Finally, limitations of the present study should be discussed. First, NIRS parameter changes may be caused by changes in the blood flow of the scalp, since NIRS measures the blood oxygenation changes within the illuminated area, which includes both intracranial and extracranial tissues. Indeed, a recent study demonstrated that hemodynamic changes in the scalp were associated with functional activation tasks.43 In the present study, however, we evaluated asymmetry of oxy-Hb changes in the right and left PFC. Although blood oxygenation changes in the extracranial tissues could affect the oxy-Hb changes, it would be difficult to explain the relation between asymmetry of the oxy-Hb changes (LIR) and STAI scores only in terms of the extracranial factor. In addition, Tanida et al.18 observed minimal changes in the skin blood flow during mental arithmetic tasks using a thermograph. However, further studies are necessary to clarify whether these effects might have significantly influenced the present NIRS data. Second, NIRS does not allow the measurement of cerebral blood oxygenation changes in the whole brain. In the present study, we measured cerebral blood oxygenation changes only in the PFC; however, this may be reasonable, as the PFC plays important roles in emotion and emotional response. Third, NIRS analysis using the modified Beer–Lambert law only allows us to assess the combined effect of blood oxygenation changes and light path lengths. The observed asymmetry of the prefrontal fluctuation may reflect individual differences of the light path lengths in the right and left prefrontal areas. Although previous studies could not find a significant difference between these path lengths,14 it would be desirable to exclude any effect of the path lengths. Finally, a possible limitation of the present study is concerning the baseline period should be mentioned. Our baseline period (3 min) was relatively short compared with that used in studies on spontaneous hemodynamic oscillation (e.g., in the range of 0.01 to 0.15 Hz), which was evaluated by means of power spectral analysis.35–39 However, it should be noted that we analyzed averaged values of oxy-Hb changes with respect to the minimum values during the baseline period [see Eqs. (1) and (2)], and defined left/right asymmetry of activity (i.e., LIR) based on the averaged values [Eq. (3)]. We consider that this approach would have been adequate to evaluate the relationship between LIR and STAI, but we cannot rule out the possibility that the use of a longer baseline period might have influenced the observed relationship.