24 CHAPTER ONE
Habit, Learning, and Synaptic Plasticity
The term plasticity was used in eighteenth- century materials science to
describe the malleability of matter, and in the nineteenth century to denote
the ability of organisms to change in response to environmental demands
(Berlucchi and Buchtel 2009). Early scientific conceptions of neural plas-
ticity seem to have some relation to both meanings of the term. William
James (1890), for example, noted that all matter, including nervous tissue,
changes structure in the face of a “modifying cause.” He defined plasticity
as the possession of a structure weak enough to yield to an influence, but
strong enough not to yield at once. Matter changes, and it resists change;
James argued that the dual ability of neurobiological matter to both modify
and stabilize, in relation to the behaviors of persons, explains why people
develop habits or characteristic propensities.^1 James assumed a hydraulic
model of the nervous system, which allowed him to consider plasticity in
broad physical terms. With the establishment of the neuron theory, later
known as the neuron doctrine, the brain’s plasticity was tied to the synapse.
The neurons that Ramon y Cajal, an anatomist at the University of Bar-
celona, described in 1887 were unlike other cells in the body, having two
different kinds of “processes” (dendrites and axons) extending from them,
with points of near meeting between the axon of one cell and the dendrite
of another.^2 These synapses opened up the question of how connections are
made across them, and to what extent such connections are stable or can
change.^3 Cajal’s student Tanzi hypothesized that the strengthening of syn-
aptic connections is linked to the consolidation of memories and the learn-
ing of motor skills, whereas Cajal proposed that the brain, at least early in
life, could strengthen neuronal connections with mental exercise (DeFelipe
2006). He thought this could explain “the great intellectual capacity of cer-
tain individuals and how an individual with a small brain could become a
genius” (812 – 13). There was considerable interest in synaptic plasticity in
Cajal’s time, but the topic was then abandoned for much of the first half of
the twentieth century. Enthusiasm was dampened in part by fierce debate
on how the connections between neurons are made in the first place, which
was unresolved until the 1950s.^4
A number of conceptual proposals midcentury renewed interest in syn-
aptic plasticity as the mechanism of learning and memory. These include
Donald Hebb’s 1949 theory of associative learning, which argues that when