For example
<picture
alcoholR>
/ items = ("1P_6.jpg","1P_7.jpg","1P_8.jpg","1P_9.jpg","1P_10.jpg", "1P_15.jpg", "1P_16.jpg", "1P_17.jpg", "1P_18.jpg")
/ select = noreplace
...
</picture>
<picture nonalcL>
/ items = ("2P_6.jpg","2P_7.jpg","2P_8.jpg","2P_9.jpg","2P_10.jpg", "2P_15.jpg", "2P_16.jpg", "2P_17.jpg", "2P_18.jpg")
/ select = picture.alcoholR.currentindex...
</picture>
defines 9 pairs. However, *two* of your <trial> elements sample from those stimulus elements above:
With
left arrow:
<trial alcoholBL>
...
/ stimulustimes = [0 = fixation; 500 = erasefixation,
alcoholR, nonalcL, arrowL; 700 = blankscreen, drink_nonalc, drink_alc]
...
</trial>
With
right arrow:
<trial alcoholBR>
...
/ stimulustimes = [0 = fixation; 500 = erasefixation,
alcoholR, nonalcL, arrowR; 700 = blankscreen, drink_nonalc, drink_alc]
...
</trial>
Think of the two paired <picture> elements as a single deck of 9 cards labeled A to I. Think of <trial alcoholBL> as your left hand and <trial alcoholBR> as your right hand. Now start drawing cards from the deck (without replacement) randomly with your left and your right hand. Once there are no more cards left in the deck, reshuffle it and repeat once more.
Question: Is there any reason why you should have ended up with having drawn the cards {A,B,C,D,E,F,G,H,I} once with your left hand and the also cards {A,B,C,D,E,F,G,H,I} once with your right hand? No.
What you need to do is set up separate stimulus elements -- one set for each <trial> element to draw from. I.e. you need to do
<picture
alcoholR_arrowL>
/ items = ("1P_6.jpg","1P_7.jpg","1P_8.jpg","1P_9.jpg","1P_10.jpg", "1P_15.jpg", "1P_16.jpg", "1P_17.jpg", "1P_18.jpg")
/ select = noreplace
...
</picture>
<picture nonalcL_
arrowL>
/ items = ("2P_6.jpg","2P_7.jpg","2P_8.jpg","2P_9.jpg","2P_10.jpg", "2P_15.jpg", "2P_16.jpg", "2P_17.jpg", "2P_18.jpg")
/ select = picture.alcoholR_arrowL.currentindex...
</picture>
for use in
<trial
alcoholBL>
...
/ stimulustimes = [0 = fixation; 500 = erasefixation,
alcoholR_arrowL, nonalcL_arrowL, arrowL; 700 = blankscreen, drink_nonalc, drink_alc]
...
</trial>
and
<picture
alcoholR_arrowR>
/ items = ("1P_6.jpg","1P_7.jpg","1P_8.jpg","1P_9.jpg","1P_10.jpg", "1P_15.jpg", "1P_16.jpg", "1P_17.jpg", "1P_18.jpg")
/ select = noreplace
...
</picture>
<picture nonalcL_
arrowR>
/ items = ("2P_6.jpg","2P_7.jpg","2P_8.jpg","2P_9.jpg","2P_10.jpg", "2P_15.jpg", "2P_16.jpg", "2P_17.jpg", "2P_18.jpg")
/ select = picture.alcoholR_arrowR.currentindex...
</picture>
for use in
<trial
alcoholBR>
...
/ stimulustimes = [0 = fixation; 500 = erasefixation,
alcoholR_arrowR, nonalcL_arrowR, arrowR; 700 = blankscreen, drink_nonalc, drink_alc]
...
</trial>
In other words: You need *two* identical decks of 9 cards -- one to draw from with your left hand, one to draw from with your right hand.