Brain functions both in an integrated and segregated manner. This study employs the concept of neural complexity to investigate these two interrelated properties of the brain. It is also known that various parts of the brain regulate the performance of the cardiovascular system and the hemisphere asymmetry which has been observed for some cognitive tasks is known to be determinative in this regulative role. In this study, we studied the complexity of the near infrared spectroscopy signals obtained during mental arithmetic (MA) task from the prefrontal cortex and we have investigated the relationship between complexity and heart rate increase during MA. We used the concept of functional clustering in neural complexity. Thus we determined the lateralization of the brain activity during MA and the clustering pattern of the subjects. It is shown that information theoretic approaches can be fruitful for investigating the pan-whole relationship in the brain.