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.

Is implicit sequence learning impaired in Parkinson's disease? A meta-analysis.

The aim of the present study was to examine impairment of implicit learning in Parkinson's disease (PD) by means of a meta-analysis of studies that used the serial reaction time (SRT) task. The authors performed a systematic review and meta-analysis of published journal articles (1987-2005) that used the SRT task with patients with PD. The principal outcome measures used to compare studies were (a) the difference in reaction time between the last block of ordered sequence trials and the randomized block for PD and control participants and (b) fixed and random effects pooled estimates by the inverse weighting method. Six studies, including 67 patients with PD, met the inclusion criteria. The meta-analysis showed that implicit learning was impaired in PD, relative to healthy controls, with a standardized mean difference of 0.73 (95% confidence interval = 0.38, 1.07). Implicit sequence learning appears to be impaired in patients with PD. Some common methodological weaknesses and limitations in the reporting of statistical data are discussed.