Resistance to extinction in an odor–20% CO₂ inhalation paradigm: Further evidence for a symptom learning account of multiple chemical sensitivity

Abstract 

Objective

Multiple chemical sensitivity (MCS) is characterized by a variety of symptoms in response to nontoxic concentrations of chemicals. To further test a laboratory model of MCS based on symptom learning, we used a stronger respiratory challenge in this study than in previous studies to induce symptoms (20% CO₂-enriched air, unconditioned stimulus). Additionally, extinction of learned symptoms was investigated.

Methods

Diluted ammonia (AM) and butyric acid (BA) served as conditioned (odor) stimuli (CSs). In a learning phase, healthy participants received three breathing trials of BA mixed with CO₂ and three trials of AM mixed with regular air, or the reversed combination. In a test phase, the same trials were administered without CO₂. Immediately after odor onset, participants rated their anxiety and expectancy to experience symptoms during that trial. Self-reported symptoms were assessed after each trial; respiratory behavior was measured throughout the experiment.

Results

Only participants who learned to correctly anticipate symptoms (learning phase) reported elevated symptoms in response to the CS+ odor (test phase). Anticipation was more likely correct and learned symptoms were more elevated when BA had been previously associated with CO₂. Across test trials, anticipatory anxiety diminished, but learned symptoms did not. Participants failing to learn the prearranged contingencies reported overall more symptoms and anxiety.

Conclusions

Strong respiratory challenges impede extinction of learned symptoms. Conscious expectancy, which may be modulated by odor quality, determines whether learned symptoms develop in response to a specific odor or to the general context. The results further support a symptom learning account of MCS.

Keywords: Multiple chemical sensitivity, Associative learning, Extinction, Online US expectancy, Odors, CO₂