We hypothesized that novel stimuli represent salient learning
signals that can motivate 'exploration' in search for potential rewards. In
computational theories of reinforcement learning, this is referred to as the
novelty 'exploration bonus' for rewards. If true, stimulus novelty should
enhance the reward anticipation signals in brain areas that are part of
dopaminergic circuitry and thereby reduce responses to reward outcomes. We
investigated this hypothesis in two fMRI experiments. Images of complex natural
scenes predicted monetary reward or a neutral outcome by virtue of depicting
either indoor or outdoor scenes. Half of the reward-predicting and neutral
images had been familiarized the day before, the other half were novel. In
experiment 1, subjects indicated whether images were novel or familiar, whereas
in experiment 2, they explicitly decided whether or not images predicted reward
by depicting indoor or outdoor scenes. Novelty led to the hypothesized
enhancement of mesolimbic reward prediction responses and concomitant reduction
of mesolimbic responses to reward outcomes. However, this effect was strongly
task-dependent and occurred only in experiment 2, when the reward-predicting
property of each image was attended. Recognition memory for the novel and
familiar stimuli (after 24h) was enhanced by reward anticipation in both tasks.
These findings are compatible with the proposition that novelty can act as a
bonus for rewards under conditions when rewards are explicitly attended, thus
biasing the organism towards reward anticipation and providing a motivational
signal for exploration.