The operant apparatus consisted of 12 identical boxes that measured 21.6 × 19.7 × 12.7 cm inside, with 2 sides constructed of clear acrylic and 2 sides of aluminum (MedAssociates, St Albans, VT). The operant box was contained in a sound and light attenuated chamber that was equipped with a fan for ventilation and background noise. An LED/nosepoke infrared detector was centered on the 19.7 cm side at 6.3 cm above the floor, and illumination of that LED signaled the beginning of a trial. Below the LED/nose-poke detector was the sipper access hole, through which the sipper descended when mice were being rewarded. The sipper tube was a 10-ml graduated plastic serological pipette fitted with a stainless steel tip (Ancare, Belmont, NY.) The sipper tube could be extended into the box for varying amounts of time, which allowed precise control of sipper access. Consumption volumes (±0.1 ml) were visually read from the tube. The sipper tube was filled with a solution of 0.0316% (w/v) saccharin solution. Levers were mounted 2.5 cm above the floor on either side of the sipper
