The functions of the left and right hemispheres are widely misunderstood, but in brief the functions can be interpreted as follows:
The left hemisphere controls anything to do with space and time (most of what we call ‘thinking’ in the West). It is where we do our calculating, analysing and planning. It deals with pattern recognition.
For instance when you walk into a room you immediately recognise the chair, the table or other objects in the room even of you have never seen that particular chair or table. What is more, you can tell objects from shadows, and where one thing is front or behind another. The left brain deal with details rather than the big picture. If we consider the brain as a computer (and it must be made clear that this metaphor only takes us so far), the left brain has the programs.
The right hemisphere is much less well understood in the West but is better understood in Eastern cultures. It is to do with being in the present. In our computer metaphor, this controls the raw input from the peripherals. For instance, whenever you move your mouse it sends signals to the computer to indicate that it has moved a certain amount horizontally or vertically. The computer interprets these signals as a particular position on the screen. Similarly, each ear send a series of signals to the brain that indicate sounds of a particular amplitude (volume) and frequency (tone). The right hemisphere of the brain deals with these raw signals. It does not really distinguish between signals that come from the eyes, the ears, the nose , etc.
The right hemisphere deals in the big picture, rather than the details. It controls the left side of the body.
This concept of the left and right hemisphere difference is substantiated by the work of the neurologist, Dr. Vilayanur S. Ramachandran.
The doctor was examining a condition called anosognosia, which typically results from damage to the right parietal lobe (the right hemisphere) of the brain. A patient with this syndrome is convinced that although they are paralysed on one half of their body, they are normal. So a patient asked to clap their hands together will wave their right hand in the air or clap it against their chest and will be convinced that it is functioning normally. When asked to tie their shoelaces (a task that is impossible to perform with one hand without considerable practice), they will persist at the task indefinitely.
This syndrome only occurs where the damage is to the right parietal lobe, which led Dr. Ramachandran to develop a theory as to its cause. He theorised that the left hemisphere of the brain is concerned with logic and working out causes and effects. For example, when you see a door you understand what it does and how it opens even if you have never seen that particular door before..You can deduce this from your experience with other, similar, doors. This is the function of the left hemisphere. On the other hand, the right hemisphere acts as what he called the ‘devil’s advocate’ (Milton Erickson used the term ‘Pattern Interrupt’ because it interrupts the patterns executed by the left hemisphere). This is the part of the brain that questions whether that particular pattern is appropriate and questions the relevance of the deductions of the left hemisphere. This allows you to adapt to changed situations.
In the case of the patient with anosognosia, the right hemisphere is damaged so even though the situation has changed (that is, the patient is paralysed in one half of their body) the inactive right hemisphere is unable to question the deductions of the left hemisphere, which has previously worked out that the body is able to function normally.