In the interest of elucidating a relation between a human mental activity and a brain's action and in an effort to specify a lesion in brain's disease therapeutics, attempts have been made to identify a neural active site of the brain. Neurons in a neural activity control an ion concentration to form a charge distribution and transmit information through propagation of a potential created by the charge distribution, namely of an action potential (see Non-patent Reference 1). Thus, the most direct information source that can identify a site of neural activity is the action potential, more cardinally the charge distribution which the neuron creates.
While in the measurement of an action potential in the nervous system, a method is usually taken in which to directly insert an electrode into the body, use cannot be made of this method for a human body, inter alia for its brain tissue but necessarily of a noninvasive measuring method to identify an active region from the outside of the human body without harming the body part.
As the noninvasive method of measuring a neural activity, PET (positron emission tomography; see Non-patent Reference 2), fMRI (functional magnetic resonance imaging; see Non-patent Reference 3), near infrared topography (see Non-patent Reference 4) and magneto-encephalography (see Non-patent Reference 5) have been primarily put to practical utilization at present.
In any of PET, fMRI and near infrared topography, however, in which neuron's activity is indirectly detected from a change in amount of metabolism, namely in amount of bloodstream in the blood or oxygen therein, in the region of an active site, no electrical signal created by neurons is directly measured. As a result, their position and time resolutions for an active site are not sufficient in elucidating a relation between a human mental activity and a brain's action or in serving for disease therapy. They require a cyclotron accelerator for producing positrons, a high field generating apparatus for nuclear magnetic resonance or the like and are extremely high in apparatus cost.
Magneto-encephalography which detects a very weak magnetic field as induced by an intracellular electric current is a process that is high in time resolution as it detects neurons' activity more directly than do the others above. This method in which a position is estimated on the basis of a magnetic field distribution and hence determined indirectly is not enough in position resolution. Especially if a plurality of sites are active simultaneously, their identification then becomes difficult. There is also the problem that it is difficult to detect information from a deep part and an electric current passed towards a normal to a surface.
Included in properties of a material is a magnetic property. As regards the magnetization of a magnetic material, it is reported in Non-patent Reference 8, for example, that a ferromagnetic thin film irradiated with laser light of femtoseconds is observed to produce a coherent radiation in a THz band.    Non-patent Reference 1: Michikazu Matsumura “Invitation to Brain Science (The secret of a neural circuit is expounded)” (in Japanese) Science Co., Ltd., Jul. 10, 2003, first Ed. third print issue, pp. 55-65;    Non-patent Reference 2: M. I. Posner & M. E. Raichele (translated into Japanese by Takeshi Yoro, Masako Kato and Kiyoto Kasai) “Observing the Brain—The riddle of the mind that the cognitive neuroscience reveals” (in Japanese) Nikkei Science Co., Ltd., Jun. 25, 2002, third print issue;    Non-patent Reference 3: P. Jezzard, P. M. Matthews, S. M. Smith edited “Functional Mri: An Introduction & Methods”, Oxford Univ. Pr (Sd), ISBN: 01985277 3X, (2003/06);    Non-patent Reference 4: Michikazu Matsumura “Invitation to Brain Science (The secret of a neural circuit is expounded)” (in Japanese) Science Co., Ltd., Jul. 10, 2003, first Ed. third print issue, p. 173;    Non-patent Reference 5: Michikazu Matsumura “Invitation to Brain Science (The secret of a neural circuit is expounded)” (in Japanese) Science Co., Ltd., Jul. 10, 2003, first Ed. third print issue, pp. 168-169;    Non-patent Reference 6: http://www.rofuku.go.jp/hanasi/eswl.htm;    Non-patent Reference 7: http://www.edap-hifu.com/; and    Non-patent Reference 8: E. Beaurepaire and five others, Appl. Phys. Lett., Vol. 84, No. 18, pp. 3465-3467, May 3, 2004.