Can Stimulation of One Nostril Draw Visual Attention to That Side?

When a stimulus reaches the left side of the nose, a person may respond slightly faster to a visual target that then appears on the left. A 2021 study found this kind of shift in attention from the nose to vision at particular time intervals.

The researchers were not examining a deliberate search guided by a smell. They wanted to know whether stimulation of the nose could draw visual attention toward its location. Across seven experiments, the time between nasal stimulation and the appearance of the target proved important to whether an effect was detected.

Looking beyond smell to the nose’s sensitivity to irritation

Besides detecting odors through olfaction, the nose senses irritation, cooling, and other sensations through the trigeminal nerve. The study used carbon dioxide, which stimulates this trigeminal system, and eucalyptol, which acts on both the olfactory and trigeminal systems.

Carbon dioxide produces a stinging sensation inside the nose. Because its olfactory contribution is small, it is useful for initially examining how nasal stimulation changes attention. The researchers then investigated the same phenomenon with eucalyptol, a substance also relevant to everyday scents, to see whether the findings extended beyond carbon dioxide.

Accordingly, nasal stimulation here is not interchangeable with fragrance in general. Understanding the study begins with distinguishing effects of purely olfactory stimulation from effects that include a sensation of irritation in the nose.

Combining the left and right nostrils with left and right visual targets

Participants were mainly students at the University of Geneva and their acquaintances. The numbers included in the analyses of the seven experiments were 28, 27, 27, 20, 27, 27, and 25, respectively. Participants were selected on their performance in a preliminary task identifying which nostril had been stimulated. Carbon dioxide concentrations were adjusted individually to be clearly perceptible but not painful.

In the main task, participants fixated on the center of a screen. After stimulation of one nostril, a small target appeared on the left or right of the screen for just 120 milliseconds. Participants used keys to indicate whether it was vertical or horizontal. The task was to judge the target’s orientation, not to respond on the same side as the nasal stimulus.

The researchers combined 16 trials each of four conditions, totaling 64 trials: nasal stimulus and target on the same side, stimulus and target on opposite sides, and two control conditions delivering only clean air to the nose. The side of nasal stimulation did not predict the target’s location. This tested whether a cue could capture attention even though deliberately using it would offer no advantage.

To avoid interference between the delivered airflow and normal breathing, participants briefly held their breath during each trial. Eye movements were also recorded to check whether faster responses could instead be explained by participants actually moving their gaze toward the stimulated side.

Effects after about 0.68 and 1.17 seconds for carbon dioxide, and 0.61 seconds for eucalyptol

The central analysis compared reaction times when the nasal stimulus and visual target were on the same side with the combined average of the opposite-side stimulus condition and the two air conditions. It did not test superiority against each of those three conditions separately.

With carbon dioxide, responses to targets on the same side were an average of 24 milliseconds faster when the interval from stimulus onset to target onset was about 680 milliseconds. At about 1,170 milliseconds, they were roughly 30 milliseconds faster. There was no significant reduction at intervals of about 580 or 1,870 milliseconds.

With eucalyptol, responses on the same side were an average of 42 milliseconds faster at an interval of about 610 milliseconds. No significant differences were found at about 830 or 1,120 milliseconds. Nasal stimulation did not capture visual attention at every timing tested; the effect appeared in only some of the temporal conditions.

Accuracy was already high, and this comparison showed no significant accuracy difference in any of the seven experiments. A measure of efficiency combining reaction time and accuracy improved for carbon dioxide at about 1,170 milliseconds and for eucalyptol at about 610 milliseconds.

Attention may have shifted without a change in gaze

The eye-movement recordings did not show a large shift of gaze toward the stimulated side between the stimulus and target. Respiratory recordings also supported that participants held their breath as instructed. The authors interpreted the findings as nasal stimulation influencing where visual attention was directed without changing eye direction.

In the three experiments showing faster responses, responses to targets on the opposite side were also compared with the air conditions, revealing no significant delay. In these results, the benefit of attention being drawn to the stimulated side stood out; a clear cost when responding on the opposite side was not established.

Other senses may direct attention toward the location from which a chemical stimulus arrives. The authors discussed the possibility that this kind of coordination between senses could help people locate important things in their environment.

How to interpret the timing of the effect

Another aim of the study was to gain clues about the time window in which attention is captured. However, the intervals were not varied systematically in equal steps, nor were the same settings used for the two substances. Nasal-stimulus duration also changed along with the interval. It is therefore not possible to conclude that eucalyptol acts faster, or that carbon dioxide’s effect lasts longer.

The authors themselves noted the need for experiments that vary the interval in smaller steps while keeping stimulus duration constant. The contributions of finding a nasal stimulus pleasant or unpleasant were not separated either. Further work needs to test natural breathing conditions and other types of nasal stimulation.

This study used brief stimuli delivered to one side of the nose and simple targets on a screen. It did not test whether diffusing a fragrance around a room improves concentration during study or work. Its findings concern where attention is directed and when that shift occurs.

Summary

Stimulation of one nostril sometimes sped responses to a target appearing on the same side. Effects were found at intervals of about 680 and 1,170 milliseconds for carbon dioxide and about 610 milliseconds for eucalyptol, with no significant differences at the other four timings. An improvement in accuracy was not established.

Left–right information obtained from nasal stimulation can influence visual attention even without large eye movements. And stimulus timing is important for detecting that coordination. The seven experiments demonstrated these two points.

Research context and disclosures: The study received support from the SNSF-funded NCCR in Affective Sciences and a research grant from Firmenich. The paper reports that the company grant supported one author’s salary and experimental materials, with no additional role in study design, data collection and analysis, the decision to publish, or manuscript preparation. Three coauthors were affiliated with Firmenich at the time. The authors declared no potential conflicts of interest and stated that the corporate affiliation did not alter their adherence to policies on sharing data and materials.

Reference

Ischer, M., et al. (2021). Exogenous capture of visual spatial attention by olfactory-trigeminal stimuli. PLOS ONE, 16(6), e0252943. Original paper and open full text