## 1 Background  
As the world's population is rapidly aging, dementia becomes a major global health problem. Currently, it is difficult to cure patients with progressive dementia, so emphasis is placed on early diagnosis and early intervention to prevent the onset of dementia [1]. The screening test of cognitive dysfunction, therefore, is important for early diagnosis of dementia. Currently, the Mini Mental State Examination (MMSE) is the most commonly used scale in cognitive function evaluation [2,3]. The MMSE is sensitive and cost-effective screening test; however, it is a subjective examination. Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have been used for the diagnosis of dementia [4]; however, these techniques require large facilities and have high costs for examination and maintenance. A simple and less costly method to assess cognitive functions is still required for an objective screening test of dementia. Near-infrared spectroscopy (NIRS), a non-invasive optical technique, appears to be an attractive alternative method since NIRS is compact and less expensive than fMRI or PET. NIRS evaluates cognitive functions by measuring evoked cerebral blood oxygenation (CBO) changes during cognitive tasks; NIRS provides concentration changes of oxyhemoglobin (oxy-Hb) and deoxyhemoglobin (deoxy-Hb) in cerebral vessels based on measurements of the absorption spectra of hemoglobin in the near-infrared wavelength range [5]. However, it should be noted that, in general, commercially available (conventional) NIRS systems employ continuous wave (CW) light allowing only qualitative measurements of relative changes in hemoglobin (Hb) concentrations during tasks [6]. Therefore, it is difficult to apply conventional CW-NIRS systems to evaluation of cognitive functions of aged people, particularly who cannot perform cognitive tasks due to dementia. In contrast, time-resolved near infrared spectroscopy (TRS), which employs light sources of laser diodes emitting picosecond light pulses and a time-resolved detector with a picosecond time-resolution, can provide quantitative measurements of Hb concentrations as well as relevant optical parameters such as the absorption coefficient (\(\mu_{a}\)), reduced scattering coefficient (\(\mu_{s}\)') and optical pathlength (OPL) in the tissues interrogated by the light pulses [7; 8]. \(\mu_{a}\) and \(\mu_{s}\)' are the optical properties averaged over the interrogated tissues and change mainly with the change in the HB concentration associated with the brain activation. OPL is the mean total optical pathlength of the light pulses travelling from the source to detector positions on the head surface, and it will change mainly with the change in the thicknesses of the various layers in the head tissues such as the scalp, cerebrospinal fluid (CSF), gray matter and white matter layers. One measurement procedure of TRS ends within a few minutes, and portable TRS systems cost much less than fMRI or PET. Because a TRS system used in this study is at an initial stage with an assumption of an optically homogeneous medium for multi-layered tissues of the human head, it can provide less spatial information than fMRI. Although the performance of the TRS system used in this study is limited, the measured HB concentrations and relevant optical parameters are very valuable for objective evaluation of dementia. Actually, TRS can measure hemodynamic conditions at rest due to its capability of acquiring baseline Hb concentrations quantitatively, and measurements at rest help evaluate the absolute changes in CBO from the rest to task states. By employing TRS in a previous study, we measured not only the Hb concentrations but also the OPLs in various regions of the brain of healthy adults [9]. In addition, for patients with subarachnoid hemorrhage (SAH), we measured baseline Hb concentrations at rest using TRS for detection of cerebral ischemia induced by vasospasms [10].