Cognitive theorists postulate how information is processed. For example, is it like a computer where all the information is broken down into bits, processed, and then reassembled for output? Or is processing handled in a completely different manner? One prevalent way to test theories of cognitive psychology is by studying patients (whether human or animal) with brain damage. There are both advantages and disadvantages to using brain-damaged patients to test and construct cognitive theories.

Many years ago Paul Broca made certain assumptions about language processing as the result of studying someone with a lesion in a specific area of the brain. The trend of studying patients with damaged brains continues today with cognitive neuropsychology. Theorists study subjects with lesions and see what is not working properly when compared to a normal subject. If a certain part of the brain is damaged and yields very specific deficits, theories can be shaped to account for that deficit. So, if for example, you have a patient with damage to the posterior part of the left inferior frontal gyrus and he or she has speech production problems (aphasia) and if you have another patient with damage to the posterior part of the superior temporal gyrus who produces fluent but meaningless speech, you know by double disassociation that speech production and comprehension are processed differently.

In a similar manner, brain damage thus makes falsification of theories fairly easy. If, for example, a researcher theorizes that there is no difference between how nouns and verbs are processed she could look at brain-damaged people to try and falsify her theory. In doing so she would discover that there are in fact people who can read nouns and write verbs but not speak verbs. This finding would probably send her back to adjust her theory. Even though these are great advantages there are some disadvantages to using brain-damaged patients to produce theories.

One disadvantage is that brain damage is rarely focal. It is rare to have only one small process interrupted and everything else intact. This means that if a researcher uses brain-damaged patients to help him create theories, there are many possible interaction effects and other confounds due to the simple physical nature of most lesions. Another disadvantage is the assumption that a damaged brain functions just like a normal brain except for the part that is damaged. In other words, theorizing based on brain-damaged patients is like having a computer that is missing capacitors from the motherboard and trying to figure out how a normal computer works. You may see that sometimes the computer randomly shuts off and when you run a program and add 1+1 you get 3.

It may be that the missing capacitors are responsible for the computer problems or they may affect the processor which then causes the problems, or there may just be a software glitch and the hardware doesn’t matter in that case. In short, there may be many explanations for the deficits caused by brain damage. This is one thing that Gestalt psychologists really point out — a possible disadvantage to studying brain-damaged patients. Is the whole greater than the simple sum of the parts? Basing theories on brain-damaged patients can be problematic because the damage can really affect the overall cognitive processing in subtle and unforeseen ways.