The scientists studied the effect of stimulating serotonin-releasing neurons (black arrows) in the medial prefrontal cortex (mPFC), the orbitofrontal cortex (OFC) and nucleus accumbens (NAc) on waiting time, under variable time trials (e.g. food delivered after 2,6 or 10 seconds) and under fixed time trials (e.g. food delivered after 6 seconds). No effect was seen when neurons in the NAc were stimulated. A weak effect led to a small increase in waiting time when neurons in the mPFC were stimulated in variable time trials. A strong effect was seen when neurons in the OFC were stimulated in variable time trials and a weak effect was seen in fixed time trials. The thickness of the dashed arrows indicate the strength of the effect.