letter categorization) to a different task set (number categorization) were calculated by subtracting the response times on repeat trials from switch trials. The resulting switch costs are illustrated in Fig. 5a, and the average switch costs demonstrated good test-retest reliability between pre-and post-tests, ICC(C,2) = 0.68 (N = 74). A 2 (test) × 3 (group) mixed-factors ANOVA revealed no main efect of group, F(2,71) = 0.16; p = 0.85; η2G < 0.01, no main efect of test, F(1,71) = 1.10; p = 0.30; η2G < 0.01, but a marginally significant group × test interaction, F(2,71) = 3.07; p = 0.05; η2G = 0.02. The interaction is likely driven by the reduction of the relatively high switch in the control group, whereas the two working memory training did not reduce the participants’ costs during task-switching (see Fig. 5a), suggesting that the two types of n-back training did not afect set-shifting abilities. Moreover, pairwise t-tests and Bayes factors revealed no clear evidence for a differential change in switch costs between the control group and the two n-back training groups (adjusted p = 0.08; BF10 = 1.39 and BF 10 = 1.62, respectively).

Finally, transfer was assessed about non-verbal problem-solving capabilities related to fluid intelligence by having participants solve eighteen different visual matrix problems at pre-and post-test. The average number of problems solved within 10 min is shown in Fig. 5b, and these scores of fluid intelligence demonstrated good test-retest reliability across all participants, ICC(C,2) = 0.83 (N = 74). While there seems to be a slight increase of the scores in the two trained groups, compared to the control group, a 2 (test) × 3 (group) mixed-factors ANOVA revealed no main effect of group, F(2,71) = 0.51; p = 0.61; η2G = 0.01, and no significant interaction, F(2,71) = 0.38; p = 0.68; η2G < 0.01, suggesting that the two types of working memory training did not exert a reliable effect on fluid intelligence in the present study. There was only a marginally significant main effect of the test, F(1,71) = 3.13; p = 0.08; η2G = 0.01, indicating a trend for a general pre-post improvement in matrix solving performance across all three groups. Consistent with these results, corrected pairwise t-tests and Bayes factors suggest that it is unlikely that the type of training differentially affected the change of fluid intelligence scores from pre-test to post-test (p > 0.80 and BF10 < 0.37 for the three contrasts).

Discussion

The present study showed that an extended cognitive training with two adaptive versions of the demanding dual n-back task, varying in the degree of inhibitory control required, improved working memory capacity not only for the trained task (i.e., the value of n increased from the first to the eighth training session) but also for a different type of updating task. However, this improvement on the untrained memory updating task was significantly different from the passive control group (which showed general improvement on the task) only for participants who were trained on the dual n-back task with additional demands for response inhibition, but not for participants who were trained on the standard dual n-back task. Hence, the transfer of training on the dual n-back task to general working memory updating abilities seems to depend on the degree of inhibitory control involved in the training task. This funding is quite consistent with a recent meta-analysis on the effects of n-back training concluding that the magnitude of transfer from training on the standard dual n-back training to other working memory paradigms, such as operation span or running span tasks, is very small (Soveri et al. 2017). The present findings suggest that extensive training with cognitive tasks for which multiple executive functions are required (e.g., updating and inhibition) may enhance the likelihood of near transfer effects to other working memory tasks, as compared to cognitive training with tasks that address only a single executive function (e.g., working memory updating in case of the n-back task).

In addition to the near transfer effects of the present training to working memory updating, transfer was tested also concerning two separate forms of inhibition. As the two types of n-back training tasks differed only about the demand for inhibition control (i.e., to suppress the predominant response on n-back trials), more transfer to other inhibition tasks was expected for participants who were trained on the inhibitory dual n-back task, as compared to training on the standard dual n-back task. How- ever, it was unclear whether to expect the transfer to inhibitory control of pre-potent responses, resistance to distractor interference, or both (Friedman and Miyake 2004). The results indicate that the two training tasks did not differ about transfer to pre-potent response inhibition. None of the two n-back pieces of training reduced the response-time costs for key presses that were spatially incompatible with the target location in the Simon task, as compared to the passive control group. This funding may be surprising given that the inhibitory dual n-back task required participants to suppress the more frequent and hence pre-potent key presses throughout the eight training sessions. However, the training task did not require participants to solve a spatial compatibility conflict as in the Simon task. Hence, the present results suggest that the inhibition of a frequently occurring response (as during training) may depend on a form of inhibitory control that is functionally distinct from the inhibition that is required for the resolution of a stimulus-response compatibility conflict. Future research is required to determine whether the present training of inhibition within a dual n-back working memory task generalizes to more similar types of pre-potent response inhibition tasks, such as the stop-signal task (Logan 1994; Verbruggen and Logan 2009).

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By contrast, the two types of n-back training yielded differential transfer effects about inhibitory control of the interference produced by task-irrelevant speech dis- tractors in a verbal short-term memory task. Specifically, participants who were trained with the newly developed inhibitory dual n-back task, requiring continuous updating and inhibition of the contents in working memory, seem to have enhanced their resistance to an irrelevant speech during serial recall. In contrast, a working-memory training with the standard dual n-back task demanding less inhibitory control does not seem to have an effect on the magnitude of the irrelevant speech effect. This attenuation of auditory distraction after inhibitory n-back training suggests that the training-related strengthening of inhibition enabled participants to reduce the interference from the external auditory environment (Friedman and Miyake 2004). About accounts of auditory distraction, the finding indicates that inhibitory control may have prevented attentional capture by task-irrelevant speech, but it may not have reduced the disruptions due to interference-by-process (e.g., in line with the duplex-mechanism account; Hughes 2014). Specifically, the fact that the irrelevant speech effect was reduced, but not eliminated after a comprehensive inhibitory-control training indicates that enhanced inhibitory control may prevent the diversion of attention by irrelevant speech, whereas the (remaining) interference with the seriation process produced by the changing-state nature of irrelevant speech may not be susceptible to top-down control. The observation that a considerable portion of auditory distraction (presumably the changing-state effect) could not be eliminated by enhanced inhibitory control might suggest that the interference-by-process mechanism is not related to general working memory functions, and thus not susceptible to inhibitory or cognitive control (see Hughes 2014). This interpretation of the present results would be consistent also with other recent observations showing that only attentional capture, but not the changing-state effect, can be reduced through cognitive control (Hughes et al. 2013; Hughes and Marsh 2020a; Marsh et al. 2019), and is related to working memory capacity (Beaman 2004; Sörqvist 2010). The findings also fit very well with recent observations of an irrelevant speech effect to be reduced (but not eliminated) after training of auditory selective attention (using a dichotic-listening task; Kattner and Ellermeier 2020), indicating that the portion of the irrelevant speech effect which can be attributed to attentional capture may be susceptible to attentional control.

Nevertheless, since the irrelevant speech effect may comprise both attentional capture and interference-by-process mechanisms of auditory distraction, it is still possible that inhibitory control also resolves the (presumably uncontrollable) interference between changing-state sound and serial-order processing. Of course, the fact that the irrelevant speech effect was only attenuated, but not eliminated, after eight sessions of inhibitory-control training does not prove that it is the changing-state effect, which remained. Moreover, it could be argued that eight sessions of inhibitory control training may not be sufficient to eliminate the disruptive effect of irrelevant changing-state sound (an effect which appears to be very robust having survived years of every- day mental activities requiring the inhibition of irrelevant information). For instance, there is evidence that irrelevant speech does not interfere at all with short-term memory in congenitally and early blind individuals (Kattner and Eller- Meier 2014), suggesting that enhanced inhibitory control of auditory information resulting from a life-long experience with a primarily auditory environment may eliminate the interference-by-process portion of auditory distraction as well. Further research is required to determine whether the attenuation of the irrelevant speech effect in the present study was due to an attenuation of attentional capture or the task-specific interference-by-process. This could be accomplished, for instance, by contrasting the transfer effects of an inhibitory control training on the disruption produced by auditory deviants (which should be due to attentional capture alone) and changing-state sound (which should reflect interference-by-process). Alternatively, transfer effects could be investigated about auditory distraction in non-serial short-term memory tasks, which are known to be immune to a changing-state effect (e.g., the missing-item task; Bea- man and Jones 1997). If attentional capture depended on the strength of inhibitory control, then the often relatively small disruptive effect of irrelevant speech in the missing-item task (due to a diversion of attention) might be eliminated completely as a result of inhibitory n-back training (compare Hughes and Marsh 2020b for a similar observation with regard to the effect of foreknowledge).

More generally, the present results indicate that extensive cognitive training cannot be used only to enhance working memory updating (Dahlin et al. 2008a, b) and set-shifting (Pereg et al. 2013), but also to strengthen the inhibitory-control function of working memory in terms of the inhibition of auditory distractor interference. The present study is the first to demonstrate that the extent of auditory distraction in short-term memory can be reduced experimentally through working memory training with enhanced demands for inhibitory control in two stimulus modalities (i.e., inhibition of responses to visuospatial and auditory stimuli in the dual n-back task). In contrast, the same working memory training with reduced demands for inhibitory control did not afect auditory distraction. Hence, the pattern of results indicates that the training-related decrease of interference by task-irrelevant auditory stimuli was not driven by working memory capacity in general, but rather by a specifc inhibitory-control function (i.e., resistance to distractor interference; Friedman and Miyake 2004).

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Finally, the present study also investigated the possible far transfer effects of an extended dual n-back training on (a) set-shifting abilities and (b) fluid intelligence scores. Regardless of the degree of inhibitory control involved, the present dual n-back training did not reduce the response-time costs resulting from switching between two different categorization tasks. This suggests that training on the n-back task does not enhance executive set-shifting abilities. Moreover, in contrast to previous findings (Jaeggi et al. 2008), the dual n-back training did not affect fluid intelligence in the present study. Specifically, participants in all experimental groups were able to solve about 9.5–10 out of 18 problems of the short version of Raven’s Advanced Progressive Matrices test at pre-test (which is equivalent to the pre-test scores reported by Jaeggi et al. 2008). In contrast to the control group and the inhibitory dual n-back training group, the average fluid intelligence score was slightly enhanced at post-test for participants who were trained on the standard dual n-back task, but the group differences in gains on fluid intelligence did not turn out to be statistically significant. In line with several other recent findings of a lack of “far transfer” (Harrison et al. 2013; Melby-Lervåg et al. 2016; Redick et al. 2013), the present result seems to contradict the findings reported by Jaeggi et al. (2008). However, the absence of a transfer efect to fluid intelligence might also be due to differences in the spacing of training times. Specifically, participants in the present study were trained for eight 80-min training sessions (breaks not included), whereas Jaeggi et al. (2008) trained participants for either eight, twelve, seventeen, or nineteen 25-min sessions. In the study by Jaeggi et al. (2008), the training-related gain on fluid intelligence was shown to increase with the number of training sessions, and statistically significant gains of fluid intelligence relative to pre-test were found only after seventeen and nineteen 25-min sessions of training, but not after eight and twelve sessions of training. Hence, it seems that the transfer to fluid intelligence depends on the training dosage. However, in the present study, participants were trained for 640 min in total (8 × 80 min), so the total training time exceeded the nineteen training sessions in the Jaeggi et al. (2008) study (i.e., 475 min). The fact that no reliable transfer to fluid intelligence was observed in the present study suggests that temporal spacing of training sessions (i.e., multiple short sessions)

may enhance the chances of far transfer effects, as compared to massed training sessions (i.e., few long sessions).

Taken together, the present study shows that working memory capacity can be enhanced successfully with extended training on two different types of dual n-back tasks varying to the degree of inhibitory control required. In general, transfer to cognitive abilities that are not directly related to the training task was very limited. However, in contrast to the standard n-back task with relatively low demands for inhibitory control, training on the newly developed inhibitory dual n-back task was found to reduce the degree of interference produced by irrelevant speech in a serial short-term memory task. This finding indicates that the inhibitory dual n-back task enhanced not only working memory updating abilities but also inhibitory control of distractor interference, thus enabling more transfer to tasks for which these executive functions are required (e.g., inhibited processing of task-irrelevant speech). More research is required to disentangle the efects of enhanced inhibitory control on attentional capture and interference-by-process mechanisms of auditory distraction and to assess possible transfer effects of an inhibitory working memory training on other forms of inhibitory control, such as pre-potent response inhibition or inhibition of proactive interference in memory.

Funding Open Access funding enabled and organized by Projekt DEAL.

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