Olfactory interference on the emotional processing speed of visual stimuli: The influence of facial expressions intensities

Abstract

Research on olfactory stimulation indicates that it can influence human cognition and behavior, as in the perception of facial expressions. Odors can facilitate or impair the identification of facial expressions, and apparently its hedonic valence plays an important role. However, it was also demonstrated that the presentation of happiness and disgust faces can influence the emotional appraisal of odorants, indicating a bilateral influence in this phenomenon. Hence, it's possible that odor influences on emotional categorization vary depending on the intensity of expressions. To investigate this hypothesis, we performed an emotion recognition task using facial expressions of five emotional categories (happiness, fear, disgust, anger and sadness) with ten different intensities. Thirty-five participants completed four blocks of the task, each with a different olfactory condition, and we found that odorants' effects varied according to the facial expressions intensity. Odorants enhanced the Reaction Time (RT) differences between threshold and high-intensity expressions for disgust and fear faces. Also, analysis of the RT means for high-intensity facial expressions revealed that the well-known advantage in recognition of happiness facial expressions, compared to other emotions, was enhanced in the positive olfactory stimulation and decreased in the negative condition. We conclude that olfactory influences on emotional processing of facial expressions vary along intensities of the latter, and the discrepancies of past research in this field may be a result of a bilateral effect in which the odorants influence the identification of emotional faces just as the facial expressions influence the emotional reaction to the odor.

Fig 1. Visual stimuli examples.

(A) Illustrative example of the linear continuum of morphs created by morphing the neutral expression with the happiness face. (B) Illustrative example of the facial identification task. The large expression in the center is a sadness expression. These images do not correspond to the actual stimuli used in the experiment, and were created just for the purpose of illustrating the facial morphing procedure and the facial identification task in this article. The images used in the experiment (obtained from The NimStim set of facial expressions) are protected by copyrights and not freely available to be published.

Fig 2. Timeline of the experimental procedure.

Timeline showing the procedures conducted in pre-experiment, experimental session and post-experiment moments. The stages of the experimental session in which an odor condition was present (butyric acid, isoamyl acetate, lemongrass or no odor) are marked in grey.

Fig 3. Perception thresholds of each emotion category.

Mean intensities and standard errors for correct identification of each emotion for all odor conditions grouped. Sadness presented the highest perception threshold between the emotions, and fear was significantly higher than happiness. The significant differences are illustrated by asterisk brackets.

Fig 4. RT for each intensity.

RT means and standard errors for each intensity with all emotions and odor conditions grouped.

Fig 5. Analysis of RT between intensities for disgust and fear.

Graphs showing RT means and standard errors for disgust and fear at threshold, medium and high expression intensities in different odor conditions.

Fig 6. Minimum RT of each emotion for high-intensity facial expressions (80, 90 and 100).

Means and standard errors of the minimum RT of each emotion for all odor conditions grouped. Happiness was recognized faster than sadness expressions (p < .005), but no other differences between emotions were observed. Significant differences are illustrated by asterisk brackets.

Fig 7. Difference in the minimum RT between emotions in each odor condition.

Means and standard errors of the minimum RT for each emotion according to the odor condition.