# Clinical Brain Monitoring with Time Domain NIRS: A Review and Future Perspectives
## 3 Clinical Brain Monitoring with TD-NIRS
### Psychiatry and Cognitive Impairment  
Another field of clinical application of TD-NIRS is psychiatry, in which NIRS is used to evaluate functional brain activation in different neuropsychiatric disorders, notably in schizophrenic disorders, and normal and pathological aging [12]. In the context of schizophrenia, we found two publications that used Hamamatsu TD-NIRS systems to provide baseline measurement of haemoglobin concentration, tissue saturation and optical path length in this group of patients. In these studies, fNIRS was also performed but using a CW-NIRS system. We will therefore only report the TD-NIRS results. Firstly, [PERSON] and colleagues [98] used TD-NIRS to investigate the frontal lobe dysfunction in schizophrenia. The authors reported no significant difference in path length between the control group and schizophrenic group. However, in some subjects, the path length was different from the mean value of the group by more than 20%, highlighting the importance of a subject specific path length in order to retrieve accurate quantitative values of the change of the haemoglobin concentrations. [PERSON] and colleagues [99] investigated the resting hypofrontality in patients with schizophrenia. They reported that the [HbT] decrease was related to the duration of illness, and it was not observed in patients whose duration of illness was less than 10 years. The authors concluded that despite the fact that resting hypofrontality is a chronically developed feature of schizophrenia, this does not necessarily represent frontal dysfunction, but may reflect anatomical and/or functional changes in frontal microcirculation. On the other hand, several studies addressed the issue of cognitive impairments and aging using Hamamatsu systems. [PERSON] and colleagues [100] compared the haemoglobin concentration measured by a TRS-10 system, and the cerebral activity, measured by a CW-NIRS system, of the prefrontal cortex between 14 young adults and 14 elderly adults while driving. The main finding regarding the TD-NIRS results for this study was that the concentrations of all forms of haemoglobin in elderly adults were lower compared to young adults. Two papers [101; 102] reported the use of a TRS-20 system to evaluate the effect of cosmetic therapy on prefrontal cortex (PFC) activity in 61 elderly females. The [HbT] and \(\text{StO}_{2}\) were evaluated before and after a three-month program of cosmetic therapy. [PERSON] and colleagues [103] used a TRS-21 system to investigate the relationship between cognitive function and haemoglobin concentration and tissue saturation of the prefrontal cortex (PFC) during rest in 113 elderly adults. The authors quantified cognitive functions using the Mini-Mental State Examination (MMSE) score and the Touch M score, which evaluates working memory function semi-automatically on a touchscreen. A significant positive correlation between the MMSE scores and \(\text{StO}_{2}\) was found together with a significant positive correlation between Touch M scores and baseline concentrations of [HbO\({}_{2}\)], [HbT] and \(\text{StO}_{2}\). It is worth noting that the authors reported that the main advantage of TD-NIRS in that study was to measure absolute information at rest; allowing them, therefore, to assess cognitive function without any task (i.e., fNIRS using CW-NIRS). [PERSON] and colleagues [104] investigated the possibility of using the haemoglobin concentration and \(\text{StO}_{2}\) information acquired with a TRS-20 system to predict the MMSE score of a group of 202 elderly people. The MMSE was conducted first and then TD-NIRSmeasurement was performed. The authors found using a deep neural network (DNN) that the \(\text{StO}_{2}\) and the optical path lengths contributed to the prediction of the MMSE score. Finally, two publications by [PERSON] and colleagues [105] and [PERSON] and colleagues [106] investigated the effects of two drugs on the improvement of cognitive function using a TRS-20 system.