Vertical ascending connections in the isocortex.

Different fluorescent tracers were applied to the surface of the cortex of rats, marmosets and one hedgehog. Irrespective of the kind of tracer and the depth of penetration, some perikarya of layer VI were labelled in each specimen and in all cortical regions. In the rat almost all labelled neurons were packed in sublayer VIb, in the marmoset such cells were dispersed throughout layer VI, whereas in the hedgehog the degree of their segregation to sublayer VIb was intermediate. Additional experiments in the rat indicated that most of the medium-sized neurons in the VIb layer project to layer I, that most of the perikarya projecting to the thalamus are localized in sublayer VIa, that different neurons project to the thalamus and to the surface of the cortex, and that only very few perikarya in deep parts of layers III and V and of sublayer VIa send axons or axon collaterals to layers I and II.