There are two things to discuss here, first the comparative anatomy of the human and chimpanzee brain, then the genetic differences that cause them. The human brain is about 2 to 3 times larger then a chimpanzee which means it would require an adaptive evolution:
Is it a few mutations in a few genes, a lot of mutations in a few genes, or a lot of mutations in a lot of genes? (Bruce Lahn)
The problem is that brain related genes are highly conserved and do not respond well to mutations.
The 118-bp HAR1 region showed the most dramatically accelerated change , with an estimated 18 substitutions in the human lineage since the human–chimpanzee ancestor, compared with the expected 0.27 substitutions on the basis of the slow rate of change in this region in other amniotes . Only two bases (out of 118) are changed between chimpanzee and chicken, indicating that the region was present and functional in our ancestor at least 310 million years (Myr) ago. No orthologue of HAR1 was detected in the frog (Xenopus tropicalis), any of the available fish genomes (zebrafish, Takifugu and Tetraodon), or in any invertebrate lineage, indicating that it originated no more than about 400Myr ago (An RNA gene expressed during cortical development evolved rapidly in humans, Nature 2006)
Now that indicates that the gene originated no more then 400 years ago and was present and functional 300 million years ago. Then it allows only two substitutions until 2-5 million years when it gets 18. The fact is that brain related genes do not respond well to mutations, there is a long list of deleterious effects and virtual no beneficial effects from mutations in human brain related genes known to modern science.
There are some pretty good papers on the subject:
An RNA gene expressed during cortical development evolved rapidly in humans. Nature 2006
Accelerated Evolution of Nervous System Genes in the Origin of Homo sapiens, Cell 2004
Genetics and the making of Homo sapiens Nature 2003
There is a whole genome sequence of the Gorilla genome out there if it seems relevant, as well as an assortment of hominid fossilized skulls. There is a large body of work surrounding the subject matter so narrowing things down to a few comparisons and specific genomic sequences might prove helpful.
Have a nice day
Mark