Stimulation sample design in fMRI
Just as the choice of imaging parameters is important for a successful experiment, so is the design of the stimulation sample design in fMRI. Much experience is available from EEG and PET, but because fMRI has a spatial resolution between these two methods, a new perspective is available. It should be used. There are many issues in sample design, which we will briefly review here.
Early fMRI experiments were mostly in the form of PET studies, i.e., a set of images taken at rest and then a set of activity. but because the BOLD contrast is relatively fast from its onset and decline (within a few seconds), it is possible to use the time interval for shorter events that usually occur. The most common pattern of stimulation is regular epochs of stimulation and rest, usually named on and off. The length of these epochs should be long enough to match the hemodynamic response, so the value of 9 and often 01 seconds is chosen. These epochs are as long as the contrast to the noise is sufficient for swimming The activity response continues to be created. However, the total laboratory time should be such that the person can remain in the recording position easily and without movement, as well as the number of data points that are needed to obtain the contrast with sufficient noise.
There are technical limitations to testing time and the possibility of subject habituation to the stimulus and thus reducing the contrast to noise over time. Instead of epochs of stimulation, a single event can be used as the stimulus, more like what We see EEG and MEG. Again, due to the hemodynamic response, this event should be for longer periods of time than what we need in EEG, but because this type of stimulation has a major advantage, which is that it can be relative timed. to isolate the activity in different parts of the brain. One of the disadvantages of the single-event sample is that it takes more time to obtain sufficient contrast to the noise compared to its epoch-based counterpart. The choice of stimulation is critical. For example, the activation of the right visual cortex, but to determine the areas responsible for recognition and thematic Because of the colors, this is very difficult. Ideally, the design of the on and off eochs, while there is only one correct and clear difference between them, activates only those parts of the brain that are responsible for the specified task.
This can always be done. It is not done and usually a set of tests should be done. For example, to find the areas involved in task A, one test is done that includes tasks A and B, and one test that only includes B. Areas Those who are involved in task A are those who were activated in the first test and were not activated in the second test. This theory assumes that the system is linear, which is not a true assumption, or at least there are unaccounted cases for the difference between two samples, which can affect the result. Another major problem when working with cognitive events. such as memory, it is that visual stimulation must be accompanied, or that the individual’s response is required, which is often accompanied by motor activities. The load is minus the specific cognitive activity in the previous experiment, it was compensated. Alternatively, the stimulus can be presented in a different way, for example, in the form of auditory-real and the response is also presented in the form of language, and in this way, it can be assumed that Common areas between both types of presented stimulation are responsible for the cognitive task in question. There are few stimuli that are difficult to implement in fMRI experiments.
An obvious case is auditory stimulation. Fast imaging scanners usually generate a lot of noise during operation, especially if an EPI sequence is used. Although acoustic signals are heard, they are not as clearly detectable as visual signals, and on the other hand, detection of activity in the cortex Early hearing is very difficult because there are many sounds during both the on and off periods. Responding verbally is also difficult because it is often accompanied by head movement, which can coincide with stimulation. All movement The responses should be small and slow to reduce the movement of the head during the scan. In the test, we need the test subject to follow the instructions well, and remain in the lying position and concentrate throughout the scan. Many stimulations are better. They show that if the answer is made. Many more things can be said about a suitable sample design, while many other things need to be understood, however, for each new experiment, careful checks are needed.