only continue after pressing two keys?

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only continue after pressing two keys?

https://forums.millisecond.com/Topic13715.aspx

By neusch - 7/1/2014

Hello everyone!

I designed an experiment, where participants should react to stimuli by releasing one out of two keys (here: "A" and "L"). My question is: How can I make sure, that both keys are pressed, before the stimulus is presented? I already created a message ("Please press the keys "A" and "L" now" = presskeyremind).

I only want to measure the time from the presentation of the stimulus until the release of one of the keys.

The trials look like this so far:

/ stimulustimes = [0=presskeyremind; ????=stimulus]

/ correctresponse = (-30)

/ validresponse = (-38)

</trial>

Thanks for your Help!

By Dave - 7/1/2014

I'm having trouble understanding the question.

#1: If you have *two* possible response options (1=release of key A, 2=release of key B), your <trial>'s /validresponse attribute should reflect that:

/ validresponse = (-30,-38)

#2: Re. "I only want to measure the time from the presentation of the stimulus until the release of one of the keys."

If your trial is set up to register key release, it is a logical necessity that the respective key(s) have been pressed beforehand. The way your trial is set up, it will terminate upon the first release of one of the two allowable keys detected *after* the presentation of "stimulus". Latency is measured relative to the presentation of "stimulus", which appears to be exactly what you want. Or am I missing something here?

#3: As a general matter re. "How can I make sure, that both keys are pressed, before the stimulus is presented?"

You cannot. A <trial> in Inquisit collects a *single* response, i.e. you cannot effectively make sure that both keys are in depressed state prior to presenting a stimulus -- it's impossible to do within a single <trial> element, and could be done only imperfectly using several "chained" <trial> elements

<trial presskeytrial>
/ stimulusframes = [1=presskeyremind]
/ validresponse = (30,38)
/ branch = [trial.releasekeytrial]
</trial>

<trial releasekeytrial>
/ stimulusframes = [1=stimulus]
/ validresponse = (-30,-38)

/ correctresponse = (-30)

</trial>

which can only ensure that *at least one* of the keys was pressed prior to starting the stimulus-dislay trial that registers a single key's release.

Hope this clarifies.

By neusch - 7/2/2014

Thanks for your answer Dave! This is exactly what I needed. I have only two questions left:

1. / validresponse = (30,38) means validresponse = "A" OR "L"

Is it possible to have something like validresponse = "A" AND "L" to ensure that both keys are pressed?

2. In your version with "chained" <trial> elements: How can I randomize the stimuli in the second trial in my <block>?

/ validresponse = (30,38)

By Dave - 7/2/2014

Re. #1: No. A <trial> can only ever register a *single* response event, i.e. a single key press or release, a single mouse click, etc. It cannot be made to register several events and treat them as a single response.

Re. #2: You do that in exactly the same way as you would without it. Not sure what exactly the misconception is here, so please elaborate.

By neusch - 7/2/2014

Thank you for your help!

I found a solution for the randomization now and I solved the problem with the "press TWO keys" by constructing three chained trials (Presskey_a --> Presskey_l --> present stimulus).

Have a nice day!

<trial presskeytrial>
/ stimulusframes = [1=presskeyremind]
/ validresponse = (30,38)
/ branch = [trial.releasekeytrial]
</trial> - See more at: http://www.millisecond.com/forums/Post.aspx?TopicID=13715&PageIndex=1&Task=PostTopicReply&ParentID=13715&Update=1#sthash.5VZEjV3W.dpuf

By Dave - 7/2/2014

Great, thanks for the update. One remark:

> I solved the problem with the "press TWO keys" by constructing three chained trials (Presskey_a --> Presskey_l --> present stimulus).

The reason I did not propose something like this is that it suffers from the same problem as the "two-chained-trials"-approach: Nothing prevents you from releasing the A-key in the Presskey_I-trial. Thus there is no guarantee that both keys are indeed in depressed state when you start the "stimulus presenation"-trial.