The application of electroencephalographic techniques to the study of human olfaction: a review and tutorial.

The use of a variety of electrophysiological techniques to determine the effects of odor on the nervous system is reviewed. Methods and problems associated with the collection of on-going EEG, chemosensory event-related potentials, and contingent negative variation data are discussed in depth as is the use of odors as modulators of brain potentials produced by other senses. In addition, the advantages of several seldom used EEG analysis techniques are discussed with respect to the unique problems of understanding olfaction.

A computer-controlled olfactometer for fMRI and electrophysiological studies of olfaction.

A design for an inexpensive and reliable olfactometer is presented. The design has several advantages for fMRI and electrophysiology investigators. These advantages include relatively rapid odorant rise times, computer control, multiple odor administration, and no ferrous materials near the subjects. In addition, the device is contamination resistant, and, because the air is neither warmed nor humidified, it is unlikely to become an incubator for bacteria. The olfactometer is constructed of off-the-shelf chromatography parts that require little modification.

On the similarity of odor and language perception.

The association between olfaction and language is discussed. The effects of odor on human behavior and cognitive processing are reviewed as are electrophysiological studies of odor/language interactions. Also reviewed are specific effects of odor administration on language-dependent tasks. The hypothesis is advanced that odor information processing shares some of the cortical resources used in processing language and that interference between these two types of stimuli occurs when they are simultaneously processed. The reason for this overlap in resources is thought to be due to the similarities in the spatio-temporal patterns produced in the neural coding of odors and language.

Chemosensory alteration of information processing.

Previous research has found both facilitation and interference when odors are administered during the performance of cognitive tasks. The present research was designed to investigate the possibility that odors might differentially effect different varieties of cognitive tasks. Twelve subjects performed matched verbal, numeric, and spatial tasks as event-related potentials were measured. Odors and tones were introduced as subjects completed the tasks in a paradigm optimized to produce disruption of performance. Results indicated that P300 amplitude to the task stimuli depended upon the type of task and the modality of the distractor. P300 amplitude was relatively consistent across tasks for the tone distractor. During the olfactory distractor, P300 amplitude was greater for the symbolic stimuli (verbal and numeric) and lower for the spatial. Reaction times to the various tasks did not differ as a function of the modality of the distractor.

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.

Spatio-temporal display of event-related potential data in three dimensions.

A technique for visualizing event-related potential data which preserves both spatial and temporal dimensions is presented. This technique can be implemented by several different types of software and may be useful when spatio-temporal information is of critical importance in a data set. It is especially useful for comparing spatio-temporal differences between experimental conditions.

EEG and ERP studies of low-level odor exposure in normal subjects.

Evidence from four EEG/ERP experiments is presented. Findings of these experiments consistently demonstrate the ability of low-level and undetected odors to alter neurophysiology. Behavioral and cognitive effects are also described. These data may have applicability to theories of MCS since they illustrate the possibility that low-level or undetected odors affect central nervous system activity and may precipitate or cue MCS symptoms.

The effects of light reinforcement and noise on young and old squirrel monkeys (Saimiri sciureus).

In two age groups of squirrel monkeys, subjects performed an operant response for light onset in the presence or absence of white noise. Results of the study indicate that light is an effective reinforcer for both younger and older monkeys, but differentially. The study also indicates that the presence of white noise affects responding in both age groups dissimilarly.