# Alternately show canvases and record keypresses,
# until a key has been pressed.
self.set('flag',0)
for i in range(N):
    t1 = self.time()
    canvas_list[i].show()
    key, t2 = my_keyboard.get_key()

# wait for
# the first keypress post the position in the loop that is marked by the variable 'critops'...
    if key is not None and i > self.get('critpos') and self.get('flag' ) == 0:
        if key == self.get("correct_response"): # if it's the correct key ...
            correct_sound.play()
            self.sleep(SOA-(t2-t1))     # wait for the next canvas for SOA minus RT
            self.set('flag',1)      # a button has been pressed, stop checking
        else:                   # otherwise ...
            incorrect_sound.play()
            self.sleep(SOA-(t2-t1))     # wait for the next canvas for SOA minus RT
            self.set('flag',1)      # a button has been pressed, stop checking