A meta-analysis of performance on simple span and more complex working memory tasks in Parkinson's disease.

The working memory (WM) concept has stimulated substantial research since Baddeley and Hitch advanced their model in 1974. There has also been growing interest in WM in Parkinson's disease (PD) where the brain structures considered important for WM are often compromised. However, it remains unclear how and to what degree WM is affected in PD. The authors used meta-analysis to clarify the research findings on WM in PD. The results confirmed that people with PD are impaired on tests of WM. This impairment is small for verbal span but moderate on complex verbal and both simple and complex visuospatial tasks. These data do not support the belief that WM impairment in PD is solely at the level of the central executive. However, our findings support the notion that impairment is more pronounced for visuospatial than verbal WM. A number of different interpretations of these results are discussed. It remains to be established what these statistically significant differences mean in terms of clinically significant levels of impairment in WM. Another important methodological issue that demands greater consideration in this area is that of sampling and the generalizability of results.