In Heidelberg, Crusio became interested in the inheritance of variations in the neuroanatomy of the mouse hippocampus. He studied the intra- and infrapyramidal mossy fiber (IIPMF) projections in this brain area. Crusio showed that about 50% of the variation found between different inbred mouse strains in the sizes of their IIPMF were due to genetic differences. Herbert Schwegler and Hans-Peter Lipp had found that these variations are correlated with learning a two-way active avoidance task. Animals with smaller projections learned much faster than animals with larger IIPMF. Together with Schwegler and Lipp, Crusio showed that an inverse correlation could be found for spatial learning in a radial arm maze task. Animals with larger IIPMF learned better in this task. Injecting mouse pups with thyroxine results in larger IIPMF projections. When mice from a strain with small IIPMF projections zere treated in this way, they could learn better in the radial maze. Crusio and his collaborators think that this correlation is causal. Not everybody believes this.