Our findings support the main hypothesis of this study that attentional deficit is related to patients' performance in ADL. Attention contributed to the prediction of physical ADL skills such as bathing, eating and getting dressed, as well as social interaction skills such as participating in conversations, keeping appointments, watching TV and reading. The effects of attention were still present after controlling for sex, age, educational level, motor functions and other aspects of cognitive functions, indicating that the effects of attention were not simply a reflection of general cognitive or motor status. Thus, given the clinical importance of ADL, these results show the prognostic significance of attention deficits in patients with PDD.
The main strengths of the study are the large and well‐described sample of patients with PDD from multiple clinics in different countries, the extensive measures of different aspects of cognition, including attention, and a validated measure of ADL.
Perhaps the most difficult aspect of the study is the precise content of the attention factor and its relationship with other cognitive variables. The variable with the highest factor loading on the attention factor was response accuracy on a digit vigilance test, measuring ability to correctly detect and respond to a stimulus matching a target stimulus, while ignoring non‐matching stimuli. The second highest loading measure was choice reaction time. The factor solution closely replicates a similar factor analysis with patients with DLB,8
where the same CDR measures were used. All variables contributing to the attention factor required a motor response and monitoring of external visual stimuli. Although we controlled for motor functions, it could be argued that the attention factor could still be influenced by motor speed. However, given that the highest loading variable on this factor is an accuracy score not dependent on simple motor reaction time and the very low correlation of 0.148 between the attention factor score and total motor score on the UPDRS‐III, this seems unlikely. The attention factor seemed mostly to include functions related to sustained attention (vigilance) and attentional focus, while not being solely a strict measure of vigilance as defined by Parasuraman,15
owing to the shorter duration of the CDR tests than typical vigilance tasks. Cognitive speed could explain some of the variability of the attention factor. However, it has been shown in patients with Parkinson's disease that prolonged simple reaction time may reflect deficits of focused attention,34
and that even complex attentional control functions probably depend on cognitive speed.35
Thus, we do not see a justification for correcting for simple reaction time.
The factor verbal cognition was more heterogeneous. Both letter fluency and the serial 7 task from MMSE, which loaded on this factor, are tests considered to require executive attentional control. These tests also loaded weakly on the attention factor. The most obvious differences between these tasks and the tasks contributing to the attention factor are the executive control demands, the verbal component, the lack of monitoring of external stimuli and the response modality being speech rather than a simple motor response using the dominant hand. The attentional demands of this factor are of a more executive nature, requiring internal control rather than monitoring of external events. The attentional components of this factor indicate that we should be careful not to treat the attention construct as a single entity. However, as the CDR tests are designed to assess attention independently of working memory, the attentional factor can be seen as a pure assessment of attention, whereas the attentional tests that involve working memory loaded separately, supporting this differentiation.
Our study shows that attentional dysfunction may be one of the primary cognitive factors associated with functional impairment in patients with PDD. Several implications can be drawn from this finding. More work should be carried out on the nature of attentional deficits in PDD, as several theoretical issues remain. Although earlier neuropsychological work on PDD focused on dopaminergic depletion and frontal, executive dysfunctions,36,37
more interest has recently been directed towards impairment in cholinergic38,39
and noradrenergic pathways.40,41
The recent staging of Parkinson's disease proposed by Braak et al42
indicate that noradrenergic and cholinergic networks related to control of arousal and vigilance are affected early in Parkinson's disease. Perhaps most important in this regard is the nucleus of Meynert in the basal forebrain. This nucleus has large cholinergic projections to the cortex and is part of an important afferent regulatory system of the cortex, affecting arousal and selective attention.43
This nucleus has been proposed as central to the fluctuating level of consciousness seen in patients with DLB,44
and may contribute to attentional deficits in those with PDD. Although the nucleus of Meynert is also affected in patients with Alzheimer disease, the cholinergic deficit seems to be more severe in those with PDD.38,39
Thus, a larger cholinergic deficit in PDD than Alzheimer's disease, may explain the improvement of attentional functions under treatment with rivastigmine in DLB45
In a recent study, ADL, attention and other cognitive measures improved,22
and patients with the most severe attention deficits responded best to rivastigmine.46
We believe that our findings indicate that attention is of fundamental importance for ADL performance in PDD. A practical implication is that we could more often use a test of attention in diagnosing cognitive impairment and treatment efficacy in patients with PDD. A standard computer‐based vigilance test with adequate norms could be used, as digit vigilance accuracy loaded most strongly on our attention factor.
The effect of impaired attention may be mediated by the known attentional requirements of complex motor tasks10
and of a wide range of cognitive processes.14
There may also be a direct effect of attentional deficits on ADL. Further studies are justified to explore whether positive effects on ADL and cognition under treatment with rivastigmine in patients with PDD22
are mediated by an improvement in attention.