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Brief communication How much larger is the relative volume of area 10 of the prefrontal cortex in humans.

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Brief Communication: How Much Larger Is the Relative
Volume of Area 10 of the Prefrontal Cortex in Humans?
Ralph L. Holloway*
Department of Anthropology, Columbia University, New York, New York 10027
brain; frontal lobe; prefrontal cortex; area 10; residuals; allometry
It has long been thought that the prefrontal cerebral cortex has been greatly expanded in the human brain. Semendeferi et al. ([2001] Am. J. Phys. Anthropol. 114:224 –241) showed that Brodmann’s area 10 is
relatively larger in the human compared to pongid brains.
The question is: how much larger relatively is it? Using
their data, it can be shown that the relative increase for
human prefrontal area 10 is only 6% larger. Looking at
the data base of neural structures provided by Stephan et
al. ([1981] Folia Primatol. (Basel) 35:1–29), it is apparent
that 6% is a relatively low residual value from a predicted
value based on allometric considerations between total
brain weight and any given neural structure. When this
small increase is combined with their earlier findings on
area 13 of prefrontal cortex (Semendeferi et al. [1997] J.
Hum. Evol. 32:375–388), it appears that the prefrontal
cortex in humans is not some 200% larger as claimed by
some researchers (Deacon [1997] Symbolic Species, New
York: W.W. Norton; cf. Holloway [1998] Am Sci 86:184 –
186), and that the findings of Semendeferi et al. ([2001]
Am. J. Phys. Anthropol. 114:224 –241) are in agreement
with the earlier work (Semendeferi and Damasio [2000] J.
Hum. Evol. 38:317–332; Semendeferi et al. [1997] J. Hum.
Evol. 32:375–388), showing that the human frontal lobe
volume is what would be expected for a primate of its
brain size. While the prefrontal cortex may have increased
relatively in Homo sapiens, the increase is likely to have
been far less than currently believed. Am J Phys Anthropol 118:399 – 401, 2002. © 2002 Wiley-Liss, Inc.
In their recent research paper on a comparative
study of Brodmann’s area 10 of the prefrontal cortex,
Semendeferi et al. (2001, p. 224) showed that the
human value for area 10 is “. . . larger relative to the
rest of the brain than it is in apes.” This observation,
if true when replicated with larger sample sizes, is
an important empirical finding, particularly since
Semendeferi and Damasio (2000), von Bonin (1948),
and Holloway (1964, 1968) suggested that the frontal lobe of humans is essentially the size one would
expect for a primate of its brain size. Thus one of the
long-standing myths regarding relatively larger
frontal lobes in humans appears to have been replaced with excellent empirical studies showing that
the situation is more complex. Still outstanding,
however, is the possibility that the prefrontal cerebral cortex might be enlarged in Homo relative to
the great apes, for which Deacon (1997) has strongly
argued, mostly on the basis of earlier tables by Brodmann (1909), based on surface areas and his cytoarchitectonic maps which were then used by Blinkov
and Glezer (1969, their Table 196) to arrive at the
relative expansion for Homo. Not all observations
suggest this to be the case. Uylings and van Eden
(1990), using Pongo as their pongid example,
showed that frontal lobe expansion in Homo was not
statistically significantly enlarged, and that the prefrontal cortex of Homo was almost exactly on the
regression line for the primates in their sample, and
that allometric slopes were essentially 1.0 These
figures contrast strongly with those of Deacon (1997)
(cf. Holloway, 1998), which suggested that the human
prefrontal cortex expanded ⫹200% from a primate
ancestor during hominid evolution. The work of Brodmann (1909), and the compilations of Blinkov and
Glezer (1968), did not use allometry to test whether
or not the absolute increases in surface area/volume
of prefrontal cortex are really significantly larger
than expected for a primate with a human brain
size. Given the increasing perceived importance of
the prefrontal cortex in human complex cognitive
behavioral patterns coming out of MRI, fMRI, and
PET studies, it would be useful to have a more
accurate assessment of the volumetric differences
between humans and pongids, to which Semendeferi
et al. (2001) have been adding newer information.
Using the data base of both Semendeferi et al.
(2001) and Stephan et al. (1981), the amount of
*Correspondence to: Ralph L. Holloway, Department of Anthropology, Columbia University, New York, NY 10027.
Received 16 April 2001; accepted 28 December 2001.
DOI 10.1002/ajpa.10090
Published online in Wiley InterScience (www.interscience.wiley.
TABLE 1. Residuals for Homo sapiens of various neural nuclei, based on Stephan et al. (1981) (volumes in mm3)
% Difference
relative increase of neural structures is calculated.
Using the data of Semendeferi et al. (2001) on area
10 without the human data points, one can calculate
the regression equation for available primate area
10. This equation allows one to calculate the expected human value based on the size of the brain.
Subtracting the actual volume of area 10 from the
expected volume provides a measure of the residual
value, which can be compared with other known
residuals from the large data base provided by
Stephan et al. (1981) for other neural structures. In
this case, the neural structures, e.g., the neocortex
and cerebellum, had the log (base 10) values regressed against the log (base 10) of brain volume.
Total brain volumes were not corrected, and included the volume of the particular structure, which
in the case of the neocortex, varies from about 60 –
76% of total brain volume. The other structures are
relatively small, and correcting them does not appreciably alter the residuals.
Table 2 of Semendeferi et al. (2001, p. 234) shows
that in absolute terms, the human volume of area 10
is approximately seven times as large as the chimpanzee’s volume of area 10, and their Figure 7 (Semendeferi et al., 2001, p. 235) shows graphically the
large absolute increment of human over other great
ape volumes, being ca. 1.2% of brain volume in
Homo, but only ca. 0.6 – 0.7% in the chimpanzee.
However, their Figure 8, a log-log plot of volume of
area 10 upon total brain volume, shows that the
human point is almost exactly on the regression line
they calculated. If one asks what the residual value
is for the human value, using only the ape values
and predicting what the expected human value of
area 10 should be, the amount is 903.03 mm3. The
equation for predicting the human volume is
Y ⫽ ⫺ 5.808 ⫹ 1.639 共X兲
Where X is log base 10 volume,
using only the five ape values for area 10 and their
respective brain volumes, the predicted volume for
Homo of area 10 would be 13,314.96 mm3, and the
actual volume would be 14,218 mm3. Thus
共14,218 ⫺ 13315兲/14218 ⫽ .0635, or 6.35%
This amount is certainly not dramatic, and we
await more sampling before knowing whether 6% is
indeed a significant relative increase. Put in the
perspective of residuals known for other structures
of the human brain, Table 1, based on the data set of
Stephan et al. (1981), gives some examples of human
residuals, expressed in percent difference between
observed and predicted values. The human cerebellum comes closest, being 6.20% larger if all 44 species of primate are used, but ⫺9.5% if only anthropoids are used. The human neocortex is ⫺11.33%
less than expected, and yet it is universally accepted
that the human value of the neocortex lies directly
on the total primate regression line of volume of the
neocortex against total brain volume. There is no
claim in the literature that these residuals are truly
significant in a statistical way. More dramatic residuals are ⫺121% for the primary visual cortex (area
17) and ⫺144% for the lateral geniculate nucleus
(for more examples, see Holloway, 1997), and these
suggest that the residuals are significant. The 95%
confidence intervals curve sharply away from the
regression lines at the ends of the distributions
when using log-log plots, making it difficult to assess
significant departures.
The work being done by Semendeferi et al. (1997,
2001) is important and represents long-overdue and
valuable additions to our understanding of comparative primate neurology and human brain evolution.
As the residuals in Table 1 suggest, it is very likely
that while brain size has important relationships
with the conservation of various neural structures
(e.g., see Finlay et al., 2001), there have been important shifts of other neural nuclei and fiber systems
in the human brain during its evolutionary course,
representing reorganization of the brain. It is quite
likely that the human cerebral cortex has also undergone mosaic evolutionary changes, but it remains
to be demonstrated exactly which regions have
changed the most in terms of volumes, and cytoarchitectonic organization of layers and cell types. For
the time being, however, I see these data as more
strongly suggesting that it will be connectivity patterns rather than volumes of neural tissues that
make the human brain distinct from those of our
pongid cousins. In any event, it is important to use
allometric tests for residual values between observed and predicted volumes if we are to assess the
relative increase or decreases of neural structures in
both the comparative and evolutionary senses.
I thank Chet Sherwood, Francys Subiaul, Michael
Yuan, Doug Broadfield, and the anonymous reviewers for useful comments and suggestions. The views
expressed here are solely those of the author.
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