April 12, 2005

Brain test assoc. with function

Mismatch negativity deficits are associated with poor functioning in schizophrenia patients.
Arch Gen Psychiatry. 2005 Feb;62(2):127-36.
Light GA, Braff DL.

The objective of this paper was to determine if an objectively measured deficit in “mismatch negativity” could be associated with clinical deficits in schizophrenia. The defects in schizophrenia are widespread throughout the brain and do not necessarily always show up as obvious symptoms. However, these changes are often at the root of difficulties in working memory, executive function (complex planning), and lead to social/occupational deficits. Traditionally, neuropsychological tests are used to evaluate for these types of problems in the brain. A patient is asked to complete various tasks that are designed to isolate different functions of the brain and depending on how well the patient is able to complete the tasks, one can evaluate for level of dysfunction. The scores on these types of test batteries can be correlated with social/occupational functioning; however there are limits to these types of tests. They must be conducted in controlled environments, require large amounts of concentration and some are subject to changes (learning effects) the more times someone is asked to complete the tasks.

Testing MMN, investigators use ERP (event related potentials which are measures of the electrical activity between neurons or the cells of the brain) which means that it is possible to record the electrical activity in a particular region at the exact time the stimulus is offered. Therefore, one can tell with a greater accuracy exactly what is being stimulated and when. MMN is a process of auditory ERP (ERP can be generated from any type of stimulus including visual, smell, etc) in which a sound is played repetitively until another, “oddball” sound is also played instead and the discrimination between the regular sounds and the brain’s reaction to the novel stimulus is measured. In this study, the “oddball” sound is a tone that is the same note, but of longer duration. MMN is useful because it is highly reproducible and a reliable measure. Also, it is not something that people have a conscious control over meaning that it is not necessary to have people focus on the task for a long period of time in order to achieve a consistent result. Interestingly, MMN is not improved or changed by the addition of antipsychotic agents either though normal results have been seen in first episode patients. MMN may have utility for screening high risk patients however as it is sometimes altered in the families of people with schizophrenia even if they do not manifest the illness.

Ultimately, the authors found that the more significant a disturbance in MMN, the more significant dysfunction the patient had in terms of social and occupational functioning. They were less likely to be living on their own or participating in the community with a worse MMN. The authors took great care in surveying consumers of all levels of functioning – from those living in long term locked facilities to those able to provide their own housing and transportation. Without having done that, it would have been more difficult to create a meaningful correlation between MMN and functional status. The authors also point out that they did not see a significant difference on various “tests” of functional ability but rather the difference was on empirically derived measures of functional status. This calls into question the empirical validity of some of the tests used, but also points out that the overall level of functioning is more important than performance on some surrogate measures.

Click here to access the article on PubMed


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