Human cortical areas activated by odorants: A study by MEG and EEG

The olfactory projections into the cerebral cortex and their function in humans have not been revealed until recently because of the absence of appropriate measurement of human brain activities. However, recent development of brain imaging technique allows us to investigate the cerebral activities of living humans and the study of olfactory processing has already started. Zattore et al. [1] reported that by using PET, cerebral blood flow increased in the piriform cortices of both hemisphere and in the right orbitofrontal cortex after various kinds of odors were presented to human subjects. Other studies also noted the same results by using fMRI [2, 3]. Those PET and fMRI techniques strictly show the activated area of brain, however, they have the defect of low temporal resolution. By using the electrophysiological approaches, such as EEG and magnetoencephalography (MEG), it will be possible to estimate the activated areas by odor stimuli with the rigid latencies. Several studies using MEG have been reported [4-7]. But olfactory monomodal stimulation has been very difficult. Using a sophisticated olfactometer (Kobal's olfactometer) by which only olfactory sensation is caused without any other sensation, such as tactile, Kettenmann et al., reported that the region between the superior temporal plane and the parainsula cortex in both hemispheres was identified at latencies corresponding to P1 component of the olfactory evoked potentials (OEP), anterior central parts of insular cortex in dominantly left hemisphere to N1, and superior temporal sulci (STS) bilaterally to P2 [5]. In the study, these neocortical activities in each hemisphere were separately measured by 37channnel SQUID system. The main aim of the present study was to trace the activated cortical areas by olfactory stimulation from moment to moment by using a whole head type of SQUID system. We attempted to record olfactory evoked magnetic fields (OEMf) and OEP at the same time. Thus we investigated the activated cortical areas corresponding to the OEP responses. 2 Methods