including stimuli that only appears in half trials, randomly

[signy]

Hi,
I am trying to figure out how to present a stimuli (and record data)
that appears on only 1/2 the trials of the experiment?
Specifically, I am planning on presenting a word to which participants
have to make 12 responses (key presses). A participant will do this
for 40 different words, but for 20 of them (picked randomly), I would
like for them to respond to an additional stimuli that is presented
mid-way through their 12 responses?
Visually,
[item]
resp1
resp2
resp3
resp4
resp5
resp6
          <-- here, for half the trials, participants will be
presented with another screen/stim asking
               about their strategy of the 6 responses.
resp7
resp8
resp9
resp10
resp11
resp12

Any ideas on how to do this?

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