Sherrington (1906) predicted that the neocortex mediates all program changes to movement (which is regulated by consciousness/learning, Hebb 1949, 1968), whereas the cerebellum maintains a steady flow of movement once the changes are put in place (that is, once the efference-copy code is reset in the cerebellum via neocortical intervention, 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 et al. 2021; Wang et al. 2023). That the human brain is composed of 86 billion neurons with the neocortex accounting for 16 billion and the cerebellum accounting for 69 billion leaving some one billion for the remaining structures (Herculano-Houzel 2009) is well accepted. The remaining one billion neurons (1% of the total) are left for sorting out functional details as they pertain to the olfactory bulb for the sense of smell and the thalamus for relaying gustatory, somatosensory, vestibular, auditory, and visual information. Moreover, the superior colliculus is for orienting toward and away from external stimuli, and the hypothalamus is connected to the hormonal and vascular system, and finally various brain stem nuclei such as the locus coeruleus are for transitioning between wakefulness and sleep and the substantia nigra is for mediating behavioral drives, i.e., the speed of emotive responses. Lastly, we cannot forget innervations of the autonomic and ocular and skeletal nuclei situated in the brain stem and spinal cord to finalize glandular secretions and muscle contractions. The 1% of neurons in the human brain—without which death would ensue—is present in all vertebrates. Thus, what distinguishes humans from other vertebrates is the ratio of neurons utilized for information storage in the telencephalon (i.e., neocortex of mammals) and cerebellum versus the neurons in the brain stem and spinal cord, which are required to maintain the organism (see Figure 28). It is obvious that consciousness must scale with this ratio in vertebrates (Hebb 1968; Koch 2013; Morgan 1900). As for invertebrates, a similar segregation between the capacity to store information and to maintain the organism must exist. Just how segregated these two properties are amongst the ganglia of invertebrates needs clarification. Indeed, ants have a communication system based on pheromones with a throughput of 1.4 bits per second (Tehovnik et al. 2021), which is based on a 20-item pheromone alphabet (Hölldobler and Wilson 1990; McIndoo 1914).[1] It is unclear where this alphabet is stored, but some have suggested that information related to pheromone communication is house separately from the general olfactory sense (Nishikawa et al. 2012).

Footnote:

[1]The bit-rate is low because the olfactory system is slow acting, taking over a second to be engaged (McIndoo 1914).

Figure 28. The vertebrate brain is made up of the telencephalon (cerebrum that includes the hippocampus), the cerebellum, the optic tectum, and the olfactory bulb. Not labelled is the brain stem, and not shown is the spinal cord. The cerebellum in the lamprey and amphibian is small and therefore not marked; it sits on top of the brain stem. The telencephalon co-evolved with the cerebellum, since the two structures work in tandem for regulating sensation and movement and they are connected anatomically in all vertebrates (Cheng et al. 2014; Murakami 2017; Murray et al. 2017; Nieuwenhuys 1977; Xu et al. 2016). The sizes of the brains are not to scale.