Mirror-touch synaesthesia: the role of shared ... - UCL Discovery

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Chapter 6
There are also important differences between lesion and TMS studies, which
lead to a number of benefits over patient-based research. Firstly, the nature of lesion
studies means that the experimenter is required to make inferences about the normal
architecture based on an abnormal system. This has a number of caveats including the
influence of mechanisms of compensatory plasticity in the abnormal system, which
may lead to changes in function or task performance. For example, it is often months
after brain injury that the experimenter is able to examine patient performance
systematically and it is difficult to disentangle whether one is measuring the removal
of a region or the ability of other brain regions to compensate the function being
investigated (Lomber, 1999; Robertson and Murre, 1999). Further, brain lesions are
rarely spatially discrete and removal of a brain area may also incur damage at distal
sites (e.g. via severed vessels, ablated white matter; Robertson and Murre, 1999;
Walsh and Pascual-Leone, 2003). In contrast to this, TMS permits investigation of
spatially specific brain regions in normal subjects and overcomes problems of neural
compensation / reorganization because the main effects occur in a discrete temporal
window (lasting a few tens of milliseconds to minutes depending on the type of
stimulation used). In addition, because behavioural performance can be measured
within-subjects (during both the application and the absence of stimulation), it is
possible for subjects to act as their own control group, thereby strengthening the
validity of the conclusions that can be drawn from a TMS experiment.
To consider the method further, this chapter discusses the principles, ethical
aspects, spatial resolution, temporal constraints, and alternative TMS paradigms.