The study of patients with motor impairment requires careful consideration. Many early studies were performed in fully recovered patients so that comparisons with healthy controls could be made. However, to address the issue of whether differences between stroke patients and controls are related to recovery, patients with a wide range of outcomes must be studied. In this way, correlations between task-related brain activation and outcome can be identified. There are two consequences of this consideration. Firstly, outcome must be characterised in a detailed and appropriate way. Secondly, patients must perform the same task during the acquisition of fMRI data so that a meaningful comparison can be made across subjects or scanning sessions. Equality of task may be interpreted in a number of ways. In patients with different abilities a task may be the same in terms of absolute or relative parameters. For example a simple motor paradigm can be equated in terms of absolute parameters of force and rate of task performance across subjects, or in terms of how effortful the task is. The experimenter is interested in different states of cerebral reorganisation. However depending on paradigm selection, changes in brain activation patterns across subjects may also be attributable to differences in absolute performance parameters (if effort is equated) or differences in effort (if absolute task parameters are equated).
Most studies have used finger tapping as the motor task. This task is problematic as many stroke patients with poor outcome are unable to perform fractionated finger movements adequately. As a result neither effort nor absolute task parameters can be controlled for. The return of fractionated finger movements represents an excellent outcome and is therefore of clinical interest but this does not mean that it is the best paradigm to use, for the reasons described above. An important concept in experimental design is that it is not possible to scan patients in order to determine the neural correlates of something they cannot do (Price and Friston, 1999). An experimenter must find alternative ways to probe the system of interest. An alternative task is dynamic hand grip which is used in several studies described in this chapter (Ward and Frackowiak, 2003; Ward et al., 2003a,b). Hand grip can be performed by patients with even minimal recovery and its performance correlates well with other measures of upper limb recovery (Heller et al., 1987; Sunderland et al., 1989). In order to control as much as possible for the effort involved in performing a hand grip task subjects (controls and patients) can be set target forces that are a fixed percentage of their own maximum hand grip. This is therefore a task that all can perform. In our series of experiments all patients were able to perform repetitive hand grips at 10%, 20%, 40%, etc. of their own maximum exertable force. The results of these studies pertain to how a damaged motor system is able to perform this task. Thus by using a motor task (hand grip) that reflects intrinsic motor recovery more than adaptation (Sunderland et al., 1989) and by controlling for motor effort as much as possible, the results within known motor-related regions are then more likely to reflect cerebral reorganisation rather than change of strategy after focal brain damage. The interpretation of results from all functional imaging studies involving patients with differing abilities needs to take these factors into consideration (Poldrack, 2001).
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