Trace conditioning: insights from invertebrate models on bridging the temporal gap

Trace conditioning is the ability to form associations between a conditioned stimulus (CS) and an unconditioned stimulus (US) separated in time. This fundamental cognitive process allows organisms to predict outcomes, avoid danger and exploit resources even when predictive stimuli are temporally discontinuous. This process has been thought to depend on complex neural structures such as the mammalian hippocampus, marking it as exclusive to vertebrates. However, emerging evidence from honeybees, fruit flies and molluscs challenges this notion, showing that trace conditioning can occur in invertebrates with relatively simple nervous systems. We highlight some current findings on trace conditioning across invertebrate species and explore promising directions for future research. We propose a two-tier mechanistic model that integrates stimulus salience and attentional processes into trace learning. We highlight the great pond snail, Lymnaea stagnalis, as a model for utilizing this framework. Our framework is based on rapid, stimulus-specific encoding of the conditioned stimulus and attention-based memory. By varying CS-US intervals, stimulus salience and distractor presence, we propose a paradigm to explicitly test how sensory inputs are transformed into persistent, attention-based memory during trace learning. This work lays a foundation for a broader, mechanistically informed framework for learning and memory across the animal kingdom.