See:
http://en.wikipedia.org/wiki/Small-world_network
Important reviews:
Full text: http://www.nonlinearbiomedphys.com/content/1/1/3
http://www.citeulike.org/user/AbnerCYH/article/1704979
Small-World Brain Networks
http://nro.sagepub.com/cgi/content/abstract/12/6/512
Alzheimer disease is characterized by a loss of small-world network characteristics
http://cercor.oxfordjournals.org/cgi/content/abstract/17/1/92
Small-World networks and schizophrenia
http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=16875801&cmd=showdetailview&indexed=google