Baselines and contrasts.

What is an appropriate comparison? Donders subtraction logic Only works if pure insertion is met – i.e., that inserting or subtracting a given component does not alter the other component processes. do you know the component processes involved in your critical task? (i.e., look to behavioural precedents, conduct your own behavioural experiments outside and inside the magnet) is your subtraction / insertion appropriate given your fMRI design? (i.e., given the temporal constraints involved are your comparisons appropriate – may be less of a concern for ER-fMRI)

Fixation or rest – is the brain ever really doing nothing? Early on the most common comparison or baseline condition was rest or simple fixation. In most cases this will be inappropriate. Noise – men think about sex 4,392 times a day! How does this affect their fMRI baseline? Would assume this would be constant across conditions and so should not show up in fMRI signal. But how confident can you be of this assumption? Especially important in memory research. No control over what the subject is doing during such a condition. Sleeping subjects – with only a little fixation cross to look at for 20 seconds subjects can and do go to sleep in the magnet! They then have a startle response upon waking up during the critical run that may contaminate your data! You can check for sleepers by having them make a response – i.e., no response usually indicates the subject is sleeping!

Memory and comparison conditions – is the brain always doing something? When trying to look at hippocampal activation one difficulty may be that the hippocampus is always activated for any given task. This could prove particularly difficult when examining component processes of memory – e.g., encoding vs. retrieval. Working memory poses many of the same problems – any time you have to manipulate a stimulus you must use working memory. One possible solution is to manipulate timing of data acquisition (e.g., for encoding vs. retrieval) and memory load (i.e., for working memory).

Example Experiment question – what areas are active for simple motor tasks? problems: – can’t know if these areas are active for all kinds of motor tasks or only for simple ones – given that nothing happens (stimulus wise) in the rest condition, activation may also reflect response to instruction stimulus (which is absent in baseline condition) Simple finger tapping vs. rest protocol images % s i g n a l c h a n g e simple tappingrest

Example Experiment question – what areas are active for complex vs. simple motor tasks? problems: – direct comparison is OK but baseline may still inappropriate (e.g., when looking at single main effects) Simple finger tapping vs. complex finger tapping vs. rest protocol images % s i g n a l c h a n g e simple tappingrest complex finger tapping

Example Experiment can now look at motor components that are independent of execution of movement – different question now being asked problems: – task is becoming more complex – ordering of conditions becomes important as does instructions Simple vs. complex finger tapping vs. imagined tapping vs. rest protocol images % s i g n a l c h a n g e sequential tappingrest complex finger tapping imagined finger tapping

Example Experiment now that’s just madness!!! Focus your question appropriately and remember Jody’s rule of thumb – 4 conditions as a maximum! Simple vs. complex finger tapping vs. imagined tapping vs. imitation vs. rest protocol images % s i g n a l c h a n g e sequential tappingrest complex finger tapping imagined finger tappingimitation finger tapping