Donald Hebb (1949) posited that the neocortex is an organ that mediates learning in animal (specifically mammals) and that this learning comes about by changing the synaptic weights between neurons, a change that signifies the storage of new declarative, conscious information (Tehovnik, Hasanbegović, Chen 2024). Two prominent structures in the brain mediate learning, the neocortex and the cerebellum, which together represent over 95% of the neurons in the brain (Herculano-Houzel 2009). Unlike the neocortex, the cerebellum is modified during learning by changing the executable motor code (i.e., the efference-copy code) stored by the Purkinje neurons (Bell et al. 1997; Chen 2019; Cullen 2015; De Zeeuw 2021; Fukutomi and Carlson 2020; Loyola et al. 2019; Miles and Lisberger 1981; Noda et al. 1991; Shadmehr 2020; Tehovnik, Patel, Tolias et al. 2021; Wang et al. 2023).

Glial cells that modify synapses and circuit connectivity are the astrocytes (Herculano-Houzel 2014; Um 2017). They can trigger dendritic arborization and receptor augmentation, as well as provide metabolic support. Glial cells that myelinate the axons are the oligodendrocytes, and the microglia remove debris and prune the synapses thereby facilitating plasticity (Um 2017). At least in humans, the proportion of glial cells is 20% astrocytes, 75% oligodendrocytes, and 5% microglia (Pelvig et al. 2008). During learning both synaptic connectivity and myelination are enhanced (Blumenfeld-Katzir et al. 2011; Hosoda et al. 2013; Kalil et al. 2014; Kitamura et al. 2017). For the neocortex the ratio of glia to neuron is 3 to 1, whereas for the cerebellum the ratio is 0.15 to 1 (see figure 3 of Herculano-Houzel 2014). Thus, the 20-fold increase in the glia to neuron ratio for the neocortex highlights its role in learning and consciousness (Hebb 1949, 1968). Moreover, this establishes the importance of the high metabolic rate exhibited by the neocortex in sustaining consciousness (see Figure 1; Chomsky 2023; James 1890; Herculano-Houzel 2011).

Figure 1: The illustration is modified from figure 1b of Herculano-Houzel (2011).