Abstract: In addition to positron emission tomography (PET) and functional magnetic resona nce imaging (fMRI) that can measure brain function in vivo, near-infrared spectr oscopy (NIRS) is being used in various fields of neuroscience. In the near-infra red region, light with wavelength of 760nm and 850nm have different characterist ics of absorbing two different states of hemoglobin (Hb), the oxyhemoglobin(O2Hb) and deoxyhemoglobin (HHb). The relative metabolic changes of O2Hb, HHb an d the total of them, the blood volume within the measured regions, can be detect ed. This study explored the feasibility of NIRS in detecting higher brain functi on, by investigating the role of the left prefrontal lobe in semantically encodi ng unrelated word pairs. The subjects studied unrelated word pairs that were vis ually presented under both semantic and shallow conditions. In semantic (or deep ) task, subjects were asked to form a sentence including the two words, while in perceptual (or shallow) task, they were asked to judge whether the two words ha d the same orthographic structures. Optical imager measured the relative changes of 760nm and 850nm absorption when the continuous light diffused through the le ft prefrontal lobe during the encoding tasks. In terms of intensity change, the regional blood volume change could be measured. Finally, subjects performed the recognition task. The results showed that recognition performance was better in semantic task than that in the perceptual task. Correspondingly, t he blood volume of the left prefrontal lobe, especially the dorsal part, increas ed in the semantic task as compared with the perceptual task. The proportion of the activated subjects out of the total subjects was higher in regions C and D t han in regions A and B. It demonstrated that the left prefrontal lobe contribute d to semantic encoding of unrelated word pairs. The present research also sugges ted that NIRS can be used to measure higher brain function non-invasively, for e xample, memory process.

Key words: prefrontal lobe, semantic encoding, episodic memory, near infrared sp ectroscopy, functional brain imaging