Page 42 - History of Psychology

Donald Olding Hebb (1904–1985) was born in Chester, Nova Scotia. Both of his
parents were medical doctors. He received his BA from Dalhousie University with

the lowest grade average a person could have and still graduate.

According to Hebb, the neural interconnections in a newborn’s brain are
essentially random. It is experience that causes this network of neurons tobecome
organized and provide a means of effectively interacting with the environment.
Hebb speculated that every environmental object we experience fires a complex
package of neurons, called a cell assembly. According to Hebb, it is reverberating
neural activity that allows neurons that were temporarily separated to become
associated. Hebb believed that neural activity caused by stimulation continued
for a short time after the stimulation ceases (reverberating neural activity), thus
allowing the development of successive neural associations. Once a cell assembly
exists, it can be fired by internal or external stimulation or by a combination of the
two.

Just as the various neurons stimulated by an object become neurologically

interrelated to form a cell assembly, so do cell assemblies become neurologically
interrelated to form phase sequences. Hebb (1959) defined a phase sequence as
“a temporally integrated series of assembly activities; it amounts to one current in
the stream of thought”. According to Hebb, childhood learning involves the slow
buildup of cell assemblies and phase sequences, and this kind of learning can be
explained using associationistic terminology. Adult learning, however, is
characterized by insight and creativity and involves the rearrangement of already
existing cell assemblies and phase sequences.

Roger Wolcott Sperry (1913–1994) was born in Hartford, Connecticut. He received
his BA in English from Oberlin College in 1935 and his PhD in zoology from the
University of Chicago in 1941, where he learned neurosurgical techniques from
the eminent neuroembryologist Paul Weiss. After receiving his doctorate, Sperry
studied with Lashley at the Yerkes Laboratories in Florida (1942–1946).

At Caltech, Sperry pursued his interest in the routes by which information is
transferred from one side of the cerebral cortex to the other. In a now-famous
series of experiments, Sperry and his colleagues discovered two possible routes

for such interhemispheric transfer—the corpus callosum (a large mass of fibers
that connects the two halves of the cortex) and the optic chiasm. The optic
chiasm is the point in the optic nerve where information coming from one eye is
projected to the side of the cortex opposite to that eye. Sperry taught cats and
monkeys to learn a visual discrimination with a patch over one eye. He then tested
for transfer by switching the patch to the other eye and found complete
interocular transfer. Sperry then began his search for the mechanism by which
information is transferred from one side of the cortex to the other. He found that
ablating either the corpus callosum or the optic chiasm alone or together after
training did not interfere with transfer. 38