The effects of active and passive stimulation on chemosensory event-related potentials.

During normal olfaction, stimulation is dependent upon nasal inhalation. When collecting chemosensory event-related potentials (CSERPs), inhalations may produce artifacts such as respiration-related brain potentials that confound interpretation of the data. To avoid this type of artifact, CSERPs have often been collected using stimulation that is independent of respiratory cycle. It is possible, however, that inspirations prime the olfactory tract for odor input, and traditional CSERP data acquisition techniques, obscure this neural preparation. To investigate this question, twelve subjects were tested using two different approaches to stimulation. Odorants (butanol 2% and 4%) were introduced into a warmed and humidified air stream and delivered to one nostril through a cannula. In one condition, subjects mouth-breathed and stimuli occurred asynchronously with respiratory cycle (passive). In the other condition, subjects inhaled through the nose and stimuli were delivered synchronously with nasal inhalations (active). CSERP data were collected from thirty scalp sites for 3 s following stimulation. Data were corrected for eye movements, smoothed, and averaged. Maximal amplitudes for an early negativity (N1) and later positivity (P2) were determined and submitted to separate analyses of variance. These analyses indicated that administration technique interacted with both odor concentration and recording site. Additionally, amplitude of P2 was greater in the passive condition. Such results suggest that the two administration techniques produce different neural processing of olfactory stimuli and that the passive technique may be better suited for determination of the integrity of the olfactory tract for single subjects because of its greater amplitude.

The contingent negative variation in an odor labeling paradigm.

Fifteen subjects participated in an experiment designed to assess the contingent negative variation (CNV) during the labeling of odors and shapes. Odors or shapes were presented (S1) and followed 3 s later by a lexical label (A, B, or C) (S2). In 75% of the trials, the S2 was the correct label for the odor or shape. In the remaining trials, the S2 was an incorrect label. Subjects' olfactory performance was correlated with both the CNV during in the S1/S2 interval and also the P300 following the S2 stimulus. The CNV over the left frontal area was significantly larger in the olfactory phase of the experiment. CNV activity also correlated with olfactory performance such that subjects with the largest odor-related CNVs had the best olfactory performance. Although P300 differed as a function of label matches versus mismatches, no odor-specific effects or correlations were found.