Kuehne, Zaehle and Lobmaier (2021, Scientific Reports) reported that holding a pen between the teeth — a posed-smiling facial feedback manipulation — selectively improves working memory for happy facial expressions, shifts remembered expressions in a more positive direction, and affects men more than women. Because the manipulation was held continuously across task blocks, and because the study included no frowning condition, only sparse indirect affect measurements, no identity-memory control task, and no neural measures, it could not determine where in the memory pipeline facial feedback acts or why. This gap remains open: the only direct follow-up (Ras et al. 2026) found facial-feedback effects on perception but not on long-term recognition when the manipulation was applied only at retrieval, and adjacent work manipulated facial-mimicry interference and working-memory load rather than posed smiling. The open problem is to localize which memory stage(s) — encoding, maintenance, or retrieval — posed-smiling facial feedback influences in memory for emotional faces, and characterize the effect's polarity specificity, mediation by mood (and any associated processing-style shift), feature specificity, neural correlates, and gender moderation, with adequate statistical power.

Background

The facial feedback hypothesis (FFH) holds that sensory feedback from one's own facial muscles influences emotional experience and the processing of emotional stimuli, rather than only the reverse. Its classic test is the pen-in-mouth paradigm (Strack, Martin & Stepper 1988): holding a pen between the teeth activates the muscles used in smiling (mainly zygomaticus major), whereas holding it between the lips inhibits them, and this unobtrusive manipulation was reported to shift affective judgments of cartoons. The FFH has been under intense scrutiny since a large registered replication failed to reproduce the original cartoon-rating effect (Wagenmakers et al. 2016); a meta-analysis (Coles, Larsen & Lench 2019) and a 26-laboratory, 3,878-participant multi-lab test (Coles et al. 2022) support small and variable facial-feedback effects on subjective experience and affective judgments, with the pen-in-mouth variant yielding the least conclusive evidence. A related embodied-cognition literature concerns facial mimicry — the automatic imitation of observed expressions — which is thought to support 'embodied simulation' of others' emotional states, especially when expressions are ambiguous; this literature established facial-feedback effects mostly at perceptual and electrophysiological levels. Whether facial feedback reaches into memory processing was tested directly by Kuehne, Zaehle & Lobmaier (2021). Thirty-seven healthy participants performed an emotional working-memory (WM) task modeled on Mok, Hajonides van der Meulen, Holmes & Nobre (2019): a target face (happy or sad, at graded intensities) was followed by a neutral probe face of the same actor, and participants reproduced the remembered expression by morphing the probe's expression on a response screen. Crucially, throughout alternating task blocks participants held a pen either between their teeth (smile-facilitating facial feedback manipulation, FFM) or in the non-dominant hand (control, no feedback manipulation). Posed smiling improved WM for happy faces (most strongly for highly ambiguous, low-intensity expressions) and, independent of the emotion of the face, reduced the negative bias (the tendency to remember faces as less positive/more negative than they were; this baseline bias was present for happy but not sad faces), and these influences were stronger in men. Participants' affect was tracked with the IPANAT (Implicit Positive and Negative Affect Test; Quirin, Kazén & Kuhl 2009), an indirect instrument in which participants rate how much nonsense words 'sound like' positive and negative moods; smiling reduced implicit negative affect, but the three temporally sparse measurements could not establish mood as the mediator of the memory effects. Related earlier work had shown that unconscious smiling can modulate the emotional evaluation of episodic memories (Arminjon et al. 2015), so a genuine feedback-memory link is plausible. However, because the pen was held across whole blocks, the design cannot separate effects on encoding, on maintenance/consolidation, and on retrieval; because only a smile-facilitating condition was used, polarity specificity is untested; and the neural account (a hypothesized network comprising the dorsolateral prefrontal and orbitofrontal prefrontal cortex, the superior temporal sulcus — a brain region involved in perceiving faces and social cues — the amygdala and the hippocampus) remains speculative — leading to the open problem stated below.

Problem Statement

Determine how — at which processing stage(s) and through which mechanism — posed-smiling facial feedback influences memory for emotional facial expressions, as left open by Kuehne, Zaehle & Lobmaier (2021). The single scientific objective is the localization and mechanistic characterization of this facial-feedback effect on emotional face memory. The following facets, stated by the source, specify the empirical content of that objective; they are independent questions, none of which is settled by resolving the others: (1) Stage localization: apply the facial-feedback manipulation time-locked to specific trial phases — restricted to target (encoding) presentation, to the maintenance/consolidation interval, or to the test/retrieval phase — and establish which stage(s), if any, carry the previously observed improvement in working memory for happy faces and the positive shift in remembered expressions. (2) Polarity specificity: directly compare smile-facilitating (pen-between-teeth) and smile-inhibiting (pen-between-lips) manipulations on memory for emotional faces to establish whether the effects are symmetric or specific to the smiling muscles' feedback. (3) Mediation: resolve whether and how the memory effects are carried by induced mood changes and by any consequent shift between global and local processing style (preferentially processing the overall configuration of a stimulus rather than its fine-grained local details), using systematic direct mood assessments and appropriate experimental controls rather than sparse indirect affect measurements. (4) Feature specificity: determine whether the effect selectively alters memory for changeable facial properties (expression and its intensity) or also for static face properties such as identity or gender, via identity-memory control tasks. (5) Mechanistic correlates and moderation: test whether facial feedback during emotional working memory engages the hypothesized network (dorsolateral prefrontal cortex, orbitofrontal cortex, superior temporal sulcus, amygdala, hippocampus) with targeted neuroimaging or electrophysiology, and investigate the mechanisms of the greater male susceptibility (for example baseline mimicry, expressivity, or developmental factors). Answers may legitimately find effects at one stage and none at another, or no memory-stage effect at all (perception-only), provided the design has the power and controls to license that conclusion.

References

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