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

Electrophysiologic assessment of olfactory and gustatory function

This chapter reviews approaches for assessing human and gustatory function using electrophysiologic methods. Its focus is on changes in electrical signals, including summated generator potentials that occur after nasal or oral stimulation. In the first part of the review, we describe tools available to the clinician for assessing olfactory and nasotrigeminal function, including modern electroencephalography (EEG) analysis of brain responses both in the time domain and in the time-frequency (TF) domain. Particular attention is paid to chemosensory event-related potentials (CSERPs) and their potential use in medical-legal cases. Additionally, we focus on the changes of summated generator potentials from the olfactory and respiratory nasal epithelium that could provide new diagnostic insights. In the second part, we describe gustatory event-related potentials (gCSERPs) obtained using a relatively new computer controlled gustometer. A device for presenting different pulses of electrical current to the tongue is also described, with weaker pulses likely reflecting gCSERPs and stronger ones trigeminal CSERPs. Finally, summated generator potentials from the surface of the tongue during gustatory stimulation are described that may prove useful for examining peripheral taste function.

Individuals With Autism Spectrum Disorder Show Altered Event-Related Potentials in the Late Stages of Olfactory Processing

Atypical sensory reactivities are pervasive among people with autism spectrum disorder (ASD). With respect to olfaction, most previous studies have used psychophysical or questionnaire-based methodologies; thus, the neural basis of olfactory processing in ASD remains unclear. This study aimed to determine the stages of olfactory processing that are altered in ASD. Fourteen young adults with high-functioning ASD (mean age, 21 years; 3 females) were compared with 19 age-matched typically developing (TD) controls (mean age, 21 years; 4 females). Olfactory event-related potentials (OERPs) for 2-phenylethyl alcohol-a rose-like odor-were measured with 64 scalp electrodes while participants performed a simple odor detection task. Significant group differences in OERPs were found in 3 time windows 542 ms after the stimulus onset. The cortical source activities in these time windows, estimated using standardized low-resolution brain electromagnetic tomography, were significantly higher in ASD than in TD in and around the posterior cingulate cortex, which is known to play a crucial role in modality-general cognitive processing. Supplemental Bayesian analysis provided substantial evidence for an alteration in the later stages of olfactory processing, whereas conclusive evidence was not provided for the earlier stages. These results suggest that olfactory processing in ASD is altered at least at the later, modality-general processing stage.

The intranasal trigeminal system

Many odors activate the intranasal chemosensory trigeminal system where they produce cooling and other somatic sensations such as tingling, burning, or stinging. Specific trigeminal receptors are involved in the mediation of these sensations. Importantly, the trigeminal system also mediates sensitivity to airflow. The intranasal trigeminal and the olfactory system are closely connected. With regard to central nervous processing, it is most interesting that trigeminal stimuli can activate the piriform cortex, which is typically viewed as the primary olfactory cortex. This suggests that interactions between the two systems may form at a relatively early stage of processing. For example, there is evidence showing that acquired olfactory loss leads to reduced trigeminal sensitivity, probably on account of the lack of interaction in the central nervous system. Decreased trigeminal sensitivity may also be responsible for changes in airflow perception, leading to the impression of congested nasal airways.

Disrupted Odor Perception

Olfactory loss is frequent. However, in public not many people complain of that, or they are even not (fully) aware of it. This indicates that it is possible to live a life without a sense of smell, albeit it is more dangerous, less pleasant, and food tastes much less interesting. Most common causes for smell loss are sinunasal disease (chronic rhinosinusitis with and without nasal polyps), acute infections of the upper airways, head trauma, and neurodegenerative disorders. In many people smell loss seems to be due to the aging process. Before treatment olfactory disorders are diagnosed according to cause with the medical history being a big portion of the diagnostic process. Olfactory disorders are in principle reversible, with a relatively high degree of spontaneous improvement in olfactory loss following infections of the upper respiratory tract. Medical treatment is according to cause. It also involves surgical approaches as well as conservative treatments including the use of corticosteroids, antibiotics, or smell training. Because today olfactory dysfunction seems to receive more attention than in previous years it can be expected that tomorrow we will have more specific and effective treatment options available.

Methods for olfactory fMRI studies: Implication of respiration

Human olfactory system is under-studied using fMRI compared with other sensory systems. Because the perception (intensity, threshold, and valence) and detection of odors are tightly involved with respiration, the subject's respiration pattern modulates and interacts with the experimental paradigm, which presents difficulties for olfactory fMRI data acquisition, post-processing, and interpretation. Based on our investigation on the interactions of odor presentation and subject's respiration, we propose a respiration-triggered event-related olfactory fMRI technique and a data post-processing method that effectively captures precise onsets of olfactory blood-oxygen-level-dependent (BOLD) signal in the primary olfactory cortex. We compared the olfactory BOLD signals from seventeen normal healthy adults with diverse respiratory patterns and showed that the subjects' respiratory patterns modulated the olfactory stimulation paradigm, which significantly confounded the BOLD signal. The presented experimental technique provides a simple and effective means for generating reliable olfactory fMRI results.

Adiposity measures predict olfactory processing speed in older adult carriers of the apolipoprotein E ε4 allele

OBJECTIVE

The current study investigated the relationship between adiposity and P3 latency.

METHODS

Fifty-one adults in two age groups (18-25 and 65+) participated. Odor stimuli were delivered via olfactometer as participants focused on a computer screen. Each stimulus was followed by presentation on the screen of four odor identification choices. Participants attempted identification by button press. Olfactory event-related potentials (OERPs) were recorded. BMI and waist circumference were measured as indicators of adiposity.

RESULTS

In bivariate analyses with all participants included, positive correlations for P3 latency with both BMI and waist circumference were observed, indicating that as adiposity increased latencies also increased. When each age group was separately examined, correlations between adiposity measures and latency remained statistically significant for older adults. Furthermore, ApoE ε4 allele status was examined. Latencies remained positively correlated with adiposity in older adult ε4 carriers; but not in non-carriers.

CONCLUSIONS

This study indicates that adiposity predicts olfactory processing speed in older adults, specifically in ε4 carriers.

SIGNIFICANCE

The results suggest that olfactory processing speed may be a useful measure for detecting and following the effects of adiposity on brain integrity and cognitive function in those at genetic risk for AD.

Time-frequency analysis of chemosensory event-related potentials to characterize the cortical representation of odors in humans

Background

The recording of olfactory and trigeminal chemosensory event-related potentials (ERPs) has been proposed as an objective and non-invasive technique to study the cortical processing of odors in humans. Until now, the responses have been characterized mainly using across-trial averaging in the time domain. Unfortunately, chemosensory ERPs, in particular, olfactory ERPs, exhibit a relatively low signal-to-noise ratio. Hence, although the technique is increasingly used in basic research as well as in clinical practice to evaluate people suffering from olfactory disorders, its current clinical relevance remains very limited. Here, we used a time-frequency analysis based on the wavelet transform to reveal EEG responses that are not strictly phase-locked to onset of the chemosensory stimulus. We hypothesized that this approach would significantly enhance the signal-to-noise ratio of the EEG responses to chemosensory stimulation because, as compared to conventional time-domain averaging, (1) it is less sensitive to temporal jitter and (2) it can reveal non phase-locked EEG responses such as event-related synchronization and desynchronization.

Methodology/Principal Findings

EEG responses to selective trigeminal and olfactory stimulation were recorded in 11 normosmic subjects. A Morlet wavelet was used to characterize the elicited responses in the time-frequency domain. We found that this approach markedly improved the signal-to-noise ratio of the obtained EEG responses, in particular, following olfactory stimulation. Furthermore, the approach allowed characterizing non phase-locked components that could not be identified using conventional time-domain averaging.

Conclusion/Significance

By providing a more robust and complete view of how odors are represented in the human brain, our approach could constitute the basis for a robust tool to study olfaction, both for basic research and clinicians.

Lateralisation in wine olfactory threshold detection: comparison between experts and novices

The study of olfactory lateralisation in humans has given rise to many publications, but few studies have been focused on possible differences in relation to the experience towards specific odorants. The aim of the present study was to compare unilateral detection thresholds for three wines between expert and novice judges. Additionally, irritation and hedonic valence were also evaluated using monorhinal stimulations. Results showed that the novices had lower detection thresholds with the left nostril--whatever the wine--compared to the experts. Concerning hedonic rating, no nostril difference existed in the expert group contrary to the novice group, which evaluated wines as more pleasant with the left than with the right nostril. Irritation rating appeared not to be lateralised in both groups. However, the novices rated the three wines as more irritant than the experts with the right as well as with the left nostril. These findings suggest that the level of experience induced specific differences in terms of lateralisation between wine experts and novices.

Neuronal generator patterns of olfactory event-related brain potentials in schizophrenia

To better characterize neurophysiologic processes underlying olfactory dysfunction in schizophrenia, nose-referenced 30-channel electroencephalogram was recorded from 32 patients and 35 healthy adults (18 and 18 male) during detection of hydrogen sulfide (constant-flow olfactometer, 200 ms unirhinal exposure). Event-related potentials (ERPs) were transformed to reference-free current source density (CSD) waveforms and analyzed by unrestricted Varimax-PCA. Participants indicated when they perceived a high (10 ppm) or low (50% dilution) odor concentration. Patients and controls did not differ in detection of high (23% misses) and low (43%) intensities and also had similar olfactory ERP waveforms. CSDs showed a greater bilateral frontotemporal N1 sink (305 ms) and mid-parietal P2 source (630 ms) for high than low intensities. N1 sink and P2 source were markedly reduced in patients for high intensity stimuli, providing further neurophysiological evidence of olfactory dysfunction in schizophrenia.

Responses to olfactory and intranasal trigeminal stimuli: relation to the respiratory cycle

The aim of the study was to investigate whether the perception of intranasal chemosensory stimuli changes in relation to the respiratory cycle. We investigated 40 healthy subjects with normal olfactory function who participated in four sessions. The first session was used to adapt subjects to the experimental conditions, and, specifically, to train a certain breathing technique (velopharyngeal closure) which prevents intranasal respiratory air-flow. In each of the following three sessions one of three stimulants was tested, namely phenyl ethyl alcohol (PEA), hydrogen sulfide (H(2)S), or the trigeminal stimulant carbon dioxide (CO(2)). The sequence of testing the three stimulants was randomized across all participants. Sessions were separated by at least 1 day. Chemosensory event-related potentials (ERP) were recorded in response to 80 stimuli each. Following each stimulus subjects rated its intensity using a computerized visual analogue scale. Respiration was recorded using a probe in front of the subjects' mouth. While presentation of chemosensory stimuli was performed independent of the respiratory cycle, responses were averaged off-line according to the subjects' respiratory phase when the stimuli had been presented. Intensity of olfactory or trigeminal stimuli did not differ significantly in relation to the respiratory cycle. Olfactory ERP to phenylethyl alcohol were larger when stimuli were presented during inspiration. Similarly, responses to H(2)S tended to be larger when stimuli were presented during inspiratory phases. In addition, responses to CO(2) were larger when stimuli were presented during inspiration. Differences in relation to the respiratory cycle were found specifically for early ERP components. It is important to note that the changes of chemosensory information processing were found in the absence of changes of intranasal airflow. These data indicate on an electrophysiological level that there is priming of both olfactory and trigeminally mediated sensations in relation to the respiratory cycle.

Differential effects of active attention and age on event-related potentials to visual and olfactory stimuli

Normal aging impairs olfactory functioning both centrally and peripherally. The P3 peak of the event-related potential (ERP), evoked by active response to a target stimulus, is considered a reflection of central cognitive processing. It can also be evoked in a passive task to both auditory and visual stimuli. Our goal was to investigate whether age influences amplitude and latency of the ERP differentially in active and passive tasks to olfactory stimuli. Olfactory and visual event-related potentials were elicited with a single stimulus paradigm in separate active and passive task response conditions. Participants included 30 healthy individuals from three age groups, young, middle age, and older adults. Results indicated that P3 ERP latency increased with age in both sensory modalities. P3 latencies for active versus passive tasks were similar across age groups for visual ERPs, but in the olfactory modality, older adults demonstrated significantly longer latencies in the passive task compared to the active task. Future directions should include research on specific clinical populations utilizing active versus passive task conditions.

Event-related potentials in patients with olfactory loss

CONCLUSION

The olfactory event-related potential (OERP) method provides a means of objectively assessing olfactory function. However, further validation is needed before OERPs can be routinely applied in clinical assessment of olfactory dysfunction.

OBJECTIVE

To assess OERPs in patients with olfactory impairment and in healthy controls to investigate possible dose-response effects of odor concentration on OERP parameters in these groups, thereby exploring possibilities and limitations regarding the clinical utility of the OERP method.

SUBJECTS AND METHODS

Twenty-three patients with a history of impaired olfactory function and 24 controls participated in the study. Olfactory function was assessed with psychophysical tests, i.e. assessment of the odor threshold, and odor identification. OERPs were obtained in response to the olfactory stimulant butanol at two different concentrations, presented via an olfactometer.

RESULTS

The OERP amplitudes increased and the latencies shortened with increasing stimulus concentration. Furthermore, a difference between the groups was found, with higher OERP amplitudes and shorter latencies in healthy subjects compared with patients.

Synergy and masking in odor mixtures: an electrophysiological study of orthonasal vs. retronasal perception

Perceptual interactions in a model of wine woody-fruity binary mixtures were previously reported in a psychophysical study performed through orthonasal stimulation only. However, recent studies suggested that the perception of food-like and nonfood-like odors may depend on the route of stimulation. The aim of the present study was two-fold: first to examine the neural correlates of perceptual interactions using electroencephalogram (EEG)-derived event-related potentials (ERPs) and second to test the influence of the stimulation route on quality perception. Therefore, we designed an experiment with 30 subjects to study perceptual interactions in woody-fruity mixtures and compared ortho- vs. retronasal stimulation sites on perceived odor quality and ERPs. The results revealed synergy or masking of the fruity component, depending on the woody component level. Synergy was supported by larger N1 amplitude of the ERP. Furthermore, mixtures including a medium level of the woody odor elicited a strong increase of P2 amplitude only retronasally. This study evidenced for the first time electrophysiological correlates of both perceptual synergy and masking on the early component of the ERPs and confirmed that retro- vs. orthonasal stimulation route induces different neural processes that are reflected in the late component of the ERP.

Assessment of olfactory and trigeminal function using chemosensory event-related potentials

GOALS

To give an overview on the theoretical and practical applications of chemosensory event-related potentials.

METHODS

Chemosensory event-related potentials (ERPs) may be elicited by brief and precisely defined odorous stimuli. Based on the principles of air-dilution olfactometry, a stimulator was developed in the late 1970s, which allows stimulation of the olfactory neuroepithelium and the nasal mucosa with no concomitant mechanical stimulation. Chemosensory ERPs were obtained after stimulation of the olfactory nerve (olfactory ERPs) or the trigeminal nerve (somatosensory or trigeminal ERPs). The characteristics of the stimulator for chemosensory research as well as the variables influencing the responses are discussed in this paper.

RESULTS

Implementation and normative data from our department are reported with different clinical examples from otorhinolaryngologic clinic. The bulk of the evoked response consists of a large negative component (often referred to as N1), which occurs between 320 and 450 ms after stimulus onset. This component is followed by a large positive component, often referred to as P2, occurring between 530 and 800 ms after stimulus onset. Absence of olfactory ERPs and presence (even with subtle changes) of somatosensory ERPs is a strong indicator of the presence of an olfactory dysfunction.

CONCLUSIONS

This review examines and discusses the methods of chemosensory stimulation as well as the electrophysiological correlates elicited by such stimuli. The clinical applications of chemosensory ERPs in neurology and otorhinolaryngology are outlined.

Basic and clinical aspects of olfaction

Disturbances of olfaction are a common occurrence in many neurological and neurosurgical patients and their correct diagnosis might be helpful in management and enhancement of quality of life. However, olfaction is seldom checked in most neurosurgical units and the "smell bottles" are often either absent or out of date. This chapter reviews systematically recent advances in our understanding of the anatomy, physiology (olfactory coding) and measurement of olfactory function in the human. The causes and symptoms of smell disorders, risk of damage to the olfactory system by various surgical procedures and, finally, the natural history of recovery and treatment of smell disorders, for example after trauma, are discussed.

Modulation of visual event-related potentials by emotional olfactory stimuli

The objective of the present study was to determine whether an olfactory prime could modulate behavior and visual event-related potentials (ERPs) obtained in response to a visual stimulation representing female faces. More specifically, we tested the hypothesis that a pleasant odor could have effects on face perception: behavioral effects on subjective emotional estimation of faces, and on associated response times, and electrophysiological effects on the N400 and late positive complex or LPC. Experiments were performed in which subjects had to decide whether the presented face was pleasant or not, while visual ERPs were recorded. Faces were always primed with either a pleasant odor or a neutral olfactory stimulus (pure air). In order to test the effect of subject's awareness, participants were not informed that an odor would be presented in the experimental sessions. Responses were significantly shorter for unpleasant faces. However, no behavioral effects of the pleasant odor on response time or on evaluation of face pleasantness were observed. Late ERPs evoked by faces were modulated by the presence of a pleasant odor, even when subjects were neither warned nor aware of the presence of the odor: in a frontal site and after the diffusion of the odor, the LPC (appearing 550 ms after the presentation of the visual stimulus) evoked by unpleasant faces was significantly more positive than the LPC evoked by pleasant faces. This effect could reflect an enhanced alert reaction to unpleasant faces are preceded by an (incongrous) pleasant odor.

Differential effects of the hiba odor on CNV and MMN

The aim of this study was to objectively measure some psychophysiological effects of odors, particularly the effects of odors on the contingent negative variation (CNV) and the mismatch negativity (MMN). It is generally believed that CNV reflects arousal processes and MMN reflects activity in an automatic detection system. Sixteen females were exposed to the odor of Thujopsis dolabrata (hiba), a conifer. CNV was obtained with a foreperiod of 2 s in a traditional click-flash reaction-time (RT) paradigm. Auditory MMN was measured while the subject was reading a book with SOA fixed at 500 ms. The amplitude of the early and late CNV components were significantly larger and RT to the imperative stimulus (IS) was shorter in the aroma condition than in the absence-of-odor condition, but there was no significant difference in the amplitude of MMN obtained in the two conditions. Our results indicate that the odor generates a high level of arousal within the nervous system but does not have a significant effect on automatic information processing.

Development of a continuous respiration olfactometer for odorant delivery synchronous with natural respiration during recordings of brain electrical activity

The continuous respiration olfactometer (CRO) was designed as a respiration-synchronous method for delivering odorants during recordings of brain electrical activity, providing control and monitoring of the timing of the delivery as well as the quantities of odorant involved. The CRO incorporates a purpose-built electronic system designed with very specific temporal and quantitative characteristics, and is composed of four main parts: the respiratory monitoring apparatus, the odorant/air delivery system, the serial interface device and the respiratory monitoring software. Tests were undertaken to determine the performance of the system with reference to the accuracy and precision of timing and control of odorant delivery. Tests were also undertaken to determine the effects of variations in natural respiration between subjects on the capability of the respiratory monitoring system, using a group of 50 subjects, to test the success of a variable gain control to optimize the range of the digitized respiratory output. The delivery system was able to provide information concerning quantities of air or odorant delivered, and the stimulus timing information required for integration with neurophysiological recording techniques.

Age-related changes in olfactory processing detected with olfactory event-related brain potentials using velopharyngeal closure and natural breathing

Previous olfactory event-related potential (OERP) studies often employed the Velopharyngeal Closure (VC) method, which prevents respiratory air flow in the nose during odor presentation. However, the use of VC has limited the application of OERPs to populations able to perform this artificial breathing technique. The present study investigated the effects of Natural Breathing (NB) in comparison to VC on OERP latency and amplitude in young (mean age: 24 years) and elderly (mean age: 71 years) adults. OERPs were recorded from three midline scalp electrodes (Fz, Cz, Pz) for 15 trials in each breathing condition with an interstimulus interval of 3.5 min, using amyl-acetate as stimulus. Subjects were asked to report perceived stimulus intensity. A thermistor placed inside one nostril monitored nasal respiration and performance of VC. In the NB condition, subjects were instructed to breathe normally through mouth and nose, while stimulus presentation occurred during inspiration. In both breathing conditions, elderly subjects showed significantly smaller N1-P2 and N1-P3 interpeak amplitudes and longer latencies for N1, P2, P3 than younger subjects. VC generated significantly larger N1-P2 amplitudes across all electrode sites, whereas Natural Breathing produced a trend towards shorter P3 latencies. No significant interaction was found between age and breathing technique. The present investigation showed that the OERP is a sensitive measure for detecting age-related changes in olfactory function regardless of breathing technique.

Complexity of olfactory lateralization processes revealed by functional imaging: a review

Currently available techniques used in neurosciences and particularly cerebral imaging are contributing to a better understanding of human perception and the treatment of sensorial information. In this field, the chemical senses (taste and smell) have received little attention when compared with the auditory, visual and tactile senses. Nevertheless, recent research has been trying to overcome this. Brain regions involved in the processing of olfactory information have been investigated in several studies including a large number dealing with the question of lateralization. However, functional asymmetry in olfaction has still not been resolved and the resulting data have not been homogeneous. In this field, the contribution of cerebral imaging studies is very important insofar as it shows that the processes of functional lateralization in olfaction depend on many factors (nature of stimulus, nature of task, characteristics of subjects ...) that future research will have to consider.

Event-related brain potentials to attended and ignored olfactory and trigeminal stimuli

Event-related brain potentials (ERPs) were recorded from 26 young adults, with equal numbers of male and female subjects, using attended and ignored, olfactory and trigeminal stimuli. The amplitudes and latencies of the N1, P2, and P3 components were recorded using a single-stimulus paradigm, with an inter-stimulus interval of 60 s, employing amyl acetate as the olfactory stimulus and ammonia as the trigeminal stimulus. Subjects estimated stimulus intensity in the attend condition or continued with a visual tracking task in the ignore condition. Results indicate that olfactory information is processed 30-70 ms faster than trigeminal information for the N1 and P2 potential and 100 ms faster for the P3 ERP component. N1/P2 interpeak amplitude was greater for the trigeminal than the olfactory stimuli, and greater in the attended than ignored condition. P3 amplitude was greater in the attend than ignore condition for olfactory information processing and equivalent for trigeminal information processing. These findings suggest that neuronal resource allocation is greatest for attended stimuli and that a painful stimulus demands neuronal resources even when ignored.

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.

Is odor processing related to oral breathing?

This paper addresses two questions related to the inherent association between breathing and odor perception: Does central nervous processing of odors change when an artificial breathing technique (velopharyngeal closure) is introduced and secondly, does odor processing vary with the oral breathing phase (inhalation or exhalation)? Chemosensory event-related potentials (CSERP) were obtained from eight female subjects while they were smelling an odor mixture (citral, eugenol, linalool, menthol and isoamylacetate). Each subject was required to perform spontaneous mouth breathing (120 trials) as well as the velopharyngeal closure technique (120 trials). Simultaneously, a thermistor monitored the phase of the respiratory cycle. The results reveal that the central nervous correlates of odor processing change with the breathing technique but not with the oral breathing cycle. The findings that early stimulus processing is faster (N1 latency) and late stimulus processing more pronounced (P3 amplitudes) when the subjects are breathing spontaneously are discussed with regard to attentional effects. The reduction of the N1 amplitude during the spontaneous breathing condition may be caused by larger latency variations and longer stimulus rise-times. Furthermore, it is concluded that the oral breathing cycle is less important than the nasal breathing cycle for olfactory information transmission.

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.

Traumatic brain injury assessed with olfactory event-related brain potentials

Olfactory event-related potentials (OERPs) were evaluated to develop an objective, quantitative assessment of sensory and cognitive olfactory loss following traumatic brain injury (TBI). Subjects included 25 TBI patients and 25 age/gender-matched healthy controls. Following standard clinical evaluation of smell function, TBI patients were divided into three groups: 12 anosmics (loss of smell), 6 hyposmics (reduced smell), and 7 normosmics (normal smell). Cognitive ability was assessed using the Trail Making Test (A and B). OERPs were recorded monopolarly from midline electrode sites using an amyl acetate stimulus with a 60-second interstimulus interval; subjects estimated the magnitude of each odor stimulus. Anosmic TBI patients were also tested with OERPs using ammonia to ensure trigeminal nerve function. Amyl acetate OERPs demonstrated that the sensory N1 and P2 amplitudes and the cognitive P3 amplitudes were absent in the anosmic TBI patients and greatly reduced in the hyposmic and normosmic TBI patients compared to healthy controls. The trigeminal OERPs from the anosmic TBI patients were within normal limits, indicating that the primary olfactory deficits were objectively measured with OERPs. The relationship between the OERPs and neuropsychologic test performance supports the cognitive loss associated with TBI. The present study lends support to the utility of OERPs as an objective tool for measuring sensory and cognitive loss after traumatic brain injury.

Olfactory and intranasal trigeminal event-related potentials in anosmic patients

The aim of this study was to investigate the usefulness of chemosensory event-related potentials (CSERPs) in response to both olfactory and intranasal trigeminal stimulation in the diagnosis of anosmia. Forty-four patients participated. Gaseous CO2 was used for trigeminal stimulation, vanillin and H2S were used as olfactory stimulants. Event-related potentials to olfactory stimuli could not be obtained in any of the anosmic patients, indicating the complete loss of the sense of smell. However, all patients responded to stimulation of the trigeminal nerve with CO2. These data clearly demonstrate the clinical significance of CSERPs in the assessment of anosmia.

Chemosensory event-related potentials change with age

The study examined age-related changes in the perception of olfactory and trigeminal chemical stimuli using chemosensory event-related potentials (CSERP). Three groups of healthy volunteers, each comprised of 8 men and 8 women, were tested (age ranges 15-34, 35-54, and 55-74 years). Subjects underwent extensive psychological testing focusing on impairments of memory and attention. In addition, odor identification and discrimination ability was evaluated, as well as detection threshold sensitivity for two odorants. Odor discrimination scores exhibited a significant age-related decrease. Significant age-related changes were also observed for CSERP N1P2 and P2 amplitudes, and for the N1 peak latency. The age-related decrease of CSERP amplitudes appeared to follow a different time course for responses to trigeminal and olfactory stimulants.