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This paper describes using a breath-synchronized olfactometer to trigger single- and dual-coil transcranial magnetic stimulation (TMS) during odorant presentation synchronized to human nasal breathing. This combination allows us to objectively investigate how pleasant and unpleasant odors impact corticospinal excitability and brain-effective connectivity in a given individual.
It is widely accepted that olfactory stimulation elicits motor behaviors, such as approaching pleasant odorants and avoiding unpleasant ones, in animals and humans. Recently, studies using electroencephalography and transcranial magnetic stimulation (TMS) have demonstrated a strong link between processing in the olfactory system and activity in the motor cortex in humans. To better understand the interactions between the olfactory and the motor systems and to overcome some of the previous methodological limitations, we developed a new method combining an olfactometer that synchronizes the random order presentation of odorants with different hedonic values and the TMS (single- and dual-coil) triggering with nasal breathing phases. This method allows probing the modulations of corticospinal excitability and effective ipsilateral connectivity between the dorsolateral prefrontal cortex and the primary motor cortex that could occur during pleasant and unpleasant odor perception. The application of this method will allow for objectively discriminating the pleasantness value of an odorant in a given participant, indicating the biological impact of the odorant on brain effective connectivity and excitability. In addition, this could pave the way for clinical investigations in patients with neurological or neuropsychiatric disorders who may exhibit odor hedonic alterations and maladaptive approach-avoidance behaviors.
It is widely accepted that olfactory stimulation elicits automatic reactions and motor behaviors. For example, in humans, the existence of an avoidance motor response (leaning away from the odor source) occurring 500 ms after negative odor onset has been recently demonstrated1. By recording freely moving human participants exploring odors emanating from flasks, Chalençon et al. (2022) showed that motor behaviors (i.e., speed of approach to the nose and withdrawal of the flask containing the odorant) are closely linked to odor hedonics2. Moreover, a close link between processing in the olfactory system and activity i....
All experimental procedures described in the following sections have been approved by an Ethics Committee (CPP Ile de France VII, Paris, France, protocol number 2022-A01967-36) in accordance with the Declaration of Helsinki. All participants provided written informed consent before study enrollment.
1. Participant recruitment
The representative data presented here reflect recordings from participants after completing the step-by-step protocol above to provide a preliminary insight into what we might expect.
Figure 2 shows an example of a representative participant's respiratory signals recorded with the olfactometer software. The expiratory and inspiratory phases are well detected when the thresholds are crossed. The odorant is triggered immediately after the expiration phase .......
The protocol above describes a novel method combining the use of a breath-synchronized olfactometer with single- and dual-coil TMS to investigate changes in corticospinal excitability and effective connectivity depending on the hedonic value of the odorants. This setup will allow for objectively discriminating the pleasantness value of an odorant in a given participant, indicating the biological impact of the odorant on brain effective connectivity and reactivity. The critical steps in this protocol involve both TMS.......
This work was supported by the Fondation de France, Grant N°: 00123049/WB-2021-35902 (a grant received by J.B. and N.M.). The authors would like to thank the Fondation Pierre Deniker for its support (grant received by C.N.) and the staff of the Neuro-Immersion platform for their valuable help in designing the setup.
....Name | Company | Catalog Number | Comments |
Acquisition board (8 channels) | National Instrument | NI USB-6009 | |
Air compressor | Jun-Air | Model6-15 | |
Alcohol prep pads | Any | ||
Butyric acid | Sigma-Aldrich | B103500 | Negative odorant |
Desktop computer | Dell | Latitude 3520 | |
EMG system | Biopac System | MP150 | |
Isoamyl acetate | Sigma-Aldrich | W205508 | Positive odorant |
Nasal cannula | SEBAC France | O1320 | |
Programmable pulse generator | A.M.P.I | Master-8 | |
Surface electrodes | Kendall Medi-trace | FS327 | |
TMS coil (X2) | MagStim | D40 Alpha B.I. coil | |
TMS machine | MagStim | Bistim2 | |
Tube 6 mm x 20 m | Radiospare | 686-2671 | Pneumatic connection |
USB-RS232 | Radiospare | 687-7806 | |
U-shaped tubes | VS technologies | VS110115 |
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