How do I present a letter, an SOA, then a tone and collect RT for both?

[I]I've added some detail to my question[/I]
[I]Here is how I'd like each trial in my dual task dual RT experiment to go[/I]

Black background, white letters
Fixation +
Spacebar press
400 ms delay
presentation of randomized H O or S at centre of screen make speeded
response to idenity of letter using j, k, l respectively record RT letter is
erased after key press SOA from onset of letter of 50, 111, 245, 542, or
1200 ms Presentation of randomized low or high pitched tone for 250 ms Make
speeded response to pitch of the tone using letter a or s respectively
Record RT Must respond to the letter before responding to the tone.

Note that the SOA is from the onset of the letter, not the response to the letter and thus there must be two events, the SOA and the tone presentation, before the letter response and there must be one event following the presentation of the tone but before the tone response (the letter response). This experiment is demonstrating that tone responses will be delayed by SOA.

So the SOA of the tone is relative to the onset of the letter rather than to the [I]repsonse[/I] to the letter? e.g.:

@0ms: letter
@246ms: tone
rt to the letter
rt to the tone

vs

@0ms: letter
rt to the letter
@rt + 246ms: tone
rt to the tone

Thanks for your interest
Here is the paradigm in more detail
letter presented until a speeded 3AFC response is made based on the letter's identity and then is cleared
stimulus onset asynchrony of 50, 11 245 or 542 ms following onset of letter
tone presented, speeded 2AFC response to the tone required
must respond to letter before tone

e.g.
H - wait for response
SOA of 50 ms
high tone presented - may make speeded response after response to letter
end of trial

The idea is that responses to the tone will be slower when the SOA is shorter. I will also be adding a difficulty manipulation to the tone task (high or low loudness) but I can work this out later.

OK, because you say "SOA of 50, 11, 245 or 542 ms following onset of the letter" it sounds like the SOA is relative to the letter and is thus [I]independent[/I] of the [I]response[/I] to the letter, yes?

OK, if so, then you can achieve that by creating a sound file for each of your SOAs--each having x milliseconds of silence at the beginning. E.g., a 245ms SOA would be achieved by inserting 245 ms of silence at the start of the sound file. This can be done easily in a freeware sound editor like Audacity (let me know if you have any trouble with it).

Then... you play the sound file before the letter (yes, [I]before![/I]) with a time value of [I]0ms[/I]. This will cause the letter to be presented in perfect synch with the sound file. This is because, with sound files, the time value defines how long to wait before the next stimulus is presented. If the sound has not finished playing, then it plays out while the trial continues. This is what gets you the various SOAs.

You can then list a blank stimulus after the letter to capture a second RT. If the codes for the letter and the sound responses are both listed for each stimulus then you should catch them both no matter the order in which they occur.

Just remember that the second RT will be the number of ms elapsed from the first RT. So you may need to add the first and second RT together when you process the data.

Hope that made sense, but I have a tiny sense it may have been very confusing. Let me know if you would like me to elaborate on or clarify any of this.

Hi,
I had thought of the 'attach an SOA to the sound" idea but had no idea how to create a sound file with such a specific amount of time in it.
Any thoughts on how to edit a sound file?
Thanks
Carolyn

Definitely. There is a freeware program called Audacity you can get from download.com. It gives you precise control over sound files at the millisecond level. Open you sound file and place your cursor at the start of the sound wave. Select "Insert Silence" from the "Edit" menu. Enter the SOA, e.g., ".241" for a 241ms SOA (note the decimal!!). Save the file, and that's it. Audacity will let you zoom in to see the sound amplitude at any given ms. This is a nice way to confirm that the audible sound is starting at the appropriate moment. That's it. Let me know if you have any trouble with it!