Neocortex size

Figure 1. Mean group size for individual primate genera plotted against relative neocortex size. The index of relative neocortex size used here is the ratio of neocortex volume to the volume of the rest of the brain. In most cases, each genus is represented by a single species. Source Dunbar 1992, with brain part volumes from Stephan et al. 1980.

NEOCORTEX SIZE AND SOCIAL SKILLS 2.1. Social Skills... 

This does, however, raise a number of interesting questions about the way in which social skills relate to neocortex volume. There are, for example, at least three reasons why we might find a relationship between group size and neocortex size (assuming that the latter actually does measure computing power). One is that the animal has to remember all the other individuals with whom it interacts a second is that it has to be able to manipulate the information relating to these individuals in a complex cognitive hyperspace a third is that it has more to do with how the individual relates to a smaller subset of special allies. 

Figure 2. Predicting the group size for modem humans by interpolating the observed relative neocortex size for humans (4.2) into the regression equation for anthropoid primates.

Aiello, L.C. Dunbar, R.I.M. (1993). Neocortex size, group size and the evolution of language. Current Antitropology M 184—193. 

Barton, R. (1996). Neocortex size and behavioural ecology in primates. Proceedings of the Royal Society. B, 263 173—177. 

Dunbar, R.I.M. (1992). Neocortex size as a constraint on group size in primates. Journal of Human Evolution 20 469-493. 

Dunbar, R.I.M. Bever, J. (submitted). Neocortex size determines group size in insectivores and carnivores. Ethology... 

Pawlowski, B., Dunbar, R. Lowen, C. (submitted). Neocortex size, social skill and mating success in male primates. Behaviour. 

Dunbar It seems that the really key correlation is the one between non-visual neocortex size and the period of juvenility between weaning and first reproduction, in other words the period of socialization. In humans, neocortex is the bit that s increased out of all proportion. It suggests that there is a very strong emphasis on socialization — the software programming, as it were, is becoming crucial at this point, it s not just the hardware that s the issue. 

Dunbar RIM 1995 Neocortex size and group size in primates a test of the hypothesis. J Hum Evol 28 287-296... 

Second, Kudo et al. (submitted) have shown that neocortex ratio correlates with with grooming clique size. This result suggests that species with larger neocortices can handle proportionately more relationships and keep them simultaneously functional. In this context, grooming cliques can be interpreted as alliances on which members rely in negotiat-... 

We can now return to the relationship shown in Fig. 1 and ask what implications it has for modern humans. Since the same database on which Fig. 1 is based also provides data on human neocortex volumes, it is easy to plug the values for humans into the regression equation and predict a group size for modem humans (Fig. 2). This turns out to be approximately 148. 

NeuN (Eriksson et al. 1998 Gould et al. 1999b, 2001 Kornack and Rakic 1999, 2001b Arvidsson et al. 2002 Parent et al. 2002) in an attempt to identify double-labeled cells whose size and morphology were similar to the surrounding NeuN+ neurons. We indeed found such cells, typically located in layers II-IV of the neocortex (Fig. 59) or lateral putamen in striatum (Fig. 60). Morphologically, BrdU+/NeuN+ cells (Fig. 59, 60 arrows) were similar in size and shape to adjacent BrdU /NeuN+ neurons (Fig. 59,60 arrowheads). Confocal analysis revealed that the NeuN+/BrdU+ constituted about 1% of the BrdU+ cells in either frontal neocortex or striatum (Table 10). 

Another example of an experiment showing the sizes of the physiologically defined minicolumns is the nerve regeneration study carried out by Kaas et al. (1981). These experimenters made an initial electrode penetration of the somatic sensory hand area in a monkey s neocortex in a direction more or less parallel to the surface of the cortex and showed that over a considerable distance the same modality type is observed. They then sectioned the median nerve and allowed time for the nerve to regenerate and re-innervate of the skin. The recording experiment was then... 

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