Near infrared (NIR) is a portable, safe, affordable, non-invasive and negligibly intrusive functional optical imaging modality, which enables the measurement of the metabolic changes associated with cognitive activity. In this paper, we present the experimental procedures and data analysis of the functional near infrared (fNIR) measurements acquired from the forehead during cognitive tasks. The data is collected from subjects engaged in two standardized tasks, namely, the 'target categorization' and the 'guilty knowledge test' (GKT). In the case of target categorization, the aim is to study the changes in the blood oxygenation and volume level while the participants are experiencing decrements in vigilance, increased lapses of attention, cognitive slowing. We measured 25 minutes-long blood oxygenation and volume level changes during target responses in 11 participants performing target categorization. Data analysis results revealed that the level of oxygenation changes in missed targets is higher than the level in captured targets. The blood oxygenation and volume level changes during deceptive and truthful responses were measured in 16 participants performing the GKT and analyzed using statistical data analysis. The results of our data analysis showed that the level of oxygenation changes during 'lie' task is higher than the level during 'truth' task.