In recent years, near-infrared spectroscopy (NIRS) has been used to study functional activation of various areas of the brain. This is based on the assumption that an increase in the recorded oxygenated haemoglobin (HbO2) concentration represents an increase in blood flow, which in turn reflects neuronal activation. The aim of this preliminary study was to use NIRS to monitor the activity of the olfactory cortex, as mirrored by the haemodynamic response, when subjects were exposed to olfactory stimuli.

A NIRO 300 (Hamamatsu Photonics, Hamamatsu, Japan) device was used. The optodes were placed on the right forehead and right temporal, parietal and occipital regions. Changes in the concentration of HbO2 and deoxygenated haemoglobin during olfactory stimulation were monitored. Olfactory stimulation was performed with vanilla essence, strawberry essence and scatol.

During olfactory stimulation, cerebral HbO2 concentration increased over the frontal region. However, in the temporal, parietal and occipital regions, little or no HbO2 changes were recorded.

This study shows that human brain cortical activation following olfactory stimulation can be recorded by NIRS. This NIRS analysis may therefore provide the basis for future development of an objective olfactory test in humans.

Olfaction is an important and primitive sense. As its importance has changed with evolution, anatomic adjustments have occurred in its structure and vasculature. Primates are a family of vertebrates that have had to develop their visual system to adapt to the arboreal environment and have evolved from a macrosmatic to a microsmatic species as the optic system has enlarged. This has resulted in anatomic changes of a small but critical area at the base of the brain. This paper describes the three-dimensional vascular anatomy of the olfactory organ of the Japanese monkey (Macaca fuscata fuscata). This is best understood by dividing the organ into three parts: the olfactory tract, olfactory bulb, and olfactory nerves in the nasal mucosa. The bulb can be partitioned into an outer or cortical part and inner or medullary part. The vasculature and tissue were examined grossly and with light microscopy and scanning electron microscopy of vascular corrosion casts. The olfactory tract and bulb were supplied by an arteriole from the anterior cerebral artery on each side. The tract was supplied by capillaries running spirally with a coarse network. At the olfactory bulb, the arteriole ramified into the intracortical and medullary branches that formed capillary networks. The bulbar intracortical capillaries were divided into two layers with different densities and vascular patterns. The capillaries of the superficial layer had a ladder-like pattern. The branches that ran into the medulla of the olfactory bulb were more widely spaced. Twigs from the posterior ethmoidal artery ran along the nerve fiber and formed intra- and extrafascicular networks. Each region of the olfactory organ had characteristic three-dimensional vascular patterns that were related to their cellular architecture.

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.

In a pilot study, EEG changes during odour administration were evaluated by coherence analysis. Ten normal adults were studied. Simultaneous recordings of 16 EEG channels with, and without, odour administration were stored on magnetic tape for further processing. EEG signals were analysed using a signal analyser. Coherence spectra were calculated between all possible channel pairs on the scalp. The amount of data was reduced by extracting broad band coherence values for five frequency bands: delta (2–3.9 Hz), theta (4–7.9 Hz), alpha 1 (8–9.9 Hz), alpha 2 (10–12.9 Hz), and beta 1 (13–17.9 Hz). Coherence values extracted from the control EEG recordings and those during odour administration were compared to evaluate the presence of any significant differences.

The results demonstrated significant changes in the EEG coherence between the two control recordings (control before and control after) in the theta and beta 1 bands. These frequency bands were therefore excluded from the examination. During odorant stimulation with methyl-cyclopentenolone, the coherence in the delta band decreased in the frontal region, while that in the alpha 1 and alpha 2 bands increased in the temporal region. During odorant stimulation with scatol, the coherence in the delta band decreased in the frontal region, while that in the alpha 1 and alpha 2 bands increased between the longitudinal electrode locations. It was suggested that EEG coherence mapping may provide the basis for the development of an objective test of olfactory function in humans.