Cognitive activity is associated with changes in optical properties of the brain tissue. Near infrared (NIR) based optical measurements during brain activation can assess the hemodynamic changes, namely blood oxygenation and blood volume. Furthermore, near infrared spectroscopy (NIRS) has recently attracted much interest, since the technology allows the design of portable, safe, affordable, non-invasive and negligibly intrusive monitoring systems. In this paper, we present experimental procedures and results of the functional near infrared (fNIR) measurements acquired from the forehead during cognitive tasks. The data is collected from subjects engaged in two standardized attention and working memory tasks, respectively, the 'target categorization' and the 'n-back memory' tests. In the case of target categorization, the aim is to study changes in blood oxygenation and volume level while participants are experiencing decrements in vigilance, increased lapses of attention and cognitive slowing. 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 consistently varying "loads" on working memory were measured in participants performing the n-back test. The results show that the level of oxygenation rises with the increase of working memory load.