Functional imaging of the human olfactory cortex by magnetic resonance imaging

Olfactory deficit: a potential functional marker across the Alzheimer's disease continuum

Alzheimer's disease (AD) is a prevalent form of dementia that affects an estimated 32 million individuals globally. Identifying early indicators is vital for screening at-risk populations and implementing timely interventions. At present, there is an urgent need for early and sensitive biomarkers to screen individuals at risk of AD. Among all sensory biomarkers, olfaction is currently one of the most promising indicators for AD. Olfactory dysfunction signifies a decline in the ability to detect, identify, or remember odors. Within the spectrum of AD, impairment in olfactory identification precedes detectable cognitive impairments, including mild cognitive impairment (MCI) and even the stage of subjective cognitive decline (SCD), by several years. Olfactory impairment is closely linked to the clinical symptoms and neuropathological biomarkers of AD, accompanied by significant structural and functional abnormalities in the brain. Olfactory behavior examination can subjectively evaluate the abilities of olfactory identification, threshold, and discrimination. Olfactory functional magnetic resonance imaging (fMRI) can provide a relatively objective assessment of olfactory capabilities, with the potential to become a promising tool for exploring the neural mechanisms of olfactory damage in AD. Here, we provide a timely review of recent literature on the characteristics, neuropathology, and examination of olfactory dysfunction in the AD continuum. We focus on the early changes in olfactory indicators detected by behavioral and fMRI assessments and discuss the potential of these techniques in MCI and preclinical AD. Despite the challenges and limitations of existing research, olfactory dysfunction has demonstrated its value in assessing neurodegenerative diseases and may serve as an early indicator of AD in the future.

Brain Perfusion Single Photon Emission Computed Tomography Findings in Patients with Posttraumatic Anosmia and Comparison with Radiological Imaging

Background

Different techniques in neuroimaging have been proposed for assessment of olfactory dysfunction but they are not without limitations. Recently, some studies showed the usefulness of single photon emission–computed tomography (SPECT) in evaluation of patients with posttraumatic anosmia. This study was designed to assess the possible diagnostic value of SPECT findings in patients with posttraumatic anosmia in comparison with magnetic resonance imaging (MRI)/CT imaging.

Methods

Sixteen patients who had head trauma and consequently anosmia, which was defined according to Cain's identification test, were included in this study. Two nonanosmic groups, traumatic patients and nontraumatic healthy individuals, were selected as control groups for this study. Qualitative and semiquantitative brain perfusion SPECT was performed by measuring the uptake ratio of the orbital frontal cortex to occipital pole in the sagittal projections (uptake index). All 16 target patients had a previous CT scan and/or MRI. Semiquantitative and qualitative brain perfusion SPECT were compared with radiological imaging.

Results

Semiquantitative assessment of brain perfusion SPECT revealed remarkable orbital frontal hypoperfusion as compared with two control groups. 87.5% of anosmic patients showed orbital frontal hypoperfusion (≥2 SD below the lowest level in healthy controls). In addition, the semiquantitative SPECT method detected more orbitofrontal abnormality than the qualitative method or radiological imaging (MRI and/or CT).

Conclusion

Findings suggest that by using SPECT, posttraumatic anosmia corresponds to the hypoperfusion in the orbital frontal cortex in a great number of patients. Because of its availability, rather low cost, technical ease, and possibility to obtain objective quantitative information, brain perfusion SPECT can be complementary to other diagnostic techniques in the evaluation of olfactory function, although additional neurophysiological and imaging studies are needed.

Human amygdala activations during nasal chemoreception

This review serves as a comprehensive discussion of chemosensory stimulation of the amygdala in healthy humans. Following an introduction of the neuroanatomy of chemosensory processing in primary and secondary olfactory structures, functional resonance magnetic imaging and positron imaging tomography studies are systematically categorized based on valence of stimuli, stimulus concentration, and paradigm-dependent amygdala activation. The amygdala shows patterns of lateralization due to stimulus valence. Main findings include pleasant odors being associated with bilateral or left amygdala activation, and unpleasant odors being associated with activation of the right amygdala, suggesting a crucial role of the right amygdala in evolutionary preservation. Potentially threatening social stimuli, however, might be processed apart from the olfactory system and tend to activate the left amygdala. Amygdala response to chemosensory stimuli correlated with simultaneous activation in the orbitofrontal cortex (OFC), piriform cortex (PC), and insula, suggesting a close-knit network of these areas during stimulus processing.

Evaluation of olfactory dysfunction in neurodegenerative diseases

It is known that the olfactory dysfunction is involved in various neurological diseases, such as Parkinson's disease, Alzheimer's disease, multiple sclerosis, Huntington's disease and motor neuron disease. In particular, the ability to identify and discriminate the odors, as well as the odor threshold, can be altered in these disorders. These changes often occur as early manifestation of the pathology and they are not always diagnosed on time. The aim of this review is to summarize the major neurological diseases which are preceded or accompanied by olfactory dysfunction. In addition, new instrumental approaches, such as psychophysical testing, olfactory event-related potentials (OERPs) and functional magnetic resonance imaging (fMRI) measurements, supported by olfactometer for the stimuli delivery, and their combination in evaluation of olfactory function will be discussed. In particular, OERPs and fMRI might to be good candidates to become useful additional tools in clinical protocols for early diagnosis of neurological diseases.

The human brain representation of odor identification

Odor identification (OI) tests are increasingly used clinically as biomarkers for Alzheimer's disease and schizophrenia. The aim of this study was to directly compare the neuronal correlates to identified odors vs. nonidentified odors. Seventeen females with normal olfactory function underwent a functional magnetic resonance imaging (fMRI) experiment with postscanning assessment of spontaneous uncued OI. An event-related analysis was performed to compare within-subject activity to spontaneously identified vs. nonidentified odors at the whole brain level, and in anatomic and functional regions of interest (ROIs) in the medial temporal lobe (MTL). Parameter estimate values and blood oxygenated level-dependent (BOLD) signal curves for correctly identified and nonidentified odors were derived from functional ROIs in hippocampus, entorhinal, piriform, and orbitofrontal cortices. Number of activated voxels and max parameter estimate values were obtained from anatomic ROIs in the hippocampus and the entorhinal cortex. At the whole brain level the correct OI gave rise to increased activity in the left entorhinal cortex and secondary olfactory structures, including the orbitofrontal cortex. Increased activation was also observed in fusiform, primary visual, and auditory cortices, inferior frontal plus inferior temporal gyri. The anatomic MTL ROI analysis showed increased activation in the left entorhinal cortex, right hippocampus, and posterior parahippocampal gyri in correct OI. In the entorhinal cortex and hippocampus the BOLD signal increased specifically in response to identified odors and decreased for nonidentified odors. In orbitofrontal and piriform cortices both identified and nonidentified odors gave rise to an increased BOLD signal, but the response to identified odors was significantly greater than that for nonidentified odors. These results support a specific role for entorhinal cortex and hippocampus in OI, whereas piriform and orbitofrontal cortices are active in both smelling and OI. Moreover, episodic as well as semantic memory systems appeared to support OI.

Near-infrared spectroscopy of orbitofrontal cortex during odorant stimulation

BACKGROUND

For olfaction, several studies have reported near-infrared spectroscopy (NIRS) signal changes in the orbitofrontal cortex (OFC) during odor stimulation. However, the roles of human OFC in olfactory cognition are less well understood. This study was designed to better understand the roles of OFC for olfaction.

METHODS

Hemodynamic responses for phenyl ethyl alcohol or citral in the OFCs were measured with NIRS. After the experiment, participants were asked to describe the characteristics of the odor and to rate odor intensity and hedonic valence.

RESULTS

Statistical analysis of all participants' data showed significant changes in the concentration of total hemoglobin in the left OFC during the trial (p = 0.04). The total hemoglobin signal increased significantly in the right OFC (p = 0.0008) of the participants who successfully identified the odorant stimulus.

CONCLUSION

Our findings showed that NIRS combined with a questionnaire is a useful method for studying the functional neuroanatomy of OFC in terms of olfaction.

Mapping brain activity induced by olfaction of virgin olive oil aroma

The difficulty of explaining sensory descriptors of virgin olive oil aroma by the analysis of volatile compounds is partially due to the subjective opinions of panelists and the lack of information of the neural mechanisms that ultimately produce a sensory perception. In this study the technique of functional magnetic resonance imaging (fMRI) has been applied to study brain activity during the smelling of virgin olive oil of different qualities. The volatile compounds of the samples were analyzed by solid-phase microextraction gas chromatography to explain the differences in the aromas presented to the subjects during the fMRI experiments. Comparing the pleasant and unpleasant aromas, the most evident differences in brain activity were found at the anterior cingulate gyrus (Brodmann area 32) and at the temporal lobe (Brodmann area 38). The activations were also observed when subjects smelled dilutions of heptanal and hexanoic acid, both compounds being responsible for off-flavors. Other areas were inherent to the olfaction task (e.g., Brodmann area 10) and to the intensity of the aroma (Brodmann area 6).

Functional optical hemodynamic imaging of the olfactory cortex in normosmia subjects and dysosmia subjects

CONCLUSION

These activated areas may be related to the orbitofrontal cortex, corresponding to olfactory cortices. This study shows that multichannel near-infrared spectroscopy (NIRS) enables the evaluation of brain activity of normosmia subjects and dysosmia subjects by olfactory stimulation.

OBJECTIVE

Objective olfactory testing is not common. NIRS has beenused before to study functional activations in various areas of the brain, but we wanted to investigate the difference in brain olfactory activity in normosmia subjects and dysosmia subjects using multi-channel NIRS (MNIRS).

SUBJECTS AND METHODS

This study was conducted on eight normosmia subjects and five dysosmia subjects. We employed a 22-channel near-infrared spectroscopy device with eight light incident fibers and seven light detector fibers, each with an inter-optode distance of 2.5 cm on the frontal head. Isovaleric acid was used as the odor stimulant. We measured the change in oxyhemoglobin concentrations [oxyHb], deoxyhemoglobin concentrations [deoxyHb], and total hemoglobin concentrations [totalHb] from pre-baseline values. Furthermore, we divided the frontal cortex into four areas (right upper, left upper, right lower, left lower) and measured the activity in each area. Then, the changes in [oxyHb], [deoxyHb], and [totalHb] of normosmia subjects and dysosmia subjects were compared in each area.

RESULTS

In all normosmia subjects, isovaleric acid caused remarkable changes, especially in the lower areas of the frontal cortex. However, in all dysosmia subjects, isovaleric acid caused no changes.

Functional MRI of regional brain responses to 'pleasant' and 'unpleasant' odors

CONCLUSIONS

Our findings demonstrate that functional MRI (fMRI) combined with a questionnaire is a useful method for studying the neuroanatomy of olfaction. Further studies with various odorants and questionnaires would provide an even better understanding of the mechanism of olfactory perception.

OBJECTIVES

To better understand the mechanism of odorant perception in the central nervous system.

SUBJECTS AND METHODS

fMRI was used to identify the activated regions during stimulation by two odorants, beta-phenyl ethyl alcohol and gamma-undecalactone. Participants were asked to describe the quality of the odor and to rate odor intensity and odor hedonic valence. Activation at each region was statistically analyzed according to the answers.

RESULTS

The bilateral middle orbitofrontal cortex (OFC), left lateral OFC, right insula, and bilateral anterior/middle cingulate gyri were most frequently activated by odor stimulation. Left middle OFC was significantly more often activated in the participants who could not identify the odor correctly (p = 0.016). The left middle OFC and right lateral OFC were significantly more often activated in the participants who perceived the odor stimulation as unpleasant (p = 0.03), while the right anterior cingulate gyrus was more often activated in those who perceived the odor as pleasant (p = 0.03).

Thallium Transport and the Evaluation of Olfactory Nerve Connectivity between the Nasal Cavity and Olfactory Bulb

Little is known regarding how alkali metal ions are transported in the olfactory nerve following their intranasal administration. In this study, we show that an alkali metal ion, thallium is transported in the olfactory nerve fibers to the olfactory bulb in mice. The olfactory nerve fibers of mice were transected on both sides of the body under anesthesia. A double tracer solution (thallium-201, (201)Tl; manganese-54, (54)Mn) was administered into the nasal cavity the following day. Radioactivity in the olfactory bulb and nasal turbinate was analyzed with gamma spectrometry. Auto radiographic images were obtained from coronal slices of frozen heads of mice administered with (201)Tl or (54)Mn. The transection of the olfactory nerve fibers was confirmed with a neuronal tracer. The transport of intranasal administered (201)Tl/(54)Mn to the olfactory bulb was significantly reduced by the transection of olfactory nerve fibers. The olfactory nerve transection also significantly inhibited the accumulation of fluoro-ruby in the olfactory bulb. Findings indicate that thallium is transported by the olfactory nerve fibers to the olfactory bulb in mice. The assessment of thallium transport following head injury may provide a new diagnostic method for the evaluation of olfactory nerve injury.

Functional Optical Hemodynamic Imaging of the Olfactory Cortex

OBJECTIVE

We used multichannel near-infrared spectroscopy (MNIRS) to monitor the activity of the frontal cortex as mirrored by hemodynamic responses subjected to olfactory stimulation. The aim of this study was to clarify the functional brain imaging of olfactory activity.

STUDY DESIGN

Prospective study.

METHODS

This study was conducted on eight healthy subjects aged from 22 to 39 years (4 men and 4 women; mean age, 28.8 yr). We used a 22-channel near-infrared spectroscopy device with eight light-incident fibers and seven detector fibers, each with an interoptode distance of 2.5 cm on the frontal region. Olfactory stimulation consisted of five repetitions, each lasting 5 seconds and followed by a 55 seconds rest period. Isovaleric acid was used as odor stimulation, and saline was used as a control. We measured the changes in concentrations of oxyhemoglobin [oxyHb], deoxyhemoglobin [deoxyHb], and total hemoglobin [totalHb] from prebaseline values. Furthermore, we divided the frontal cortex into four areas (right upper, left upper, right lower, left lower) and investigated the activity in each area.

RESULTS

Isovaleric acid caused changes, especially in the lower area, but saline caused no changes. [oxyHb] and [totalHb] increased after odor stimulation, but [deoxyHb] did not change. These active areas may be related to the orbitofrontal cortex, corresponding to olfactory cortices.

CONCLUSION

This study has shown that MNIRS enables evaluation of changes in hemodynamics related to brain activity by olfactory stimulation.

Validation and optimization of statistical approaches for modeling odorant-induced fMRI signal changes in olfactory-related brain areas

Recent neuroimaging studies have converged to show that odorant-induced responses to prolonged stimulation in primary olfactory cortex (POC) are characterized by a rapidly habituating time course. Different statistical approaches have effectively modeled this time course. One approach explicitly modeled rapid habituation using an exponentially decaying reference waveform that decreased to baseline levels within 30 to 40 s. A second approach modeled an early transient response by simply shortening the odorant 'ON' period to be less than the actual stimulation period (i.e., 9 of 40 s). The goal of the current study was to validate, compare, and optimize these methodological approaches by applying them to an olfactory fMRI block-design dataset from 10 healthy young subjects presented with odorants for 12 s (ON), alternating with 30 s of clear air (OFF). Both approaches significantly improved sensitivity to odorant-induced signal changes in POC relative to a square-wave model based on the actual stimulation period. Our findings further demonstrate that the 'optimal' model fit to the data was achieved by shortening the odorant 'ON' period to approximately 6 s. These results suggest that sensitivity to odorant-induced POC activity in block-design experiments can be optimized by modeling an early phasic response followed by a precipitous rather than specific exponential decrease to baseline levels. Notably, whole brain voxel-wise analyses further established that modeling rapid habituation in this way is not only sensitive, but also highly specific to odorant-induced activation in a well-established network of olfactory-related brain areas.

Improved methods for fMRI studies of combined taste and aroma stimuli

Previous neuroimaging studies of the cortical representation of gustatory and olfactory stimuli have often delivered tastants to the mouth in very small quantities or stimulated olfaction orthonasally. In studies of retro-nasal olfaction, swallowing was generally delayed to reduce head motion artefacts. The present fMRI study aims to improve upon such methodological limitations to allow investigation of the cortical representation of flavour (taste and aroma combination) as it typically occurs during the consumption of liquid foods. For this purpose we used (1) a novel, automated, sprayed stimulus delivery system and a larger volume of liquid sample (containing sweet tastants and banana/pear aroma volatiles) to achieve more extensive stimulation of the oral cavity taste receptors, (2) a pseudo-natural delivery paradigm that included prompt swallowing after each sample delivery to obtain physiological retro-nasal olfactory stimulation, (3) fMRI acquisition with wide brain coverage and double-echo EPI to improve sensitivity. We validated our paradigm for the delivery of volatiles using atmospheric pressure chemical ionisation mass spectrometry. This showed that the main retro-nasal delivery of volatiles in the paradigm occurs immediately after the swallow. Several brain areas were found to be activated, including the insula, frontal operculum, rolandic operculum/parietal lobe, piriform, dorsolateral prefrontal cortex, anterior cingulate cortex, ventro-medial thalamus, hippocampus and medial orbitofrontal cortex.

Cognitive impairment and olfactory panic from occupational exposure to VOCs

BACKGROUND

A Canadian government clerical worker in her early thirties developed frontal lobe dysfunction from inhalation of volatile organic compounds off-gassed during an office renovation.

METHODS

Pulmonary function, bronchial provocation, allergy testing, and a brain (SPECT) scan were performed.

RESULTS

SPECT scanning showed frontotemporal hypoperfusion and neuropsychologic testing revealed deficits in verbal learning and poor organizational memory.

CONCLUSIONS

A significant component of this worker's impairment was the development of "olfactory panic," a debilitating aversion to odor accompanied by symptoms of panic. The Ontario Workplace Safety and Insurance Appeals Tribunal granted entitlement for her cognitive difficulties and olfactory panic as a result of her toxic exposure.

Neuromagnetic Changes of Brain Rhythm Evoked by Intravenous Olfactory Stimulation in Humans

To identify the changes in the respective frequency band and brain areas related to olfactory perception, we measured magnetoencephalographic (MEG) signals before and after instilling intravenously thiamine propyl disulfide (TPD) and thiamine tetrahydrofurfuryl disulfide monohydrochloride (TTFD), which evoked a strong and weak sensation of odor, respectively. For the frequency analysis of MEG, a beamformer program, synthetic aperture magnetometry (SAM), was employed and event-related desynchronization (ERD) or synchronization (ERS) was statistically determined. Both strong and weak odors induced ERD in (1) beta band (13-30 Hz) in the right precentral gyrus, and the superior and middle frontal gyri in both hemispheres, (2) low gamma band (30-60 Hz) in the left superior frontal gyrus and superior parietal lobule, and the middle frontal gyrus in both hemispheres, and (3) high gamma band 2 (100-200 Hz) in the right inferior frontal gyrus. TPD induced ERD in the left temporal, parietal and occipital lobes, while TTFD induced ERD in the right temporal, parietal and occipital lobes. The results indicate that physiological functions in several regions in the frontal lobe may change and the strength of the odor may play a different role in each hemisphere during olfactory perception in humans.

Near-infrared spectroscopy of the adult human olfactory cortex

OBJECTIVE

Near-infrared spectroscopy (NIRS) is a non-invasive method for investigating activation of the human cortex. The applicability of NIRS to the olfactory cortex was investigated.

MATERIAL AND METHODS

The relative oxy- and deoxy-hemoglobin levels of the orbito-frontal cortex during olfactory stimulation in healthy subjects were measured using NIRS.

RESULTS

When perfumed strips containing the odorants beta-phenyl ethyl alcohol, iso-valeric acid and gamma-undecalactone were presented, the oxy-hemoglobin level increased but the deoxy-hemoglobin level did not change. The increase in the oxy-hemoglobin level was observed bilaterally. A placebo perfumed strip did not elicit a change in the hemoglobin level. It was also observed that the odorant intensity affected the oxy-hemoglobin level. Although the orbito-frontal cortices seemed to be activated bilaterally during olfaction, the right cortex was activated to a greater extent than the left.

CONCLUSION

NIRS appears to be an adequate method for investigating the human olfactory cortex.

Electrically stimulated olfactory evoked potential in olfactory disturbance

Olfactory evoked potential is considered a useful method of electrophysiological olfactometry for the diagnosis of olfactory disturbance. However, electrophysiological olfactometry is not as widely used as electrophysiological audiometry, such as the auditory brain stem response, because odor stimulation is difficult to perform. In contrast, electrical pulse stimulation is easy to perform, and its evoked potential is also easily recorded by the averaging method. We recorded olfactory evoked potentials from the scalp produced by electrical stimulation on the olfactory mucosa and investigated the relationship between this electrical olfactory evoked potential (EOEP) and the results of Toyoda and Takagi's perfumist's strip method (T&T) olfactometry, which is a standard Japanese means of psychophysical olfactometry. In EOEP-detectable cases, the detection and cognitive thresholds of T&T olfactometry were 1.32+/-1.99 (mean +/- SD; n = 10) and 3.02+/-1.64 (n = 10), respectively. But in the undetectable cases, the thresholds were 4.67+/-2.03 (n = 8; 5.8 means off the scale) and 5.80+/-0.00 (n = 8), respectively. The differences between the T&T thresholds of EOEP-detectable and -undetectable cases were significant. We concluded that EOEP is suitable for electrophysiological olfactometry.

Is Imaging Necessary in the Evaluation of the Patient with an Isolated Complaint of Anosmia?

Imaging studies are often obtained during the evaluation of the patient with an olfactory deficit. The purpose of this article is to describe an investigation that was conducted to assess the effectiveness of imaging studies in determining the etiology of olfactory loss in the patient with an isolated complaint of anosmia and no relevant findings on physical examination, including nasal endoscopy. The author conducted a retrospective review of the records of 20 women and eight men, aged 22 to 71 years (mean: 45), whose duration of anosmia had ranged from 1 month to 2 years (median: 3 mo). Twenty of these patients had undergone contrast-enhanced magnetic resonance imaging of the paranasal sinuses and brain, and the other eight had undergone enhanced computed tomography. The author found that these imaging studies did not add to the information obtained by the clinical history and endoscopic nasal examination and were thus not useful in determining the cause of anosmia. Based on these findings, the author does not recommend that imaging studies be included as part of the evaluation of these patients.

Imaging of the olfactory system

The olfactory system consists of the primary olfactory nerves in the nasal cavity, the olfactory bulbs and tracts, and numerous intracranial connections and pathways. Diseases affecting the sense of smell can be located both extracranially and intracranially. Many sinonasal inflammatory and neoplastic processes may affect olfaction. Intracranially congenital, traumatic, and neurodegenerative disorders are usually to blame for olfactory dysfunction. The breadth of diseases that affect the sense of smell is astounding, yet the imaging ramifications have barely been explored.

Olfaction

The main and accessory olfactory systems have received considerable attention on the part of scientists and clinicians during the last decade, largely because of (a) quantum advances in understanding their genetically expressed receptor mechanisms, (b) evidence that their receptor cells undergo neurogenesis and both programmed and induced cell death, and (c) important technical and practical developments in psychophysical measurement. The latter developments have led to the proliferation of standardized olfactory testing in laboratories and clinics, and to the discovery that smell loss is among the first signs of a number of neurodegenerative diseases, including Alzheimer's disease and idiopathic Parkinson's disease. Recent controversial claims that humans possess a functioning vomeronasal system responsive to "pheromones" has added further interest in intranasal chemoreception. This review focuses on recent progress made in understanding olfactory function, emphasizing transduction, measurement, and clinical findings.

Lateralization of brain activation to imagination and smell of odors using functional magnetic resonance imaging (fMRI): left hemispheric localization of pleasant and right hemispheric localization of unpleasant odors

PURPOSE

Our goal was to use functional MRI (fMRI) of brain to reveal activation in each cerebral hemisphere in response to imagination and smell of odors.

METHOD

FMRI brain scans were obtained in 24 normal subjects using multislice fast low angle shot (FLASH) MRI in response to imagination of banana and peppermint odors and in response to smell of corresponding odors of amyl acetate and menthone, respectively, and of pyridine. Three coronal sections selected from anterior to posterior brain regions were used. Similar studies were obtained in two patients with hyposmia using FLASH MRI and in one patient with hyposmia using echo planar imaging (EPI) both before and after theophylline treatment that returned smell function to or toward normal in each patient and in two patients with birhinal phantosmia (persistent foul odor) and global phantogeusia (persistent foul taste) with FLASH and EPI fMRI before and after treatment with neuroleptic drugs that inhibited their phantosmia and phantogeusia. Activation images were derived using correlation analysis. Ratios of hemispheric areas of brain activation to total hemispheric brain areas were calculated for FLASH fMRI, and numerical counts of pixel clusters in each hemisphere were made for EPI studies. Total pixel cluster counts in localized regions of each hemispheric section were also obtained.

RESULTS

In normal subjects, activation generally occurred in left (L) > right (R) brain hemisphere in response to banana and peppermint odor imagination and to smell of corresponding odors of amyl acetate and menthone. Whereas there were no overall hemispheric differences for pyridine odor, activation in men was R > L hemisphere. Although absolute activation in both L and R hemispheres in response to banana odor imagination and amyl acetate smell was men > women, the ratio of L to R activation was women > men. In hyposmic patients studied by FLASH fMRI, activation to banana odor imagination and amyl acetate smell was L > R hemisphere both before and after theophylline treatment. In the hyposmic patient studied with EPI before theophylline treatment, activation to banana and peppermint odor imagination and to amyl acetate, menthone, and pyridine smell was R > L hemisphere; after theophylline treatment restored normal smell function, activation shifted completely with banana and peppermint odor imagination and amyl acetate and menthone smell to L > R hemisphere, consistent with responses in normal subjects. However, this shift also occurred for pyridine smell, which is opposite to responses in normal control subjects. In patients with phantosmia and phantogeusia, activation to phantosmia and phantogeusia before treatment was R > L hemisphere; after treatment inhibited phantosmia and phantogeusia, activation shifted with a slight L > R hemispheric lateralization. Localization of all lateralized responses indicated that anterior frontal and temporal cortices were brain regions most involved with imagination and smell of odors and with phantosmia and phantogeusia presence.

CONCLUSION

Imagination and smell of odors perceived as pleasant generally activated the dominant or L > R brain hemisphere. Smell of odors perceived as unpleasant and unpleasant phantosmia and phantogeusia generally activated the contralateral or R > L brain hemisphere. With remission of phantosmia and phantogeusia, hemispheric activation was not only inhibited, but also there was a slight shift to L > R hemispheric predominance. Predominant L > R hemispheric differences in brain activation in normal subjects occurred in the order amyl acetate > menthone > pyridine, consistent with the hypothesis that pleasant odors are more appreciated in L hemisphere and unpleasant odors more in R hemisphere. Anterior frontal and temporal cortex regions previously found activated by imagination and smell of odors and phantosmia and phantogeusia perception accounted for most hemispheric differences.

Olfactory-evoked regional cerebral blood flow in Alzheimer's disease

Olfaction is impaired in Alzheimer's disease (AD). It was hypothesized that AD would reduce olfactory-evoked perfusion in mesial temporal olfactory (piriform) cortex, where neuropathology begins. Seven AD patients and 8 elderly controls (ECs) underwent olfactory threshold and identification tests and olfactory stimulation during positron emission tomography. Odor identification was impaired in AD, but threshold was not. Olfactory stimulation in ECs activated right and left piriform areas and right anterior ventral temporal cortex. AD patients had less activation in right piriform and anterior ventral temporal cortex but not in the left piriform area. Although orbital cortex did not activate in ECs, there was a significant between-groups difference in this area. Right piriform activation correlated with odor identification. Impaired odor identification likely reflects sensory cortex dysfunction rather than cognitive impairment. Given olfactory bulb projections to the mesial temporal lobe, olfactory stimulation during functional imaging might detect early dysfunction in this region.

Olfactory function evaluated by SPECT

Few articles on neuroimaging techniques in the study of central and peripheral olfactory pathways are present in the literature. By Single Photon Emission Computed Tomography (SPECT), cortical perfusion increment after sensorial stimulation can be evaluated objectively. In the present research, 10 healthy adults underwent SPECT by CER.TO.96 cerebral tomograph, before and after olfactory stimulation with lavender-water. A variable degree of cortical activation was detected in all patients. Gyrus rectus (+24.5%), orbito-frontal cortex (right +26.6%, left +25.6%), and superior temporal (right +9.9%, left +5.5%) cortical areas were always activated. A slight perfusion increase was present in middle temporal (right +3.2%, left +2.1%) and parieto-occipital (right +0.4%, left +2%) regions. Five patients affected by posttraumatic anosmia were also investigated: they showed a perfusion increment markedly inferior to 0.5% in every olfactory area. SPECT is a rather diffused, easily performed technique which yields objective semi-quantitative information on brain perfusion. Hence, it can be regarded as a promising contribution in the fields of smell neurophysiology, clinical olfactometry, and medicolegal queries.

Time course of odorant-induced activation in the human primary olfactory cortex

Paradoxically, attempts to visualize odorant-induced functional magnetic resonance imaging (fMRI) activation in the human have yielded activations in secondary olfactory regions but not in the primary olfactory cortex-piriform cortex. We show that odorant-induced activation in primary olfactory cortex was not previously made evident with fMRI because of the unique time course of activity in this region: in primary olfactory cortex, odorants induced a strong early transient increase in signal amplitude that then habituated within 30-40 s of odorant presence. This time course of activation seen here in the primary olfactory cortex of the human is almost identical to that recorded electrophysiologically in the piriform cortex of the rat. Mapping activation with analyses that are sensitive to both this transient increase in signal amplitude, and temporal-variance, enabled us to use fMRI to consistently visualize odorant-induced activation in the human primary olfactory cortex. The combination of continued accurate odorant detection at the behavioral level despite primary olfactory cortex habituation at the physiological level suggests that the functional neuroanatomy of the olfactory response may change throughout prolonged olfactory stimulation.

Reversible hyposmia caused by intracranial tumour

Two patients with hyposmia caused by an intracranial tumour recovered olfactory functions after craniotomy. The first case was a 68-year-old male with a tumour metastasized from the lung to the right frontal lobe. The second case was a 75-year-old male with meningioma of the right frontal lobe. Results of T & T olfactometry and venous olfaction tests also indicated suspected central hyposmia. Magnetic resonance imaging (MRI) indicated compression of the frontal lobe by intracranial tumour. Pressure on the olfactory centre located in the frontal lobe produced hyposmia. Decompression of the frontal lobe by craniotomy improved the sense of smell. Therefore, some cases of olfactory disturbance caused by intracranial tumour may be reversible if they are the result of simple compression of the olfactory centre.

Cerebral cortical blood flow maps are reorganized in MAOB-deficient mice

Cerebral cortical blood flow (CBF) was measured autoradiographically in conscious mice without the monoamine oxidase B (MAOB) gene (KO, n=11) and the corresponding wild-type animals (WILD, n=11). Subgroups of animals of each genotype received a continuous intravenous infusion over 30 min of phenylethylamine (PEA), an endogenous substrate of MAOB, (8 nmol g-1 min-1 in normal saline at a volume rate of 0.11 microl g-1 min-1) or saline at the same volume rate. Maps of relative CBF distribution showed predominance of midline motor and sensory area CBF in KO mice over WILD mice that received saline. PEA enhanced CBF in lateral frontal and piriform cortex in both KO and WILD mice. These changes may reflect a differential activation due to chronic and acute PEA elevations on motor and olfactory function, as well as on the anxiogenic effects of this amine. In addition to its effects on regional CBF distribution, PEA decreased CBF globally in KO mice (range -31% to -41% decrease from control levels) with a lesser effect in WILD mice. It is concluded that MAOB may normally regulate CBF distribution and its response to blood PEA.

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.

Gender effects on odor-stimulated functional magnetic resonance imaging

On standardized tests of odor identification and odor detection, women tend to score better than men at nearly all age groups. We sought to determine if these findings would translate to differences between the sexes in the volume of activated brain when odors are presented to subjects as the stimulants for functional magnetic resonance imaging (FMRI) experiments. The activation maps of eight right-handed women (mean age 25.3 years old, range 20-44, S.D. 8.3 years) were compared with those of 8 right-handed men (mean age 30.5, range 18-37, S.D. 6.5 years) given the same olfactory nerve stimuli in an FMRI experiment at 1.5 T. Olfactory stimuli were delivered to the patients in a passive fashion using a Burghart OM4-B olfactometer with a nose piece inserted into the patients' nostrils. We used agents (eugenol, phenyl ethyl alcohol, or phenyl ethyl alcohol alternating with hydrogen sulfide) that were selective for olfactory nerve stimulation in the nose. The odorants were delivered to both nostrils for 1 s every 4 s during a 30 s 'on-period'. During the 30 s 'off-period', the patient received room air at the same flow rate. The women's group-averaged activation maps showed up to eight times more activated voxels than men for specific regions of the brain (frontal and perisylvian regions). The left and right inferior frontal regions showed a statistically significant increase in activation in women at p<0.01. In general, more women showed activation than men. The results suggest that (1) FMRI activation maps in subject groups can demonstrate correlates to psychophysical tests of olfaction, and (2) one must control for gender when performing odor-stimulated FMRI experiments.

Alzheimer's disease risk factors as related to cerebral blood flow: additional evidence

In a previous report, Alzheimer's disease risk factors, including alcohol abuse, depression, Down's syndrome, cerebral glucose metabolism defect, head trauma, old age, Parkinson's disease, sleep disturbance, and underactivity, were shown to have an association with reduced cerebral blood flow. In this report an attempt is made to strengthen a hypothesis that reduced cerebral blood flow may be a required cofactor in the cause of Alzheimer's disease with examples of additional putative risks, including aluminum, ApoE 4 alleles, estrogen deficiency, family history of dementia, low education-attainment, olfactory deficit, and underactivity coupled with gender, considered to have a relationship or potential relationship with reduced cerebral blood flow. Factors, believed to ameliorate Alzheimer's disease, associated with improved or stabilized cerebral blood flow are tabulated. A tentative cerebral blood flow nomogram is shown as a potential model to possibly help predict Alzheimer's disease susceptibility.

Olfactory identification deficit in relation to schizotypy

Past studies have shown that schizophrenic men tend to be impaired on the olfactory identification task. We examined individual differences in the olfactory function in relation to schizotypy. Healthy individuals who might carry latent liability for schizophrenia participated in olfactory identification and acuity tasks. Psychometrically ascertained schizotypic men showed deficits on the olfactory identification task but not on the olfactory acuity task. The olfactory identification function partly depended on the menstrual cycle in women. But further systematic research is needed to clarify the possible role of estrogen.

The role of the piriform cortex in kindling

In epilepsy research, there is growing interest in the role of the piriform cortex (PC) in the development and maintenance of limbic kindling and other types of limbic epileptogenesis leading to complex partial seizures, i.e. the most common type of seizures in human epilepsy. The PC ("primary olfactory cortex") is the largest area of the mammalian olfactory cortex and receives direct projections from the olfactory bulb via the lateral olfactory tract (LOT). Beside the obvious involvement in olfactory perception and discrimination, the PC, because of its unique intrinsic associative fiber system and its various connections to and from other limbic nuclei, has been implicated in the study of memory processing, spread of excitatory waves, and in the study of brain disorders such as epilepsy with particular emphasis on the kindling model of temporal lobe epilepsy with complex partial seizures. The interest in the kindling model is based primarily on the following observations. (1) The PC contains the most susceptible neural circuits of all forebrain regions for electrical (or chemical) induction of limbic seizures. (2) During electrical stimulation of other limbic brain regions, broad and large afterdischarges can be observed in the ipsilateral PC, indicating that the PC is activated early during the kindling process. (3) The interictal discharge, which many consider to be the hallmark of epilepsy, originates in the PC, independent of which structure serves as the kindled focus. (4) Autoradiographic studies of cerebral metabolism in rat amygdala kindling show that, during focal seizures, the area which exhibits the most consistent increase in glucose utilization is the ipsilateral paleocortex, particularly the PC. (5) During the commonly short initial afterdischarges induced by stimulation of the amygdala at the early stages of kindling, the PC is the first region that exhibits induction of immediate-early genes, such as c-fos. (6) The PC is the most sensitive brain structure to brain damage by continuous or frequent stimulation of the amygdala or hippocampus. (7) Amygdala kindling leads to a circumscribed loss of GABAergic neurons in the ipsilateral PC, which is likely to explain the increase in excitability of PC pyramidal neurons during kindling. (8) Kindling of the amygdala or hippocampus induces astrogliosis in the PC, indicating neuronal death in this brain region. Furthermore, activation of microglia is seen in the PC after amygdala kindling. (9) Complete bilateral lesions of the PC block the generalization of seizures upon kindling from the hippocampus or olfactory bulb. Incomplete or unilateral lesions are less effective in this regard, but large unilateral lesions of the PC and adjacent endopiriform nucleus markedly increase the threshold for induction of focal seizures from stimulation of the basolateral amygdala (BLA) prior to and after kindling, indicating that the PC critically contributes to regulation of excitability in the amygdala. (10) Potentiation of GABAergic neurotransmission in the PC markedly increases the threshold for induction of kindled seizures via stimulation of the BLA, again indicating a critical role of the PC in regulation of seizure susceptibility of the amygdala. Microinjections of NMDA antagonists or sodium channel blockers into the PC block seizure generalization during kindling development. (11) Neurophysiological studies on the amygdala-PC slice preparation from kindled rats showed that kindling of the amygdala induces long-lasting changes in synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility to evoked burst responses. The epileptiform potentials in PC slice preparations from kindled rats seem to originate in neuron at the deep boundary of PC. Spontaneous firing and enhanced excitability of PC neurons in response to kindling from other sites is also seen in vivo, substantiating the fact that kindling induces long-lasting changes in the PC c

Illusory contours activate specific regions in human visual cortex: evidence from functional magnetic resonance imaging

The neural basis for perceptual grouping operations in the human visual system, including the processes which generate illusory contours, is fundamental to understanding human vision. We have employed functional magnetic resonance imaging to investigate these processes noninvasively. Images were acquired on a GE Signa 1.5T scanner equipped for echo planar imaging with an in-plane resolution of 1.5 x 1.5 mm and slice thicknesses of 3.0 or 5.0 mm. Visual stimuli included nonaligned inducers (pacmen) that created no perceptual contours, similar inducers at the corners of a Kanizsa square that created illusory contours, and a real square formed by continuous contours. Multiple contiguous axial slices were acquired during baseline, visual stimulation, and poststimulation periods. Activated regions were identified by a multistage statistical analysis of the activation for each volume element sampled and were compared across conditions. Specific brain regions were activated in extrastriate cortex when the illusory contours were perceived but not during conditions when the illusory contours were absent. These unique regions were found primarily in the right hemisphere for all four subjects and demonstrate that specific brain regions are activated during the kind of perceptual grouping operations involved in illusory contour perception.