By 1950, the results of attempts to relate brain
processes to mental experience appeared rather
discouraging. Herring suggested that different modes of
sensation, such as pain, taste, and color, might be
correlated with the discharge of specific kinds of nervous
energy. However, subsequently developed methods of
recording and analyzing nerve potentials failed to reveal
any such qualitative diversity. Although qualitative
variance among nerve energies was never rigidly
disproved, the doctrine was generally abandoned in favor
of the opposing view, namely, that nerve impulses are
essentially homogeneous in quality and are transmitted as
"common currency" throughout the nervous system.
According to this theory, it is not the quality of the
sensory nerve impulses that determines the diverse
conscious sensations they produce, but rather the different
areas of the brain into which they discharge, and there is
some evidence for this view. In one experiment, when an
electric stimulus was applied to a given sensory field of
the cerebral cortex of a conscious human subject, it
produced a sensation of the appropriate modality for that
particular locus, that is, a visual sensation from the visual
cortex, an auditory sensation from the auditory cortex,
and so on. However, cortical locus, in itself, turned out to
have little explanatory value.