Sets of neurons in somatic cerebral cortex of the cat and their ontogeny.

The process of set formation is briefly reviewed and five monothetic schemes for classification of neurons in the somatic cerebral cortex are described. Criteria for evaluation of neuronal sets are presented and applied to the five different monothetic classification schemes. Classification by size and distribution of peripheral receptive fields orders existing data on cortical neurons better than classification by possession of an axon in the pyramidal tract, by modality, by lability of receptive field, or by 'lemniscal properties'; however, no monothetic scheme orders all the data. A useful polythetic scheme, using s and m terminology is suggested. The ontogeny of the cerebral cortex is reviewed in detail. It is suggested that sa neurons are Golgi type II neurons while m neurons are Golgi type I neurons. The hypothesis is presented that wide-field or m neurons develop and are recognizable before small-field or sa neurons in ontogeny. Evidence regarding this hypothesis is indirect, often conflicting, but suggestive that the hypothesis may be correct. The idea that m neurons may also be phylogenetically older than sa neurons is presented and shown to be consistent with ontogenetic data and interpretations.