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Generalization of Acquired Somatic Symptoms in Response to Odors: A Pavlovian Perspective on Multiple Chemical Sensitivity

From the Department of Psychology (S.D., W.W., K.S., I.V.D., P.E., O.V.) and Faculty of Medicine (H.V., B.N., K.V.), University of Leuven, Leuven, Belgium.

Address reprint requests to: Omer Van den Bergh, PhD, Department of Psychology, University of Leuven, Tiensestraat 102, B-3000 Leuven, Belgium. Email: omer.vandenbergh{at}psy.kuleuven.ac.be

OBJECTIVE: Somatic symptoms that occur in response to odors can be acquired in a pavlovian conditioning paradigm. The present study investigated 1) whether learned symptoms can generalize to new odors, 2) whether the generalization gradient is linked to the affective or irritant quality of the new odors, and 3) whether the delay between acquisition and testing modulates generalization.

METHODS: Conditional odor stimuli (CS) were (diluted) ammonia and niaouli. One odor was mixed with 7.4% CO₂-enriched air (unconditional stimulus) during 2-minute breathing trials (CS+ trial), and the other odor was presented with air (CS- trial). Three CS+ and three CS- trials were conducted in a semirandomized order (acquisition phase). The test phase involved one CS+-only (CS+ without CO₂) and one CS- test trial, followed by three trials using new odors (butyric acid, acetic acid, and citric aroma). Half of the subjects (N = 28) were tested immediately, and the other half were tested after 1 week. Ventilatory responses were measured during and somatic symptoms were measured after each trial.

RESULTS: Participants had more symptoms in response to CS+-only exposures, but only when ammonia was used as the CS+. Also, generalization occurred: More symptoms were reported in response to butyric and acetic acid than to citric aroma and only in participants who had been conditioned. Both the selective conditioning and the generalization effect were mediated by negative affectivity of the participants. The delay between the acquisition and test phases had no effect.

CONCLUSIONS: Symptoms that occur in response to odorous substances can be learned and generalize to new substances, especially in persons with high negative affectivity. The findings further support the plausibility of a pavlovian perspective of multiple chemical sensitivity.

Key Words: classic conditioning • generalization, • psychosomatic symptoms • multiple chemical sensitivity.

Abbreviations: ANOVA = analysis of variance; CR = conditioned response; CS = conditioned stimulus; f = frequency per minute; MCS = multiple chemical sensitivity; NA = negative affectivity; NEM/PEM = Negative and Positive Emotionality Measure; UR = unconditioned response; US = unconditioned stimulus; V_E = minute ventilation; V_T = tidal volume.

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