Calvin uses the term superficial layers (see figure 12)7.1 to refer to the three top layers of pyramidal neurons, as identified by cell size and axon-packing density: level one is situated immediately beneath the pial-glial membrane, level two, of small pyramidal cells, and level three, of larger pyramidal cells, whose apical dendrites reach upward to level one, and whose axons extend to lower (cortical and sub-cortical) regions. The distribution of the neurons (c. 5mm apart), their repetitive firing, and their mutual re-excitation and consequent entrainment provide, as he supposes, significant clues to the nature of the self-organization of neuronal firing patterns.
The consequent behavior examined in superficial layers exhibits characteristics not unlike the emergent synchronicity in a local population of firing fireflies, say those in a single tree. Individual fireflies light up in independent sequences but the proximity of the nearest firefly provides a weak entrainment influence that eventually propagates to the entire tree. Eventually all fireflies light-up synchronously. This kind of phenomena is related to feedback behavior; a concept that will explored later in this chapter.
It is this kind of organization that the Calvin pattern-propagation model seeks to illustrate in the context of the superficial layers of neocortex.