Olfactory event-related brain potentials (OERPs) are neuro-electrical
recordings from the skull to an olfactory stimulus. OERPs possess
a high temporal resolution and are a representation of brain processes.
OERPs can tell us a great deal about the way humans process smell information.
In our laboratory we are using this method to systematically investigate olfaction in health and disease.

The following are excerpts from a recent study conducted in our lab:

Electroencephalographical recordings of brain responses to auditory, visual, and somato-sensory stimuli have received much attention by researchers. Findings from these studies contributed to the understanding of human information processing in health and disease, and Event-Related Potentials (ERPs) of these modalities are already well established assessment tools in clinical settings (Moecks, 1981). Comparably, the development of ERPs to olfactory stimuli progressed much slower. This was mainly due to previous negligence of the chemical senses in research, and the difficulties associated with the control of olfactory stimuli. Reliable recording of Olfactory Event-Related Potentials (OERP) requires rapid rise of odor concentration without stimulation of sensory modalities other than olfactory; features that were firstly incorporated in a stimulus delivery device for odors by Kobal and colleagues (Kobal, 1981). Rise times below 20 ms are generally achieved with this and similar olfactometers, yielding reliable neural responses in an ERP paradigm.

A look inside the olfactometer reveals it's complexity. Three different odors can be administered at rapid successions for
usage in an odd-ball paradigm. However, due to the strong adaptation and habituation properties of the olfactory system,
we are mostly using a single stimulus paradigm to elicit cognitive ERP components.

In addition, smell and taste are generally the least understood human senses. However, in the last decade a growing body of research has emphasized the importance of the chemical senses in mate selection, quality of life, nutrition, as well as in disease, such as in Parkinson's Disease, Huntington's Disease, Korsakoff's Syndrome, Depression, and Alzheimer's Disease (AD) (Harrison & Pearson, 1989). For example, recent evidence suggests that areas in the central nervous system processing olfactory information are affected at the early stages of AD, even before the onset of cognitive decline, and that olfactory dysfunction might be an early indicator of AD (Murphy, 1999). It is this more profound understanding of the importance of olfaction and the constant improvements in OERP research methods which have rendered the OERP a promising candidate to be included in standard clinical assessments.

A happy subject in an OERP experiment. The 'white arm' you see in this picture delievers the stimulus
into the subject's nose in a constantly flowing air stream, and extends to the olfactometer.

Much of the previous literature on OERPs used the Velopharyngeal Closure (VC) method as the breathing technique, where subjects are trained to use the levator veli palatini muscle to elevate the soft-palate in order to isolate the pharyngeal cavity from the nasal cavity (Geisler, Morgan, Covington, & Murphy, 1999; Huang, Lee, & Rajendran, 1998; Kobal, 1981; Kobal, 1991; Murphy, Nordin, de Wijk, Cain, & Polich, 1994; Pause, Sojka, Krauel, & Ferstl, 1996; Wetter & Murphy, 1999).

Schematic drawing of human head showing velum position for natural breathing and velopharyngeal closure.
No respiratory airflow enters the nasal cavity during velopharyngeal closure.

This procedure prevents intranasal respiratory airflow and ensures the absence of interference from respiration on stimulus presentation (Kobal, 1991). However, some populations may not be able to perform VC reliably, and therefore, the use of VC has limited olfactory assessment with OERPs in populations unable to perform this artificial breathing technique. These patients, in particular, should not be automatically excluded from OERP assessment, since OERPs require minimal effort and cooperation from the patient compared with traditional psychophysical testing, and are therefore especially well-suited for these populations. However, when using an alternative to VC, the OERP components can be expected to differ in amplitude and latency from those recorded with VC.

Two procedural variables that have been identified in OERP studies are breathing technique and administration of the stimulus with respect to the respiratory cycle. In a recent study, Pause et al. (Pause, Krauel, Sojka, & Ferstl, 1999) recorded OERPs with a short ISI (8 s) from 8 subjects performing VC and normal mouth breathing. In both instances, odor presentation occurred non-synchronously to the breathing cycle. In an off-line analysis, trials were separated into whether the odor was delivered during phases of inspiration or expiration. In this study, mouth breathing yielded shorter N1 latencies and larger P3 amplitudes than VC, which the authors attribute to the division of attention between stimulus processing and maintenance of breathing technique. Lorig et al. (Lorig, Matia, Peszka, & Bryant, 1996) examined the difference between synchronous and non-synchronous odor presentation. If the stimulus was delivered during nasal inspiration, the amplitude of the positive peak at approximately 800 ms latency was generally found to be smaller than if stimulus presentation in the nose occurred randomly during mouth breathing. However, these studies used only normal, young adults with similar olfactory function as participants. To this point, no previous study had investigated the effects of the above mentioned variables in populations with different olfactory function.

This led us to designe a study that looked at the effects of Natural Breathing (NB) in comparison to Velopharyngeal Closure on OERP latency and amplitude in two populations who were known to exhibit different olfactory function, but were able to perform Velopharyngeal Closure: young and elderly adults.

Psychophysical measures have demonstrated an age-related decline in olfactory function, including odor detection threshold (Murphy, 1983), odor identification (Murphy & Cain, 1986) and odor memory (Murphy, Cain, Gilmore, & Skinner, 1991; Murphy, Nordin, & Acosta, 1997). Moreover, anatomical changes across the life span in peripheral olfactory structures (e.g. olfactory receptor cells) and central olfactory areas such as temporal lobe, entorhinal cortex, hippocampus, and amygdala have been identified (Cowell et al., 1994; Liss, 1958; Price, Davis, Morris, & White, 1991). Recent OERP studies reported decreased amplitudes and longer latencies from older subjects compared to younger subjects (Morgan, Covington, Geisler, Polich, & Murphy, 1997; Morgan, Geisler, Covington, Polich, & Murphy, 1999; Murphy, 1999; Murphy et al., 1994).

An elderly subject during an experiment. The task is easy to perform and requires minimal cooperation.
A thermistor inside one nostril monitors nasal respiration.

Procedure

Participants were seated comfortably in a reclining chair adjacent to the olfactometer arm to reduce muscle movement. Before each trial, participants placed the nostril on the nasal piece. Stimulus onset occurred randomly within a 10 - 25 s time window. The time window was chosen to reduce expectancy effects (Loveless & Sanford, 1974). All participants were exposed to both VC and NB in a counterbalanced block design. Before each session, participants were trained to perform VC using a thermistor (tc = 6s; Model F-TCT, Grass Instruments, USA) placed inside one nostril, which monitored nasal air flow at all times. Nasal respiration was displayed on an oscilloscope not visible to the participant and recorded simultaneously with EEG activity. In the VC condition, participants were instructed to use the soft palate to close the nasal cavity from the pharyngeal tract until correct performance of VC was achieved. Stimulus presentation in the VC condition was triggered manually non-synchronously to the breathing cycle. In the NB condition, participants were instructed to breathe normally through mouth and nose while stimulus presentation was triggered manually during inspiration. The experimenter judged the phase of inspiration heuristically based on oscilloscope readings.The relationship between stimulus administration and inspiration was not explicitly stated. Correct performance of VC was monitored throughout the experiment. Trials showing nasal respiration were rejected and repeated. If necessary, VC was practiced between trials.

Magnitude estimation and single-stimulus paradigm:

Immediately after each trial, participants were asked to report the perceived intensity of the stimulus they had just received on the Labeled Magnitude Scale (LMS). The LMS is a semantically labeled scale with logarithmic spacing of its verbal labels (e.g. strongest imaginable, strong, weak, barely detectable, etc.), developed by Green et al. (Green, Shaffer, & Gilmore, 1993) for magnitude estimation of oral somatosensation and gustation, and was subsequently validated for olfaction (Green et al., 1996). In addition to eliciting a subjective measure of the participant's olfactory perception, the estimation of odor magnitude for each stimulus insured that the participant was attending to the stimulus, eliciting cognitive OERP components in a single-stimulus paradigm. Generally, a P3 component can be elicited when subjects attend to a novel stimulus (Donchin & Coles, 1988), and is usually produced with an oddball paradigm. However, P3 components can also be obtained with a single-stimulus, where only one stimulus is presented to which the subject has to respond. Longer ISI's are needed in this paradigm to allow for the memory trace to be updated, rendering each trial 'novel' (Polich, Eischen, & Collins, 1994). This paradigm produces virtually identical scalp topographies as the oddball paradigm, and has been shown to exhibit the same effects for many experimental variables (Cass & Polich, 1997; Polich & Heine, 1996). The single-stimulus paradigm is especially useful for cognitive ERP testing in the olfactory modality (Geisler et al., 1999; Geisler & Murphy, 2000; Morgan et al., 1999), where a rapid succession of stimuli would produce strong adaptation and habituation effects.

Results

The findings from the Velopharyngeal Closure condition were consistent with those observed in previous studies in which OERPs were recorded in different age groups using VC (Morgan et al., 1997; Morgan et al., 1999; Murphy et al., 1994; Murphy, Wetter, Morgan, Ellison, & Geisler, 1998b). Increasing age was associated with smaller N1-P2 and N1-P3 amplitudes across all electrode sites. For both age groups, maximum amplitudes were observed at Cz and Pz electrode sites. Elderly subjects also showed prolonged latencies for N1, P2, and P3.
Similar effects were observed for Natural Breathing. In this condition, older participants showed significantly smaller N1-P2 and N1-P3 amplitudes and prolonged P2 and P3 latencies relative to younger participants. However, NB consistently yielded smaller N1-P2 amplitudes at Fz, Cz, and Pz than Velopharyngeal Closure. No difference in N1-P3 amplitude was observed between the two breathing techniques. Means for P3 latency showed a trend towards shorter latencies for NB. No interaction between age and breathing technique was found in this study.

The present investigation showed that the OERP is a sensitive measurement for detecting age-related changes in olfactory function regardless of breathing technique. Results support the use of Natural Breathing in OERP paradigms, and thereby make electrophysiological assessment of olfactory processing possible in populations that were previously excluded from OERP experiments due to their inability to perform Velopharyngeal Closure. However, in populations where it is feasible, Velopharyngeal Closure will produce a higher amplitude response and results that can be compared with normative data (Murphy, 2000). Thus, in many cases it will be the technique of choice.

This study will be published in the International Journal of Psychophysiology under the title: Age-related changes in olfactory processing detected with event-related brain potentials using velopharyngeal closure and natural breathing. The authors are Thomas Thesen and Claire Murphy.

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