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

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Chapter 6
is infinite (with the induced electrical field decreasing from the centre of the
stimulation focal point), in practice the size of the electrical field capable of disrupting
normal neuronal activity is limited (i.e. it is anatomically and functionally specific).
For example, TMS of the motor cortex results in visible and measurable (via MEPs)
motor twitches in the muscles. This effect however is neither random nor non-
specific – TMS to the motor cortex has been shown to evoke muscular twitches from
selective muscle groups in a manner compatible with the functional layout of the
motor homunculus, with stimulation at target sites varying from 0.5 to 1 centimetres
apart leading to selective activation of each muscle type (Singh, Hamdy, Aziz, and
Thompson, 1997; Wassermann, McShane, Hallet, and Cohen, 1992). The functional
focality of the method is further demonstrated in the visual domain, where a similar
spatial resolution (less than 1 cm on the scalp) has been reported through the study of
the physiological effects induced by TMS stimulation of the primary visual cortex
(V1; c.f. Walsh and Cowey, 2000). Moreover, stimulation to V1 leads to the
generation of phosphenes, which are spatially distributed in a manner that corresponds
with the retinotopic organisation of V1 (Kammer, 1999).
Outside of the primary motor and sensory areas, the effective spatial
resolution 3 of TMS has regularly been demonstrated by functional dissociations
following TMS to spatially discrete regions of the cortex. For example, in the same
subjects, TMS targeted at the right occipital face area (thought to be functionally
specialised for face processing) has been shown to impair face but not body or object
recognition, while stimulation at a region of lateral occipital cortex (LO; thought to be
functionally specialised for object processing) has been shown to impair object but
not face or body recognition, whilst stimulation at the right extra-striate body area
3 It is of note, that TMS does not only stimulate the neuron in a 1cm region, rather, it is that this
represents the physiologically effective resolution of TMS.