fMRI Journal Club August 31st, 2004. Experimental Design for Brain fMRI Chapter 30 of Functional MRI By G.K. Aguirre & M.D’Esposito Block Design – Nelson Klahr Event Related – Paul Wright. Goals of an fMRI Experiment. Be inferentially capable of rejecting a hypothesis of interest.
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fMRI Journal ClubAugust 31st, 2004 Experimental Design for Brain fMRI Chapter 30 of Functional MRI By G.K. Aguirre & M.D’Esposito Block Design – Nelson Klahr Event Related – Paul Wright
Goals of an fMRI Experiment • Be inferentially capable of rejecting a hypothesis of interest. • Maximize sensitivity for a predicted effect.
Temporal Autocorrelation • Distribution of power is characterized by 1/f (sampling frequency) • “High” enough to avoid the elevated noise range due to physiological fluctuations
Limitations of BOLD Output • BOLD fMRI signal has no absolute interpretation • weighted by its concentrations and is not a direct measurement of [deoxyhemoglobin] • Generally tests for differences in the magnitude of the signal between different conditions within a scan. • Can vary between subjects, scans, and voxels.
Cognitive Subtraction • Differences in neural activity between the two conditions can be attributed to the cognitive process of interest. • Can maximize sensitivity by minimizing inter-trial interval and by evoking cognitive processes in a homogenous manner.
Assumptions • Pure Insertion: adding to a pre-existing set of cognitive processes without affect. Ex. Button response • Linearity: transformation of neural activity into fMRI signal is linear. The response to the temporally adjacent stimuli is equal to the total response of the separated stimuli.
The “Cons” of Block Designs • Constrained to grouping trials of the same type with each other in time. Does not allow for randomization of the order of stimuli. • Imaging signal may be the result of anticipatory behaviors in which the subject engages before the presentation of each stimulus. • Fixed: A-B-C-A-B-C-A-B-C • Random: A-C-C-B-A-B-C-B-A
The “Pro’s” of Block Designs • Superior Statistical Power: • Easy to position fundamental frequency • Optimal block length for a boxcar design is 30 sec but if the subject’s hemodynamic transfer function is known then 14-20 sec is more desired.
Multi-Conditioned, Caterogical, Blocked Experiments • Can compromise previously mentioned problems with randomization by fixing the order of blocks within a subject but varying (counterbalance) this order across subjects.