Previous studies have demonstrated that autonomic arousal is increased following correct task performance on a difficult, relative to an easy, task. Here, we hypothesized that this arousal response reflects the (relative) surprise of correct performance following a difficult versus an easy task. Following this line of reasoning, we would expect to find a reversed pattern following erroneous responses, because errors are less expected during an easy than during a difficult task. To test this, participants performed a flanker task while pupil size was measured online. As predicted, the results demonstrated that pupil size was larger following difficult (incongruent) correct trials than following easy (congruent) correct trials, but smaller following difficult than following easy incorrect trials. Moreover, participants with larger congruency effects, and hence a larger difference in outcome expectancies between the two trial types, showed larger differences in pupil size after both correct and incorrect responses, further corroborating the idea that pupil size increased as a measure of performance prediction errors.

Lactobacillus reuteri is a commensal, beneficial gut microbe that colonises the intestinal mucus layer, where it makes close contact with the human host and may significantly affect human health. Here, we investigated the capacity of linoleic acid (LA), the most common polyunsaturated fatty acid (PUFA) in a Western-style diet, to affect L. reuteri ATCC PTA 6475 prevalence and survival in a simulated mucus layer. Short-term (1 h) survival and mucin-agar adhesion assays of a log-phase L. reuteri suspension in intestinal water demonstrated that the simulated mucus layer protected L. reuteri against the inhibitory effects of LA by lowering its contact with the bacterial cell membrane. The protective effect of the simulated mucus layer was further evaluated using a more complex and dynamic model of the colon microbiota (SHIME®), in which L. reuteri survival was monitored during 6 days of daily exposure to LA in the absence (L-SHIME) and presence (M-SHIME) of a simulated mucus layer. After 6 days, luminal L- and M-SHIME L. reuteri plate counts had decreased by 3.1±0.5 and 2.6±0.9 log cfu/ml, respectively. Upon supplementation of 1.0 g/l LA, the decline in the luminal L. reuteri population started earlier than was observed for the control. In contrast, mucin-agar levels of L. reuteri (in the M-SHIME) remained unaffected throughout the experiment even in the presence of high concentrations of LA. Overall, the results of this study indicate the importance of the mucus layer as a protective environment for beneficial gut microbes to escape from stress by high loads of the antimicrobial PUFA LA to the colon, i.e. due to a Western-style diet.