Frontoclaustral interrelations in cats and dogs.

In 13 cats and 5 dogs various parts of the frontal lobe were removed. As a result terminal degenerations in the rostral quarter of the insular claustrum were found by Nauta and Fink-Heimer II methods. In cats with motor cortex lesions, the degenerations were localized more dorsally than in cats with removals of the frontoorbital cortex, but there is a large overlap between degenerating areas of the claustrum in both groups of animals. In the dog it even seems possible to differentiate claustral regions in which axons from various gyri of the frontal lobe mainly terminate. It was found under the electron microscope that degeneration affects asymmetrical axodendritic synapses which make contacts with small dendritic branches. Comparing these results with severe cellular degenerations found in the claustrum following large neocortical removals it seems that the rostral part of the claustrum is abundantly interconnected with the frontal cortex and interrelations between the frontal granular cortex and claustrum may be considered as close. There are two hypotheses concerning frontoclaustral relations: either the rostral pole of the claustrum belongs, like the dorsomedial nucleus of the thalamus mainly to the frontopetal ascending system which has frontoclaustral feedback connections or it is incorporated into a descending system through which the frontal cortex controls the functions of other subcortical structures.