To investigate whether pattern similarity in the hippocampus is associated with univariate VTA activity  during the Question presentation and Face presentation phase, we conducted an exploratory analysis using Bayesian multilevel logistic regression. We compared three models: Model 1 included pattern similarity, curiosity, and their interaction as predictors (see Fig. 5); Model 2 added the main effects and interaction of VTA activity and curiosity; and Model 3 included a three-way interaction between pattern similarity, curiosity, and VTA activity.
When investigating activation during the Question presentation phase, model comparison using leave-one-out cross-validation (LOO-CV) revealed that Model 2, which included the main effects and interaction of VTA activity and curiosity, had the best predictive performance (ELPD difference = 0.0, SE = 0.0). Model 3, with the three-way interaction between pattern similarity, curiosity, and VTA activity, performed worse than Model 2 (ELPD difference = -2.3, SE = 1.4). Model 1, which only included pattern similarity, curiosity, and their interaction as predictors, had the poorest performance (ELPD difference = -3.7, SE = 3.5). The LOO-CV model weights further supported the superiority of Model 2 (weight = 0.808), followed by Model 1 (weight = 0.192) and Model 3 (weight = 0.000). These results suggest that including VTA activity and its interaction with curiosity improves the prediction of subsequent recognition accuracy moderately, indicating a potential association between hippocampal pattern similarity and VTA activity during the Question presentation phase. However, the three-way interaction between pattern similarity, curiosity, and VTA activity did not improve the model’s predictive performance.
Next, we investigated the relationship between hippocampal pattern similarity, VTA activity, and curiosity during the Face presentation phase. Model comparison using LOO-CV revealed that Model 3, which included the three-way interaction between pattern similarity, curiosity, and VTA activity, had the best predictive performance (ELPD difference = 0.0, SE = 0.0) (Fig. 6). Model 2, which included the main effects and interaction of VTA activity and curiosity, performed worse than Model 3 (ELPD difference = -8.5, SE = 4.6). Model 1, which only included pattern similarity, curiosity, and their interaction as predictors, had the poorest performance (ELPD difference = -13.0, SE = 5.7). The LOO-CV model weights further supported the superiority of Model 3 (weight = 0.889), followed by Model 1 (weight = 0.111) and Model 2 (weight = 0.000). Upon closer inspection the mean estimate for the three-way interaction effect is 90.01 (95% CI [37.50 to 144.07], P+=100%). To interpret this result, we can say that when curiosity is high, the effect of pattern similarity (during Face presentation) on the subsequent recognition accuracy increases as univariate VTA increases. In other words, at high levels of curiosity, higher VTA activity is associated with a stronger relationship between hippocampal pattern similarity and subsequent memory.

> reviewer comment: maybe we focus first on Question presentation given that we have a clear motivation to do that following up the Fig. 5 findings (see my comment below), and then we explore Face presentation later (given that we don't find that strong evidence during Question presentation that VTA activity drives hippocampal pattern similarity in support of incidental memory.?