Nucleus accumbens shell activity reflects reward magnitude and training duration during Pavlovian and operant conditioning

• Reward size is encoded in nucleus accumbens shell mainly at reward delivery. • Cue responses shift with learning, showing inhibition that tracks reward magnitude. • Action tasks reveal pre-action ramping and stronger inhibition for larger rewards. The nucleus accumbens shell (NAcs) is central to reward-guided behaviour, yet how it encodes reward magnitude across different learning modes and training stages remains unclear. Using fibre photometry, we recorded calcium activity in NAcs neurons of rats during Pavlovian and operant tasks with small and large rewards in early and late training. NAcs activity consistently reflected reward magnitude after reward receipt, but the timing and form of this signal depended on task and experience. In Pavlovian conditioning, cue-evoked responses initially increased but shifted to magnitude-dependent inhibition with extended training. In operant tasks, activity ramped before actions regardless of reward size, while post-reward inhibition was stronger and longer after large rewards. These findings demonstrate that NAcs neurons encode reward magnitude primarily at the point of reward receipt, with limited cue-related encoding, and that this encoding adapts with both learning type and training duration. This work identifies dynamic, context-specific roles of the NAcs in shaping reward-driven behaviour.