Problem with readmouse and background

Hello everyone,

I have run into a problem with using readmouse.

For my experiment I need a white background. Consequently, I am using in the task

  show background 250 250 250

which works perfectly.

I show a few stimuli around the center and after a few ms they disappear. An arrow points into a direction and I am asking which item was shown in that direction.

To answer the question I show all stimuli again in a row at the bottom of the screen. I wanted to use readmouse to answer by clicking on the correct item.

Despite using range in readmouse (readmouse l 1 60000 range 12 15) it appears that only the background is recognised. I use set $bit_nr bitmap-under-mouse MOUSE_X MOUSE_Y after readmouse in order to identify the chosen item. I saved the variable $bit_nr to see if it shows the correct bitmap number but the only number that is saved in the variable $bit_nr is 1, which is the background first shown at the start of the task.

I am not really sure how to proceed from this way forward. I tried a while loop (see below)

while $bit_nr == 1
readmouse l 1 60000 range 12 15
set $bit_nr bitmap-under-mouse MOUSE_X MOUSE_Y
while-end

but this freezes the task completely.

Had anyone else this problem and found a solution to get the correct bitmap number when using a different background color and readmouse with its range function?