Abstract: Seven studies with a total of 225 participants met the inclusion criteria for this review. All seven studies compared the effects of a cognitive training intervention to a control intervention at the end of treatment periods lasting four to eight weeks. Six studies included people with PD living in the community. These six studies recruited people with single-domain (executive) or multiple-domain mild cognitive impairment in PD. Four of these studies identified participants with MCI using established diagnostic criteria, and two included both people with PD-MCI and people with PD who were not cognitively impaired. One study recruited people with a diagnosis of PD dementia who were living in long-term care settings. The cognitive training intervention in three studies targeted a single cognitive domain, whilst in four studies multiple domains of cognitive function were targeted. The comparison groups either received no intervention or took part in recreational activities (sports, music, arts), speech or language exercises, computerised motor therapy, or motor rehabilitation combined with recreational activity. We found no clear evidence that cognitive training improved global cognition. Although cognitive training was associated with higher scores on global cognition at the end of treatment, the result was imprecise and not statistically significant (6 trials, 178 participants, standardised mean difference (SMD) 0.28, 95% confidence interval (CI) −0.03 to 0.59; low-certainty evidence). There was no evidence of a difference at the end of treatment between cognitive training and control interventions on executive function (5 trials, 112 participants; SMD 0.10, 95% CI −0.28 to 0.48; low-certainty evidence) or visual processing (3 trials, 64 participants; SMD 0.30, 95% CI −0.21 to 0.81; low-certainty evidence). The evidence favoured the cognitive training group on attention (5 trials, 160 participants; SMD 0.36, 95% CI 0.03 to 0.68; low-certainty evidence) and verbal memory (5 trials, 160 participants; SMD 0.37, 95% CI 0.04 to 0.69; low-certainty evidence), but these effects were less certain in sensitivity analyses that excluded a study in which only a minority of the sample were cognitively impaired. There was no evidence of differences between treatment and control groups in activities of daily living (3 trials, 67 participants; SMD 0.03, 95% CI −0.47 to 0.53; low-certainty evidence) or quality of life (5 trials, 147 participants; SMD −0.01, 95% CI −0.35 to 0.33; low-certainty evidence). There was very little information on adverse events. We considered the certainty of the evidence for all outcomes to be low due to risk of bias in the included studies and imprecision of the results. We identified six ongoing trials recruiting participants with PD-MCI, but no ongoing trials of cognitive training for people with PDD. This review found no evidence that people with PD-MCI or PDD who receive cognitive training for four to eight weeks experience any important cognitive improvements at the end of training. However, this conclusion was based on a small number of studies with few participants, limitations of study design and execution, and imprecise results. There is a need for more robust, adequately powered studies of cognitive training before conclusions can be drawn about the effectiveness of cognitive training for people with PDD and PD-MCI. Studies should use formal criteria to diagnose cognitive impairments, and there is a particular need for more studies testing the efficacy of cognitive training in people with PDD.

Plain-language summary: We found seven studies that randomly allocated a total of 225 participants to cognitive training or to a control group. Treatment lasted from four to eight weeks. All the cognitive training interventions were delivered by computer. The control groups received either no intervention or a control intervention such as language or motor exercises or participation in recreational activities. We found no difference between people who received cognitive training and people in the control groups in global cognition shortly after treatment ended. There was no convincing evidence of benefit in specific cognitive skills and no benefit shown in activities of daily living or quality of life. However, these findings were based on a small number of participants in a small number of studies. The overall certainty of the evidence was low, meaning that the results of further research could differ from the results of this review. We found no good evidence that cognitive training is helpful for people with Parkinson's disease and dementia or MCI. The included studies were small and had flaws that may have affected the findings. The certainty of the results was low, and further studies are needed before we can be confident whether or not cognitive training is effective for this group of people.