Parametric design

Parametric designs are so called because the same task is presented throughout the experiment but some continuously variable parameter of the task is experimentally manipulated.(6,7) For example, we could ask subjects to perform the semantic analysis task for 5 min, but continuously vary the interval between consecutive stimuli (words) from 10 s at the start of the experiment to 1 s at the end. Here we are assuming that as the task becomes more difficult, i.e. the interstimulus interval becomes shorter, blood flow to the regions specialized for semantic analysis will increase. The main advantage of this design is that it avoids the assumption of cognitive subtraction; the main disadvantage is that it may lack specificity. Motor and visual cortex, as well as brain regions specialized for semantic analysis, will probably show an increased blood flow as the rate of stimulus presentation is increased.

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