Edoardo Bisiach
Institute di Clinica Neurologica dell' Universit0 di Milano

Somewhere along the line-and it is not easy to determine just when and by whom--
the hypothesis of cerebral dominance took origin. The term is an unsatisfactory one,
for it suggests a suzerainty of the cerebral hemispheres over some other unnamed
structure of the brain. What is no doubt implied, though not explicitly stated, is a
relative preponderancy of one half of the cerebrum over the other. ( Critchley, 1972)

Critchley's words indicate the mild annoyance with which the worn-out
concept of cerebral dominance may nowadays be received. Although a faithful
reconstruction would indeed be laborious, we may trace the root of this concept
back to Jackson's ( 1874) principle of hemispheric leadership and view its further
development as a partial distortion of this principle.

In his writings, Hughlings Jackson explicitly exonerates himself of any
attempt to imply a misconceived segregation of brain activities. His idea of a
leading side is a dynamic one and refers to the migration of the primary stages of
an activity to the locus where suitable brain structures can be found for the
execution of that activity. Moreover, the leadership is not absolute but appor-
tioned: the left hemisphere is held by Jackson to lead the "voluntary" expression
of language, to the "automatic" inception of which the right hemisphere con-
tributes; the reverse applies, in his view, to the "revival of images."

In the fervent years of rising neuropsychology, however, the apparent
implication of left-hemisphere pathology in a series of conspicuous behavioral
disorders besides dysphasia was sufficient to cloud the role of the right hemi-
sphere in highest nervous functions. In 1926, Henschen, stressing the slight
share of the right hemisphere in a series of mental functions, wondered whether it
was a regressing organ or, to the contrary, an organ capable of training and
"reserved for future higher development and possibly for new facilities." Such a

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