**2. Utilization of NIRS for functional brain imaging**

Information processing in the brain occurs via two different systems, a neuroelectric transmission system and an energy-supplying system to neurons (Guiou et al., 2005). Nutrient arteries around the neurons supply them with blood containing the oxygen and glucose necessary for neural activity. Thus, changes in the ratio of oxygenated hemoglobin (oxy-Hb) to deoxygenated hemoglobin (deoxy-Hb) due to increased blood flow for such activity should be observable in tissues adjacent to the activated neurons. This relationship between neural activity and subsequent changes in cerebral blood flow is known as neurovascular coupling (Guiou et al., 2005; Rasmussen et al., 2009).

Similar to fMRI and PET, NIRS indirectly measures local cortical activity in vivo by measuring the differential concentration between oxy- and deoxy-Hb in the blood vessels. Specifically, it measures the difference in the absorption rate of near-infrared light by oxy-

Effects of Sleep Debt on Cognitive Performance and Prefrontal Activity in Humans 27

performance of executive functions, one of the higher cognitive functions that includes divided attention (Drake et al., 2001; Frey et al., 2004; Lim & Dings, 2010; Stenuit & Kerkhofs, 2008) and working memory (Bartel et al., 2004; Binks et al., 1999; Choo et al., 2005; Frey et al., 2004; Lim & Dings, 2010; Tucker et al., 2010; Wimmer et al., 1992) varies among studies; some report a significant effect of sleep loss (Bartel et al., 2004; Choo et al., 2005; Drake et al., 2001; Frey et al., 2004; Stenuit & Kerkhofs, 2008), while others report no such effect (Binks et al., 1999; Lim & Dings, 2010; Tucker et al., 2010; Wimmer et al., 1992).A discrepancy has also been seen in the influence of sleep loss has on behavioral performance versus its influence on neural activity; functional neuroimaging has revealed that sleep loss deteriorates not behavioral performance but neural activity (Choo et al., 2005). Such a discrepancy could point to a difference in neural substrates between basic and higher cognitive functions and/or possible personal differences in vulnerability of executive functions to sleep loss. We describe here two of our studies conducted using NIRS in order to explore the influence of sleep loss on basic and higher cognition associated with the

**4. Influence of sleep loss due to total deprivation of a night's sleep on time** 

When elapsed time is comparatively brief (within several minutes), humans can typically perceive the passage of time accurately without referring to an artificial time keeping device such as a wristwatch (Ivry, 1996; Rammsayer, 1999; Treisman, 1963). Human short-time perception is modulated by a robust neural basis consisting of subcortical structures, such as the cerebellum and basal ganglia, together with the right prefrontal cortex (Harrington et al., 1998; Pouthas et al., 1999). Moreover, a circadian pacemaker located in the suprachiasmatic nucleus of the hypothalamus, which is driven by a self-sustaining oscillator with a period of about 24 h and provides the time of day, participates in short-time perception (Aschoff, 1998; Ashoff & Daan, 1997; Kuriyama et al., 2003). As such, short-time perception is not independent of the influence of the circadian pacemaker; under a condition where zeitgebers are strictly controlled, short-time perception fluctuates on around a 24-h cycle and correlates with circadian markers such as core body temperature and melatonin, and consequently shows diurnal variation (Kuriyama et al., 2005). It has been confirmed that short-time perception shortens from morning into night, and is prolonged again from night to the next morning under a 30-h constant routine (Kuriyama et al., 2005; see Fig. 1). For sleep deprivation on the other hand, it has been reported that there is less diurnal variation

frontal functions mentioned above.

Fig. 1. Diurnal fluctuation of short-time perception

**4.1 Short-time perception** 

**perception** 

and deoxy-Hb, and the scalp and skull are high permeable to near-infrared light (Obrig et al., 2000). When such light is locally irradiated from an irradiation probe, it diffuses in the cerebral tissue up to a depth of 20-30 mm. A detection probe located 30 mm from the irradiation probe can detect the light diffusely reflected by the oxy- or deoxy-Hb, making it possible to estimate local changes in oxy-, deoxy- and total-Hb concentrations (Ferrari et al., 2004). For high-resolution detection of oxy- and deoxy-Hb concentrations, multiple channels of wavelengths (2 or 3) of near-infrared light (700-1000 nm) are usually simultaneously irradiated and detected.

NIRS has been widely used for several years in medical and biological studies of the brain. Although NIRS uses an accessible, non-invasive neuroimaging device, it should be applied to measure local cerebral metabolic rate of oxygen consumption with consideration given to its strong and weak points, which are listed in Table 1.

#### **Strengths (compared with MRI or PET)**


#### **Weaknesses (compared with MRI or PET)**


Table 1. Strengths and weaknesses of NIRS
