> In a paper we have submitted recently, one suggestion was that we should
> “covary out” reaction times to exclude that differences between our 3
> conditions are due to reaction time effects (all 3 conditions differ with
> respect to RTs) Although i am not
> convinced that covarying Rts out is necessarily a good idea, i do think
> that i will have to do this due to the reviewers comments.
> From previous discussions on this list and from my own limited
> understanding, i think there are in principle a number of ways to do this.
> I would be very grateful if somebody more knowledgeable than me could <inform
> me, whether my ideas are correct and if there any other ways to achieve <this
> (i.e. “covary out” RTs to show that condition differences “can't” be
> explained as simple RT effects)
The simplest way to do this is to model the three trial types with stimulus
functions as in the original model but now add a fourth trial type that includes
all three conditions. Modulate this with the trial-specific RTs. This will
explain away
any evoked responses that can be explained by RTs. Any contrast comparing the
three conditions now tests for condition-specific effects that cannot be
explained
by simple RT differences. Note you can only covary out the RT effect at the
first level because the RTs are specific to each [within-subject] trial.
On a philosophical note, adjusting for RT effects has to be motivated carefully.
This is because RT is a dependent behavioral measure, not an independent
explanatory variable. RTs might be the psychophysical correlate of the
neuronal response you are interested in (and should not be treated as a
confound).
In some cases people consider that non-specific activity between a cue and
response is
uninteresting. In this case, RT can be used as a surrogate measure for this
confounding
effect.
I hope this helps - Karl
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