cognitive subtraction in functional imaging research

What is functional Magnetic Resonance Imaging (fMRI)?

            “Functional Magnetic Resonance Imaging or fMRI is a non-invasive brain imaging technique which captures images of brain areas that are active at a given time due different kinds of physical sensation such as sound, sight, smell, touch, and taste” (What is Functional Magnetic Resonance Imaging or FMRI?).  It measures the small metaolic changes that occur when a part of the brain is active.  It is used as a brain mapping tool in cognitive science.  It provides the means to observe mental processes.

            “There are basic processes involved in functional Magnetic Resonance Imaging”. (Atherton & Bart, 2001).    The two types of images that fMRI produces are functional and anatomical which depends on the type of molecules the signals are coming from.  Functional scans measure oxygen amount change in the blood over time and generate images of areas with the greatest oxygen consumption, while anatomical scans measure bone and soft tissue signals which produce images similar to x-rays (Atherton & Bart, 2001).  The assumption in fMRI is that the areas that used up the most oxygen are the most neurally active.  The functional images are placed on top of anatomical images so that the specific brain areas can be identified.  Computer software will then be used to create images that are three-dimensional.

            Functional Magnetic Resonance Imaging aside from its important role in scientific research has other uses.  Physicians use fMRI to observe the anatomy of the brain, to monitor the function and growth of brain tumors, to guide the preparation for brain treatments which require surgery, and to assess the effects of trauma, stroke or degenerative diseases on brain functions (Radiological Society of North America, 2007).  It is also used in brain mapping where areas of the brain related to the functions of speech, thought, sensation, and movement are determined.

What is the Method of Cognitive Subtraction in fMRI?

“Functional Magnetic Resonance Imaging (fMRI) research has two fundamental design paradigms”. (Functional Imaging of Neurocognition, 2000).  These design paradigms include block and event-related.  The two tasks selected in the block design are experimental task and control task.  To come up with the cognitive component that is used in observation, a process called cognitive subtraction is used.  Cognitive subtraction is the process of calculating the difference between the experimental task and the control task.  It seeks activation differences between two tasks.

Blocked designs are favored in experiments if the experiments aim to address cognitive processes with certain conditions.  The cognitive process should be a phenomenon which could not be subjected to parametric manipulation, homogenous in its ideas where there are no correct or incorrect trials, and could not be separated from other cognitive processes by several seconds in time (Functional Imaging of Neurocognition, 2000).

In studies using cognitive subtraction, both the experimental condition and the control condition which may be images of interest to the subject or person being studied, will be shown to the person in alternate time blocks.  Time blocks may be a matter of seconds or minutes.  This is done while the person is in the MRI scanner where the levels of brain activation are recorded.  The data are then analyzed using special statistical software.  The difference in the activation between the experimental task and the control task is calculated by the software.  The difference is assumed to be the cognitive factor of interest, which may be a thought process or other elements depending on the case being studied.

An important element of block designs is that a stimulus response is comprised of an average set of responses over a time block.  Because the design involves a summary value, specific individual responses are lost.  Specific studies such as determining the behavioral stimuli associated with the active parts of the brain at a given time would require other measures or additional methods.  Other stimulus may be introduced to the subject or person while he or she is in the scanner.  If the goal is to observe and determine fMRI signals in response to a particular stimulus, a different design should be used.

The other design paradigm of fMRI is event-related which observe activity changes immediately after the introduction of one stimulus.  The process involved in the event-related design enables researchers to correlate a specific behavioral stimulus to a particular response.

What are the Problems faced by the Cognitive Subtraction Method?

            The main problem encountered by researchers or scientists when using the cognitive subtraction method is the factors involved which makes it hard for them to interpret results or outcomes.  One factor is its inability to link behaviors to specific activity of brain areas.  It can identify which areas of the brain are active at a given time, but it is unable to link this activity to behavior.  Another point is the presumption that learning can result to lower levels of activation instead of higher levels due to the increased efficiency of metabolic and neurological processes, and that other forms of cognitive processing may involve inhibition instead of activation (Atherton & Bart, 2001).  This now can lead to misinterpretations if analyzed without any other supporting methods.

            In cognitive subtraction, there exists a difficulty in determining and placing appropriate baseline tasks or control tasks.  There is always a possibility that a baseline task may include inherent usage of a cognitive process that is not specifically required, which may lead to errors in distingusihing the cognitive differences between the baseline task and the experimental task (Caplan & Moo, 2003).  The difficulty often lies in trying to isolate a single cognitive operation.

            The assumption involved in cognitive subtraction is that a new cognitive component introduced to an existing one is unique and would have no interactive effect on the existing component.  This is the concept of pure insertion.  Pure insertion revolves around the idea that adding a cognitive process to an existing set of cognitive processes will not have any effect on the existing processes.  The assumption is hard to prove as an independent measure of the existing processes without the new process is needed.  A failure of pure insertion will lead to a possibility of the difference in the two conditions’ neuroimaging signal being interpreted not as a resulting cognitive component of interest, but as an interaction between the two (Functional Imaging of Neurocognition, 2000).

            Cognitive subtraction is limited by its inability to allow the order randomization of stimuli.  It is also restricted to grouping the same type of trials to each other.  The predictability of trial types may alter outcomes or confuse interpretations of outcomes.  The influence of the trial order on the data on functional neuroimaging can happen on two levels: within the trial or during the inter-trial interval.  The trial presentation order may have an effect on the cognitive processes involved during the trial itself, and a random or blocked presentation may have an effect on the cognitive processes involved during the inter-trial interval (Functional Imaging of Neurocognition, 2000).  As a result, experiments using block designs such as cognitive subtraction is distracted or confused by behaviors that may have possibly resulted from the presentation of similar trial groups, rather than outcomes resulting from effects of individual trials.

References

Atherton, M., & Bart, W. (2001, April). Education and MRI: Promises and Cautions. Retrieved

Oct 22, 2007, from University of Minnesota:

http://www.tc.umn.edu/~athe0007/EducationAndfMRI.pdf

Functional Imaging of Neurocognition. (2000). Retrieved Oct 22, 2007, from Medscape:

http://www.medscape.com/viewarticle/410866_4

Radiological Society of North America. (2007, JUl 3). Functional MR Imaging (fMRI) – Brain.

Retrieved Oct 22, 2007, from RadiologyInfo:

http://www.radiologyinfo.org/en/info.cfm?pg=fmribrain&bhcp=1

What is Functional Magnetic Resonance Imaging or FMRI? (n.d.). Retrieved Oct 22, 2007, from

California State University Long Beach:

http://www.csulb.edu/~cwallis/482/fmri/fmri.html

 

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