“Yerkes (1912) trained an earthworm to choose one arm of a T-maze, using electric shock as punishment for error and the moist burrow as reward for correct choice. The habit was acquired in twenty trials, 2 days at ten trials per day, about what might be necessary for the laboratory rat. No errors were made on the third day, though the behaviour was somewhat inconsistent in the following week as between good days and bad days (even worms have them). Yerkes then removed the brain, or principal ganglia, by cutting off the head—the anterior four and a half segments. The animal continued to respond correctly, showing that there were sufficient synaptic modifications in the remaining ganglia to mediate the response—until the new head regenerated, at which time the habit was lost. The noise generated by the new ganglia, the irrelevant neural activity of the uneducated brain, was sufficient to disrupt learning completely.” (Hebb 1961, p. 78)
Mammals, unlike earthworms, after reaching adulthood have a very limited capacity for neurogenesis (Charvet and Finlay 2018; Hebb 1961; Sorrells et al. 2018). A major difference between mammals and the other classes of vertebrates such as fishes, amphibians, and reptiles (but not birds) is that the vertebral column of mammals stops growing shortly after sexual maturity (Hariharan et al. 2016; Ngwenya et al. 2013; Williams 2017), and the number of cells contained in the brain remains relatively constant, with major alterations for learning occurring by way of changes at synaptic connections (Bernocchi et al. 1990; Font et al. 2001; Hebb 1949; Kandel 2006; Zupanc et al. 1995). Consequently, memories can be stored within the brain over the lifetime of a mammal, since the same neurons remain in place (Hebb 1961; Square 2001); the same is not true of an earth worm that has been neurally assaulted. Finally, if the neurons within the plastic portion of the telencephalon of the songbird is partially silenced then this should disrupt the entire repertoire of songs learned due to alterations in the connectivity between neurons, which introduces noise into the system (Hebb 1961, p. 78).
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