Brief Peppermint Odour Exposure and Immediate Free Recall of a Visual Object Array: A Three-Arm Randomised Experiment

Abstract

Background: Odours reach limbic and prefrontal targets through a uniquely direct pathway and are often claimed to enhance cognition, yet experimental evidence for specific essential oils is mixed and frequently confounded by expectancy. We tested whether a brief inhalation of peppermint or clove essential oil, compared with no odour, alters immediate free recall of a visual object array. Methods: Sixty healthy adults aged 18-50 years were randomly allocated to peppermint oil, clove oil or no odour (n = 20 each). After 30 s of inhalation (or none), participants studied a 4 x 4 array of 16 pictured items for 20 s and then wrote down every item they could recall, without time limit. Groups were compared with one-way and Welch ANOVA, Welch-corrected pairwise tests with Holm correction, Hedges' g with 95% confidence intervals and Bayes factors. Results: Recall differed across conditions (Welch F(2, 37.0) = 8.53, p < .001; omega2 = 0.14). Participants exposed to peppermint recalled more items (11.20 +/- 2.14) than those exposed to no odour (8.45 +/- 2.04; g = 1.29, 95% CI [0.61, 1.97], p(Holm) < .001, BF10 = 134). Clove (9.55 +/- 3.19) did not differ reliably from either no odour (g = 0.40, p(Holm) = .203) or peppermint (g = 0.60, p(Holm) = .127; BF10 = 1.29). Conclusions: A single brief exposure to peppermint oil was associated with substantially better immediate recall than no odour. Because the peppermint-clove contrast was inconclusive and no placebo odour was used, the data do not establish an odour-specific effect; general arousal and expectancy remain plausible contributors. Pre-registered, laboratory-based replications with placebo odours, blinding and arousal measures are required.