Noise reduction on the olfactory neuromagnetic measurements using SSP method

The successful signal space projection (SSP) method was firstly performed by Ilmoniemi for separating the differenct components of evoked responses and spontaneous activity in brain signals as well as signals measured from the heart [1] and applied by Huotilainen et al. for the rejection of eye-blink [2]. The effective noise reduction method is one of the most important measuring techniques in olfactory neuromagnetic experiments because we must use very few summation in the acquisition of olfactory evoked sensations which have the larger adaptation effects. The aim of this study was comparison between active olfaction and passive olfaction using magnetoencephalography (MEG). We used a whole-cortex type biomagnetometer (122-channel SQUID gradiometer) in these olfactory experiments. In the passive olfaction amyl acetate gas (1%) was administered for 300 ms into either the right or left nostril in synchronization with respiration using a mask and an optical fiber sensor. We obtained the clear olfactory responses on the both sides of forehead in all six subjets (all righthanded). The generators of olfactory magnetic fields were estimated at two regions located fairly asymmetrically near the bilateral frontal deep areas. Almost all subjects showed ipsi-lateral dominancy with the response of the same side with stimulation the largest. In contrast, active olfaction showed the dominacy of right hand side responses. We found also the responses of the moving-related magnetic fields accompanied with active respirations, since about 500 ms before the starting time of the subjec's sniffing to odorants. In this reseach we applied to analyse the odorant responses using principal component analysis (PCA) for the sniffing data processing. The difference of two olfaction methods suggests us the difference of detection and cognition of human olfactory mechanism. 2 Methods