Event related fMRI

Event-related functional magnetic resonance imaging (efMRI) is a technique in magnetic resonance imaging that can be used to detect changes in the BOLD (Blood Oxygen Level Dependent) hemodynamic response to neural activity in response to certain events. Within fMRI methodology, there are two different ways that are typically employed to present stimuli. One method is a block related design, in which two or more different conditions are alternated in order to determine the differences between the two conditions, or a control may be included in the presentation occurring between the two conditions. By contrast, event related designs are not presented in a set sequence; the presentation is randomized and the time in between stimuli can vary. efMRI attempts to model the change in fMRI signal in response to neural events associated with behavioral trials. According to D'Esposito, "event-related fMRI has the potential to address a number of cognitive psychology questions with a degree of inferential and statistical power not previously available." Each trial can be composed of one experimentally controlled (such as the presentation of a word or picture) or a participant mediated "event" (such as a motor response). Within each trial, there are a number of events such as the presentation of a stimulus, delay period, and response. If the experiment is properly set up and the different events are timed correctly, efMRI allows a person to observe the differences in neural activity associated with each event.

Positron Emission Tomography (PET), was the most frequently used brain mapping technique before the development of fMRI. There are a number of advantages that are presented in comparison to PET. According to D’Esposito, they include that fMRI “does not require an injection of radioisotope into participants and is otherwise noninvasive, has better spatial resolution, and has better temporal resolution." The first MRI studies employed the use of “exogenous paramagnetic tracers to map changes in cerebral blood volume”, which allowed for the assessment of brain activity over several minutes. This changed with two advancements to MRI, the rapidness of MRI techniques were increased to 1.5 Tesla by the end of the 1980s, which provided a 2-d image. Next, endogenous contrast mechanisms were discovered by Detre, Koretsky, and colleagues was based on the net longitudinal magnetization within an organ, and a “second based on changes in the magnetic susceptibility induced by changing net tissue deoxyhemoglobin content”, which has been labeled BOLD contrast by Siege Ogawa. These discoveries served as inspiration for future brain mapping advancements. This allowed researchers to develop more complex types of experiments, going beyond observing the effects of single types of trials. When fMRI was developed one of its major limitations was the inability to randomize trials, but the event related fMRI fixed this problem. Cognitive subtraction was also an issue, which tried to correlate cognitive-behavioral differences between tasks with brain activity by pairing two tasks that are assumed to be matched perfectly for every sensory, motor, and cognitive process except the one of interest. Next, a push for the improvement of temporal resolution of fMRI studies led to the development of event-related designs, which according to Peterson, was inherited from ERP research in electrophysiology, but it was discovered that this averaging did not apply very well to the hemodynamic response because the response from trials could overlap. As a result, random jittering of the events was applied, which meant that the time repetition was varied and randomized for the trials in order to ensure that the activation signals did not overlap.

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