Inter-response times and semantic similarity within and between cluster transitions
We explored semantic retrieval patterns during the PolyFT in relation to the predictions of the marginal theorem value. The mean IRT across participants and cue words was 3.6 s (SD = 1.4 s). We identified a mean of 3.7 switches across participants (SD = 1.2, representing 28% of the mean retrieved associations). We first examined whether retrieving items during between-cluster rather than within-cluster transitions takes more time.
On a global average, the switching-IRT was 4.87 s (SD = 1.97 s), while the mean clustering-IRT was 2.89 s (SD = 1.49 s). The Wilcoxon test showed that IRT was lower when people stayed in a cluster than switching to a different cluster. This result was significant for the three cue words (cue1, glace: W = 351, p < .001; cue2, rayon: W = 461, p < .001; cue 3, somme: W = 542, p < .001; Supplementary Figure S1A), and for the mean of the three cue words (W = 312, p < .001; Fig. 1A). We also examined whether semantic similarity when retrieving items during between-cluster transitions differs from within-cluster transitions. Words related to the same meaning are expected to be closer in terms of semantic distance, and we aimed to capture this effect. On a global average, the switching-IRS was 0.06 (SD = 0.05), while the mean clustering-IRS was 0.25 (SD = 0.06). The Wilcoxon test showed that IRS was lower when people switch to a different cluster than when staying in a cluster. This result was significant for the three cue words (cue1, glace: W = 3664, p < .001; cue2, rayon: W = 3357, p < .001; cue 3, somme: W = 2541, p < .001; Supplementary Figure S1B), and for the mean of the three cue words (W = 3740, p < .001; Fig. 1B).
Second, we explored whether participants leave a cluster when they reach their long-term IRT (i.e., when the ratio between the IRT for retrieving an item and the long-term IRT [IRTr] equals 1) as predicted by the marginal value theorem. Consistent with this theory, the mean IRTr was greater than 1 for switches (IRTr = 1.44; Fig. 2A), while it was lower than 1 for non-switching items (IRTr were 0.78 and 0.84 for responses in positions − 2 and − 1 before switching, and 0.63 and 0.83 for the position 2 and 3 after switching, respectively. The IRTr was significantly different for switches than non-switching responses (p < .001; Fig. 2A). The results of statistical tests are reported in Supplementary Table S1.
Third, we explored whether a comparable pattern is observed for IRSr, but in the opposite direction, by comparing if responses generated during within-cluster transitions are more similar to each other than the ones in between-cluster transitions. We observed higher IRSr in responses during within-cluster transitions compared to between-cluster transitions. The mean IRSr during switching (position = 1; Fig. 2B) was 0.35, while for responses before switching, IRSr was higher (1.27 for position − 2 and 1.21 for position − 1). IRSr were also higher for responses after switching (1.65 and 1.18 for positions 2 and 3, respectively). The IRSr for switching was different from all other values (p < .001). Thus, as expected, the first response of a new cluster is highly dissimilar (< 1) to the last response of the previous cluster (pre-switch response) but highly similar (> 1) to the next one in the same cluster (Fig. 2B). The results of statistical tests are reported in Supplementary Table S1.
Finally, following Hills et al. (2012), we explored whether participants with IRT close to reaching their long-term IRT before switching generated more responses during the task (higher PolyFT_fluency), as it would be expected from the marginal value theorem. The value of the IRTr before switching (responses in position − 1 in Fig. 2A) reflects how close the participants are to reaching their threshold or long-term IRT. We regressed the absolute difference between the IRT in position − 1 and the long-term IRT on the total number of responses retrieved (for each PolyFT cue word and each participant). The results show that the closer participants were to their long-term IRT before switching, the more responses they retrieved during the PolyFT, R2 = 0.108, F(1,81) = 9.83, p < .01 (Fig. 2C).
We further examined the link between IRS and IRT for clustering and switching responses. Mixed regression models showed that time and similarity correlated significantly for clustering and switching responses (clustering: R2 = 0.08, b = − .018, p < .001; switching: R2 = 0.07, b = − .004, p < .01). This result indicates that time and similarity covary during clustering and switching; it takes longer to retrieve distant responses in both clustering and switching. Together, these results indicate that memory search during the PolyFT follows the MVT predictions.
Patterns of inter-response time and similarity of the responses categorized as clustering and switching
While IRT and IRS supported MVT on average, participants varied in their adherence to the MVT policies. As shown in Fig. 3, some switching responses occurred faster than expected by the MVT (IRTr < 1), whereas some clustering responses occurred later than expected (IRTr > 1). Therefore, we further subdivided our clustering and switching responses to take into account this incongruency. We used the IRTr of each clustering and switching response to classify them either as Fast-Switching or Fast-Clustering (when IRTr was lower than 1), or as Slow-Switching or Slow-Clustering (IRTr larger than 1) (Supplementary Figure S2-S3). At the group level, participants generated a mean of 21.24 (SD = 9.6) responses corresponding to Fast-Clustering, 5.17 (SD = 2.2) responses corresponding to Fast-Switching, 8.62 (SD = 4.8) responses corresponding to Slow-Clustering and 5.84 (SD = 2.3) responses corresponding to Slow-Switching. In total, Fast-Clustering represented 49.98% (SD = 9.95%) of all responses, Fast-Switching 14.43% (SD = 7.82%), Slow-Clustering 20.17% (SD = 7.17%), and Slow-Switching15.42% (SD = 6.95%).
To explore whether fast and slow switching involved distinct mechanisms, we compared the IRT and IRS for responses in positions − 1 and − 2 for both Fast-Switching vs. Slow-Switching, separately. (Fig. 4). The IRTr and IRSr of responses in position − 1 and − 2 before switching did not differ significantly from each other in Fast-Switching (IRTr: W = 1842, p > .05 and IRSr: W = 1358, p > .05). For Slow-Switching responses in position − 1 and − 2 before switching did not differ for IRSr (W = 2063, p > .05) but these responses were significantly different for IRTr (W = 1411, p < .05). However, as for the global switching responses described above (see Fig. 2), the IRTr and IRSr of the pre-switch responses (position = -1) were significantly different from those of the switching-response (position = 1) in either Slow-Switching (IRTr: W = 9, p < .001 and IRSr: W = 3543, p < .001). and Fast-Switching (IRTr: W = 2493, p < .01; IRSr: W = 3366, p < .001). Finally, the IRTr and IRSr of pre-switch responses (position = -1) in Slow-Switching equaled or were close to 1, while it was farther from 1 before Fast-Switching. Additional statistical analyses confirmed that participants reached their marginal value before Slow-Switching but this was not the case before Fast-Switching (See Supplementary Results S1). The results of statistical tests are reported in Supplementary Table S2. Altogether these findings suggest that fast and slow switching rely on distinct mechanisms.
To further characterize fast and slow clustering and switching responses, we explored the relationship between IRT and IRS separately for each of these types of responses. We ran mixed models to predict the IRS from the IRT values for each type of response and found that IRS was predicted from the IRT values for both Fast-Clustering (R2 = .07, b = − .05, p < .001) and Slow-Clustering (R2 = .05, b = − .01, p = .007) responses. Thus, it took longer to retrieve more semantically distant concepts within a cluster during both fast and slow clustering. We did not observe this pattern in the Fast-Switching (R2 = .09, b = − .01, p = .11) and Slow-Switching (R2 = .03, b = -0.00, p = .39) responses (Supplementary Figure S2).
Finally, to explore whether slow clustering and switching responses relate to controlled processes, we ran Spearman correlations between executive function abilities and the number of Fast-Clustering, Fast-Switching, Slow-Clustering and Slow-Switching responses generated by the participants. We found significant correlations between Slow-Switching and higher Backward-span (rs = .261, p = .015), faster TMT-shifting (rs = − .232, p = .032) and lower Stroop-interference (rs = − .257, p = .018). No significant correlations were found between executive function abilities and Fast-Clustering, Fast-Switching and Slow-Clustering. The Spearman correlation coefficients are presented in Supplementary Table S3. Although no correlations remained significant after FDR correction for multiple comparisons, these trends may suggest that individuals with more Slow-Switching responses had better executive function abilities.
Individual patterns of brain functional connectivity predict Fast-Switching and Slow-Clustering
To explore the brain correlates of clustering and switching behaviors, we performed four independent CPM analyses to determine whether individual patterns of task-based brain functional connectivity predict Fast-Clustering, Fast-Switching, Slow-Clustering, and Slow-Switching. Fast-Clustering (rs = .23, p = .036), Fast-Switching (rs = .33, p = .002) and Slow-Clustering (rs = .33, p = .002) were significantly predicted by brain functional connectivity but not Slow-Switching (rs = − .18, p = .09). After permutation testing, only Fast-Switching (p = .019) and Slow-Clustering (p = .024) remained significant. We went on to characterize the predictive networks of these two response types.
Characterization of the brain networks predicting Fast-Switching and Slow-Clustering
The positive model network predicting Fast-Switching was composed of 19 brain regions connected by 18 links (Fig. 5A). Most of these connections were localized between brain regions of DMN and visual networks (5 links), DAN and visual networks (4 links), DMN and somatomotor networks (3 links), and DMN and salience networks (3 links). The highest degree node was localized in the left inferior parietal lobule (k = 12), region that belong to the DMN network. This brain region connects to brain regions of the visual, somatomotor, salience, limbic and temporoparietal networks.
The model network predicting lower Fast-Switching was composed of 37 brain regions connected by 34 links. Most of these connections were localized between brain regions within DMN (9 links), within DAN (4 links), between DAN and salience networks (3 links), and between DMN and temporoparietal networks (3 links). The highest degree nodes were localized in the left inferior parietal lobule (k = 6), region that belong to the DMN network, and the right post central region of the DAN network (k = 6). The left inferior parietal lobule had connections to DMN brain regions, and one connection to the lateral PFC of the ECN. The right post central region connected to regions of the DAN, ECN and salience networks.
The positive network model predicting Slow-Clustering was composed of 28 brain regions connected by 26 links (Fig. 5B). Most of these connections were localized between brain regions of DAN and somatomotor networks (6 links), ECN and somatomotor networks (4 links), DAN and visual networks (3 links), and DMN and somatomotor networks (3 links). The highest degree nodes were localized in the right superior parietal lobule (k = 8), region that belong to the DAN network, and the left intraparietal sulcus of the ECN (k = 6). These nodes connected to brain regions of the visual, somatomotor and salience networks.
The positive network model predicting lower Slow-Clustering was composed of 24 brain regions connected by 29 links. Most of these connections were localized between brain regions within the DMN (13 links), between DMN and temporoparietal (8 links) and between ECN and DMN networks (4 links). The highest degree nodes were localized in the right temporoparietal regions of the temporoparietal network (k = 9), and brain regions of the DMN (left temporal: k = 8; right anterior temporal: k = 6; right dorsal PFC: k = 4). All these regions had mostly within network connections to the rest of the brain.
Relationships between PolyFT responses, brain connectivity, and creativity
To explore the relationship between PolyFT responses, brain connectivity and creativity, we first explored how individual patterns of memory search during PolyFT relate to creative abilities. We ran Spearman correlations between the number of Fast-Switching, Fast-Clustering, Slow-Clustering and Slow-Switching items generated by the participants and the different measures of the creativity. For the CAT task, we found significant correlations between CAT_CR and Fast-Clustering (rs = − .244, p = .024) and between CAT_CR and Fast-Switching (rs = .260, p = .015). No significant correlation was observed for CAT_eureka. For the AUT task, we found significant correlations between AUT-fluency and Slow-Switching (rs = .213, p = .049), Fast-Clustering (rs = .455, p < .001) and Slow-Clustering (rs = .315, p = .003), and between AUT-uniqueness and Fast-Clustering (rs = .381, p < .001) and Slow-Clustering (rs = .370, p < .001). No significant correlation was found for AUT-ratings. The correlations between AUT-fluency and AUT-uniqueness and Fast-Clustering and Slow-Clustering remained significant after correction for multiple comparisons. These results suggest that individuals with more Fast-Clustering and Slow-Clustering responses had higher divergent thinking abilities. In addition, our findings may also suggest that individuals with more Fast-Switching have better associative combination abilities, although the correlation lost statistical significance after applying FDR correction for multiple comparisons. Results for statistical tests are reported in Supplementary Table S2.
Since the frequency to which participants produce Fast-Switching and Slow-Clustering responses were predicted by individual patterns of brain functional connectivity, and these frequencies also related to creative abilities, we lastly examined the relationship between brain connectivity patterns and creative abilities. Specifically, we explored whether the relationship between brain functional connectivity (i.e., mean connectivity strength of the positive model network) and creative abilities was mediated by the individual patterns of clustering and switching behavior during semantic retrieval (i.e., semantic foraging patterns). We conducted mediation analyses that focused on the indirect effect of brain connectivity predicting Fast-Switching and Slow-Clustering on creative abilities. In all mediation analyses, we calculated the indirect effect as the product of path a (i.e., the regression coefficient between brain functional connectivity and foraging patterns) and path b (i.e., the regression coefficient between foraging patterns and creativity). We tested the significance of the indirect effect using bootstrapping methods.
Since Fast-Switching was related to the ability to combine remote associates evaluated by the CAT task, we explored the mediating role of Fast-Switching on the relationship between the brain connectivity patterns predicting it and CAT_CR. As shown in the previous analyses, the regression coefficient between the brain connectivity pattern and Fast-Switching (b = .371, p < .001), and between Fast-Switching and CAT_CR (b = .247, p < .05) were significant. The total effect represented by the regression coefficient between the brain connectivity patterns and CAT_CR was significant (b = .201, p < .05), and the direct effect was not significant (b = .109, p = .294). The bootstrapped indirect effect was (0.371) × (0.247) = .092, and the 95% confidence interval ranged from 0.019 to 0.206. Hence, Fast-Switching mediated the relationship between the positive predictive network model of Fast-Switching and CAT performance: The higher the strength of connectivity in the positive network model that predicts Fast-Switching, the higher the number of fast switches in semantic retrieval, and the higher the abilities to combine remote associates in memory.
We also explored the mediation analyses for Slow-Clustering. Because Slow-Clustering was predicted by the fluency and originality of the participants in the AUT, we explored the mediating role of Slow-Clustering on the relationship between the brain connectivity patterns predicting it and AUT_fluency and AUT_uniqueness in independent analyses. As shown in the previous analyses, the regression coefficient between the brain connectivity pattern and Slow-Clustering (b = .50, p < .001) was significant. The regression coefficient between Slow-Clustering and AUT_fluency (b = .284, p < .01) and between Slow-Clustering and AUT_uniqueness (b = .324, p < .001) were significant. The total effect and the direct effect between the brain connectivity patterns were not significant for AUT_fluency (total effect: b = .056, p = .549; direct effect: b = − .087, p = .390) or for AUT_ uniqueness (total effect: b = .036, p = .691; direct effect: b = − .126, p = .200). For AUT_fluency, the bootstrapped indirect effect was (0.50) × (0.284) = .142, and the 95% confidence interval ranged from 0.06 to 0.29, while for AUT_originality, the bootstrapped indirect effect was (0.50) × (0.324) = .162, and the 95% confidence interval ranged from 0.072 to 0.32. Hence, Slow-Clustering mediated the relationship between the positive predictive network model of Slow-Clustering and AUT performance: The higher the strength of connectivity in the positive network model that predicts Slow-Clustering, the higher the frequency of Slow-Clustering during semantic retrieval and the more participants were fluent and original in the divergent thinking task.