Research concerning the development of learning and memory suggests that there are multiple memory systems. These systems differ in complexity, underlying neural substrates, and consequently, their developmental emergence. Pavlovian conditioning, and specifically eyeblink conditioning (EBC), allows researchers to investigate both simple and complex forms of learning and memory early in development. Delay EBC, which is considered a relatively simple form of learning, involves the association of a conditioned stimulus (CS) with an unconditioned stimulus (US). Research from our laboratory suggests that the emergence of delay EBC is dependent on the development of sensory input to the pontine nucleus. Trace EBC, a more complex form of learning, involves the association of a CS with a US over a stimulus-free trace interval. Due to its relatively late emergence, the developmental time course of trace EBC has been traditionally regarded as independent of sensory system development. Rather, it is the involvement of late-developing structures such as the hippocampus which is considered the principle limiting factor in the emergence of trace EBC.
The current collection of studies investigates the developmental emergence of delay and trace conditioning. We found that both delay and trace conditioning are facilitated by using an early-developing somatosensory CS. This suggests that the sensory system development plays a role in even late-developing trace EBC. Moreover, hippocampal CA1 neuronal activity shows increased responsiveness in even very young animals when trained with an early-developing somatosensory CS compared to those trained with a tone CS. Combined, these data suggest that both hippocampal and sensory system development may play key roles in the developmental emergence of learning.