Differential retroactive interference in humans following exposure to structured or unstructured learning material: a single distributed neural network account

While retroactive interference (RI) is a well-known phenomenon in humans, the differential effect of the structure of the learning material was only seldom addressed. Mirman and Spivey (2001 , Connection Science , 13 : 257-275) reported on behavioural results that show more RI for the subjects exposed to 'Structured' items than for those exposed to 'Unstructured' items. These authors claimed that two complementary memory systems functioning on radically different neural mechanisms are required to account for the behavioural results they reported. Using the same paradigm but controlling for proactive interference, we found the opposite pattern of results, that is, more RI for subjects exposed to 'Unstructured' items than for those exposed to 'Structured' items (experiment 1). Two additional experiments showed that this structure effect on RI is a genuine one. Experiment 2 confirmed that the design of experiment 1 forced the subjects from the 'Structured' condition to learn the items at the exemplar level, thus allowing for a close match between the two to-be-compared conditions (as 'Unstructured' condition items can be learned only at the exemplar level). Experiment 3 verified that the subjects from the 'Structured' condition could generalize to novel items. Simulations conducted with a three-layer neural network, that is, a single-memory system, produced a pattern of results that mirrors the structure effect reported here. By construction, Mirman and Spivey's architecture cannot simulate this behavioural structure effect. The results are discussed within the framework of catastrophic interference in distributed neural networks, with an emphasis on the relevance of these networks to the modelling of human memory.