line.The small sizes of these polygons show that individual differences
within a species are relatively small compared with those between
species.
The data of figure 12.3 are important because they indicate that gross
brain size in living mammals is a good statistic,as it were,for estimating
total information-processing capacity.This conclusion follows from uni-
formities in the way cortex is organized into columns of neurons,and
from the way neurons are packed in the brain.The number of neurons
under a given cortical surface area is remarkably uniform in mammalian
brains that have been studied (Rockel,Hiorns,and Powell 1980).Since
these features—the number of cortical columns and the number of
neurons—are usually considered appropriate units for analyzing the
capacity of a brain to handle information,the orderly relationship shown
in figure 12.3 between surface area and brain size implies the same kind
of relationship between brain size and information-processing capacity.
From uniformitarianism we can extend the analysis to fossil brains.
This kind of evidence leads us to assume that at some levels of com-
plexity the workings of animal brains are likely to be similar,even in dis-
tantly related species.Such uniformity of function enabled us to localize
and map many brain functions and analyze them in great detail.The
functions of auditory cortex,for example,were analyzed by studying this
part of the brain carefully in house cats,and we know that it functions
in a similar way in monkeys.
My doctoral dissertation was on this subject in monkeys (Jerison and
Neff 1953),and I considered my negative result as something of a failure.
I found that my macaques were affected by brain surgery in exactly the
same way as cats in their ability to distinguish among patterns of pure
tones.I had hoped for evidence of progression or a scale of nature that
differentiated higher animals (monkeys) from lower animals (cats;cf.
Hodos and Campbell 1969).Instead I found uniformity in behavior
and brain function across species.The uniformity lay in a conditioned
response to different patterns of pure tones (three-tone melodies):abla-
tion of auditory neocortex resulted in unrelearnable loss of the habit in
both species.A comparable habit to discriminate between single pure
tones,however,although lost after surgery,could be relearned.I had run
into a uniformity of behavior,or expression,at least between cats and
monkeys.It is much more difficult to ask the same kind of question about
the nature of the experience that is correlated with a behavior,or of the
mental activity in which little or no overt behavior can be observed.We
have no idea what was on each cat’s or monkey’s mind as it did its job.
But it is reasonable to assume that the control mechanisms used by both
animals in generating their performances were homologous and perhaps
comparable with our own experience when we hear pure tones.186 Harry Jerison