Thanks Dave!
Does this seem like I'm at all on the right track? Where I'm trying to adjust "rightPic" to match "leftPic"
(excuse the poorly written code, I'm still very much a novice):
<values>
/ Brightness = 3
</values>
<item leftPicList>
/ 1 = "leftPic.PNG"
</item>
<picture leftPic>
/ items = leftPicList
/ position = (25, 50)
</picture>
<item rightPicList>
/ 1 = "rightPic_brightness1.PNG"
/ 2 = "rightPic_brightness2.PNG"
/ 3 = "rightPic_brightness3.PNG"
/ 4 = "rightPic_brightness4.PNG"
/ 5 = "rightPic_brightness5.PNG"
</item>
<picture rightPic>
/ items = rightPicList
/ position = (75, 50)
/ select = values.Brightness
</picture>
<trial myTrial>
/ stimulustimes = [1 = leftPic, rightPic ]
/ inputdevice = keyboard
/ validresponse = (28, 203, 205)
/ response = free
/ responseinterrupt = trial
/ isvalidresponse = [if (trial.myTrial.response == 205) values.Brightness += 1; picture.rightPic.item]
/ isvalidresponse = [if (trial.myTrial.response == 203) values.Brightness -= 1; picture.rightPic.item]
/ isvalidresponse = [if (trial.myTrial.response == 28) trial.myTrial.next]
</trial>
<block myBlock>
/ trials = [1-4 = myTrial]
</block>