Over the last 20 years research has found that people with chronic fatigue syndrome often have cognitive problems as well. However, different studies have found different cognitive problems and the studies themselves have often had very different methodologies. Researchers from the University of Adelaide in Australia reviewed 50 studies into this issue and found that cognitive problems mostly occured in the areas of attention, memory and reaction time; there was no evidence of deficits in 'fine motor speed,' vocabulary, reasoning and global functioning.
Cockshell, S.J. and Mathias, J.L. - Cognitive functioning in chronic fatigue syndrome: a meta-analysis Psychological Medicine, August 2010, 40(8), 1253-1267