One cognitive task can involve different processes. Humans are able to do different processes in parallel. According to [eyse93, p4,] this is the most likely explanation for the processing power of the brain. Most tasks involve a combination of serial and parallel processing.
It can be observed that while humans have the ability to do different things in parallel; some parallel tasks are easier than others. Most people have no problems with walking and talking in parallel; whereas they find listening to two different speakers at the same time very difficult. This implies that tasks which can be processed in different modules can be done easily in parallel, whereas tasks which need the same processing unit are difficult to manage concurrently.
Neuropsychologists study the information processing system of humans. They focus especially on patients with a partly damaged brain. The damage of a region of the brain has implications on the cognitive abilities which are situated in this area. The non-damaged parts are still working; in some cases their performance is improved to compensate the loss in the other parts. These results suggest that the brain has a highly modular and parallel structure, [eyse93].