When evaluating the smell of a hair-color product, combining answers about whether it was unpleasant with electroencephalography (EEG) may reveal different aspects of the response. A small 2019 study reported differences in EEG between a formulation designed to reduce pungent odor and an ammonia-free formulation, although there was no clear difference in their pleasantness–unpleasantness ratings.
The study aimed to examine responses not only while people smelled the products but also during the subsequent rest period, to explore whether EEG could be used to evaluate product odors.
The same 10 people compared three hair-color formulations
The participants were 10 healthy adults, five men and five women, with a mean age of approximately 20 years. They compared the following three cream formulations.
- ① An ammonia-free formulation using monoethanolamine
- ② An ammonia-containing formulation with a layered structure designed to reduce ammonia odor
- ③ A conventional formulation that released ammonia odor
After resting for two minutes, participants mixed a sample for 30 seconds and then smelled it for five seconds. EEG was also recorded during a further two-minute rest period. The order of formulations was randomized, with at least one day between conditions.
Because the act of mixing itself also affects EEG, the researchers included a control condition in which participants made the same movements without a sample, and subtracted those data for the analysis. EEG was measured at 15 scalp sites, and after the task participants rated the odor’s pleasantness or unpleasantness on a five-point scale.
The conventional formulation was rated more unpleasant than the other two
Higher pleasantness–unpleasantness scores indicated greater unpleasantness. Mean scores were 2.6 for ①, 3.2 for ②, and 4.2 for ③. Formulation ③ was rated statistically significantly more unpleasant than either ① or ②, but there was no significant difference between ① and ②.
In this comparison, the results indicate that the odor-reduced formulation ② was perceived as less unpleasant than the conventional formulation ③. The means for ① and ② were not identical; rather, their difference was not clear in this 10-person study.
The alpha band during smelling also differed between ① and ②
In the alpha band at the Pz recording site near the top rear of the head, activity values fell during mixing and smelling relative to rest. The largest decrease occurred with ③. During the smelling phase, ② also showed lower values than ①, revealing a difference between formulations that had not been clearly distinguished by subjective pleasantness–unpleasantness responses.
The authors interpreted this as suggesting that pungent odor might reduce a relaxed state. However, alpha-band changes are not a measure specific to a single emotion. What was directly demonstrated here was that the EEG response differed between formulations.
Different beta-band patterns remained after smelling
During rest after smelling the odors, the distribution of activity was estimated from beta-band EEG. Activity was reported to be prominent in the left parietal region for ① and ②, and in the right parietal region for ③. Comparisons between formulations also showed left–right differences between ① or ② and ③; directly comparing ① with ② showed higher left-parietal activity for ①.
Drawing on previous research on emotion and brain activity, the authors discussed the possibility that these lateral differences were related to pleasantness and unpleasantness. Their conclusion was that combining EEG during and after smelling might help assess differences in product responses that are difficult to capture with questionnaires alone.
The locations of activity were computational estimates based on 15 scalp electrodes, not direct images of the inside of the brain. In addition, the original paper contains an inconsistency between the name of the analysis method in the methods section and in the figures, so detailed interpretations of the locations and estimation method require caution.
What can differences in EEG be linked to?
The three formulations differed not only in whether they contained ammonia but also in other ingredients and the structure of the cream. The study did not measure airborne ammonia concentrations and compare controlled levels, so the differences cannot be attributed solely to the amount of ammonia.
Furthermore, the psychological question concerned pleasantness and unpleasantness; stress of which participants were unaware was not independently verified by another method. Combining EEG and ratings offers clues for product evaluation, but this trial cannot determine health harms, safety, or the prolonged experience of actually coloring hair. The authors also identified larger studies that consider age and experience as future tasks.
Summary
In this comparison involving 10 people, the conventional ammonia-odor-emitting formulation was rated more unpleasant than the other two. Although subjective pleasantness–unpleasantness did not clearly differ between the odor-reduced and ammonia-free formulations, their responses differed in the alpha band during smelling and the beta band afterward.
The study illustrates the value of examining product odors through both verbal evaluations and EEG responses over time. A challenge for future evaluation methods is to establish what each measure captures and use it accordingly.
Five of the eight authors were affiliated with Nihon L’Oréal at publication. The acknowledgments state that the company supplied the experimental products. No separate funding or conflict-of-interest statement was found.
Reference
Kodama T, et al. (2019). Effects of Odor of Hair Color Products on Psycho-physiological Index. Transactions of Japan Society of Kansei Engineering, 18(3), 195–200. https://doi.org/10.5057/jjske.TJSKE-D-19-00003 (Original PDF)
