When we smell wood while looking at a wooden wall, do we receive the information from our eyes and nose separately? Or do we experience both as information about a single object: wood?
In a study published in 2025, Viviane Gallus, Christine I. Hucke and colleagues combined pine-wood odour with photographs of wood, examining ratings of odour pleasantness and intensity alongside brain activity. They aimed to distinguish the effects of stimulating two senses at once from the effects of a meaningful connection between them.
Combining wood odour with photographs of wood
The first experiment enrolled 27 people. After excluding three because of measurement problems and three whose performance on an odour-detection task did not meet the criterion, the analysis included 21 participants aged 18–35.
The odour came from finely divided Scots pine wood, not essential oil or perfume. An apparatus delivered either wood odour or clean control air to the nose.
The visual stimuli were 24 photographs showing wood surfaces or non-wood textures resembling artificial wall surfaces. Brightness and other properties were adjusted. Four conditions paired wood odour or air with either wood or non-wood photographs.
Each participant completed 240 trials. A photograph appeared for 6 seconds, with a 1.5-second odour delivery beginning at image onset. Participants pressed a button if they detected an odour, then rated its pleasantness and intensity. Only trials with correct odour-presence responses were analysed.
A separate second experiment analysed data from 21 of 23 participants. These were different people from the first experiment, and the lemon-like fragrance compound citral replaced wood odour. This tested whether the same results would occur with an odour that was not conceptually matched to wood photographs.
The photographs influenced pleasantness
Contrary to expectations, neither wood odour nor citral was rated significantly more pleasant than air. Odour ratings varied substantially between individuals: some responded favourably and others did not.
An image effect did appear in both experiments. Odour pleasantness ratings were higher while participants viewed wood photographs than while they viewed photographs of other surfaces.
The first experiment, using wood, also showed an interaction between odour and image. However, individual condition comparisons found that wood photographs increased ratings not only with wood odour but also when only air was delivered.
Furthermore, wood odour with wood photographs was not significantly more pleasant than air with wood photographs. It also did not clearly differ from the control condition containing neither a wood image nor wood odour. Matching wood odour and imagery therefore did not invariably make the experience more pleasant than every other combination.
The authors considered it important that the images influenced ratings even though participants had been asked to evaluate their olfactory experience. A favourable impression of the wood photographs themselves may have carried over into olfactory judgements.
In both experiments, intensity ratings were higher when odour was delivered, but photographs produced no clear difference. Pleasantness and intensity did not change in the same way.
Examining brainwaves at different frequencies
The researchers also analysed electroencephalographic activity recorded by scalp electrodes. EEG contains overlapping faster and slower fluctuations. They separated these by frequency and examined when activity increased or decreased relative to the period before image presentation. The paper calls this time-frequency analysis ERSP.
Processing wood odour was associated with reduced alpha-band activity, which fluctuates roughly 8–13 times per second, and increased activity in the slower delta band. The authors considered possible links with sensory processing and attention to the odour.
In the wood experiment, viewing wood photographs also changed alpha-band activity. The researchers suggested that visual information relevant to the odour might attract attention and be processed differently from unrelated images. Reduced EEG power should not simply be interpreted as the brain becoming inactive.
Does mismatched information impose a processing burden?
The central analysis combined and compared EEG responses across the four conditions to seek a component related to conceptual congruence that could not be explained by image-only or odour-only changes.
In the wood experiment, a relative delta-band difference appeared late, approximately 4.5–4.7 seconds after image onset. Activity was greater on the incongruence-related side of the comparison involving wood odour and non-wood photographs.
The authors interpreted this as potentially reflecting additional processing needed to ignore an incongruent image or resolve competing information while evaluating the odour. Congruent information might require less of that effort, but this remains a hypothesis.
The same kind of congruence-related effect was not detected in the citral experiment. However, there was no statistical test directly comparing the two experiments. Detecting an effect in one but not the other does not by itself establish that the meaning of wood caused the difference.
Exploratory findings that need confirmation
The EEG analysis was exploratory in that it examined many times and frequencies. A significant cluster does not establish that every time point or frequency within it is individually significant. There was also a delay between activating the apparatus and the odour reaching the sensory receptors in the nose; the reported timing was not referenced to that arrival time.
Participants pressed a button only when they detected an odour, so odour and air conditions also differed in movement. The researchers removed components associated with movement, but did not guarantee that its influence had been completely eliminated.
The control conditions were also asymmetric: the visual comparison involved two image types, whereas the olfactory comparison involved an odour versus air. Alongside the sample size of 21 per experiment, this calls for comparisons between different odours and confirmatory analyses with prespecified windows.
The research received funding from Germany’s Federal Ministry of Food and Agriculture. The authors declared no competing interests to report.
Summary
In these experiments, wood photographs influenced judgements of olfactory pleasantness, not only for wood odour. EEG changes may also point to attention to visual information associated with wood and to the burden of processing incongruent information.
The goal is not simply to identify pleasant smells in isolation, but to understand how the visible environment and odour become one experience. In wooden interiors, how wood looks may matter alongside how it smells. To establish reliable design implications, the visual effects and the interpretation of the exploratory EEG findings need to be tested separately.
References
Gallus, V., Hucke, C. I., Butter, K., Ohlmeyer, M., & van Thriel, C. (2025). Conceptual processing of natural, complex odours: multisensory effects on behaviour and ERSP. Brain Research, 1863, 149839. DOI: 10.1016/j.brainres.2025.149839. Full text (Thünen Institute, PDF). © 2025 The Authors. Published by Elsevier B.V. CC BY 4.0.
