It is unknown how the response of cannabis users might compare to those of non-users in response to similar dose of physical activity. We will therefore compare brain activation during craving to cannabis, a natural reward (food) and a neutral scene (nature images). The food cues will allow us to determine if effects of exercise on brain activation and craving generalize to two different reward conditions and the nature scenes provide a neutral, no rewarding control. In addition to the analysis of predetermined regions of interest, whole-brain exploratory analyses will also be conducted to examine for additional brain regions showing associations between various conditions of interest and regional brain activation.