I suppose it depends on what your question is.

If you ask subjects to respond as quickly as possible,
then the RT can be correlated with the MEG data. This
may actually produce much better results for certain
types of experiments than waiting for a few seconds
and giving a GO signal.

Many experimental designs follow a pattern such as,
when you see the stimulus, respond according to X,
where X is unrelated to the goal of the experiment.
For example, given a speech sample, respond according
to gender. The experimenters, however, are not really
interested in gender identification, they really want
to see the brain's response to emotional valence,
or semantic category. By using such a design, the
experimenters assume the processes of interest
are automatic, pre-conscious. This was probably fine
a few years ago, when primary sensory activity was
a hotter topic than it is today. I think we should
be getting past that, personally, and go ahead and
ask people to respond according to what we are really
interested in. MMN experiments, for example, often
have the subject reading a book or something while
beeps and boops are playing ... but if the subject
actually attends to the differences, the brain
responses can be much larger.

Delaying the responses is not quite the same thing,
of course; while Piers is correct in saying that
the response activity occurring so close to the
stimulus could confound the result, using a delay
will also change the situation to include a memory
component. Maybe you are interested in that. Fine.

On the other hand, modern source localization
techniques can separate the signals sufficiently
to disambiguate the response components from the
stimulus components, I would say, because they are
going to be generated from different brain region.

Piers Cornelissen wrote:
> Dear all,
> 
> I have noticed a number of recent MEG papers which report experiments 
> that require participants to give button presses and/or overt spoken 
> responses on each trial of the task. There appears to be no attempt to 
> delay these responses, rather the investigators appear to expect 
> participants to respond within very much the same kinds of time frame as 
> standard behavioural tasks. Moreover, in some of these studies, there 
> appears to be no explicit attempt to model the explicit responses and 
> disentangle them from other signal components.
> 
> Should I be surprised? Have I missed a trick in the literature? Is this 
> now a reasonable thing to do?
> 
> I would value the MEG communities opinion.
> 
> Regards
> 
> Piers Cornelissen

-- 
Dr. Tom
---
I would dance and be merry,
Life would be a ding-a-derry,
If I only had a brain.
        -- The Scarecrow