Multitasking in simple language means that we try to perform more than one task at the same time, e.g., driving while making or receiving a mobile phone call, or trying to listen to directions by the GPS (Global Positioning System). Usually many a times we are tempted to multitask and feel that we become more productive and efficient. But on the contrary multitasking slows performance and increases the number of mistakes committed due to divided attention among the tasks. In our studies on human volunteers we found that both pro-active and post-active interference with the provided task resulted in deterioration in performance and the number of errors committed increased (Ref: Barar FSK, Vanjani S, & Ajmera RL. Ind J Pharmac. 12: 149-156, 1960).
On attention to a task, the front of the brain known as the prefrontal cortex becomes active. This area spans the left and right sides of the brain's motivational system. This helps to focus one's attention on a goal and coordinates messages with the other brain systems to carry out the task. (Ref: Charron, S. & Koechlin, E., Science 328: 360-363, 2010).
The right and left sides of the prefrontal cortex work together when focused on a single task, but the sides work independently when people perform two tasks simultaneously. MRI studies revealed that larger reward with one task increased prefrontal activity on the related side, and a larger reward for the other task led to increased activity on the other side. The findings suggest that with two goals the brain divides in two halves (left & right). Koechlin suggests that we can switch readily between two tasks, but it is difficult to juggle between three tasks to be performed together (like eating lunch, checking e-Mail, and listening to the radio). This is only possible with practice.
In the process of multitasking, one switches attention back and forth between activities, which is relatively easier while young, but it becomes tougher with advancing age. Thus, multitasking is harder in older people (Ref: Clopp W, Rubens M, Sabharwal J. & Gazzley A. Proceedings of the National Academy of Sciences, USA 108: 7212-7217, 2011).