Stevens, Laura Kate
(2009)
Disruption of spatio-temporal processing in human vision using transcranial magnetic stimulation.
PhD thesis, University of Nottingham.
Abstract
Transcranial magnetic stimulation (TMS) is a non-invasive technique used to reversibly modulate the activity of cortical neurons using time-varying magnetic fields. Recently TMS has been used to demonstrate the functional necessity of human cortical areas to visual tasks. For example, it has been shown that delivering TMS over human visual area V5/MT selectively disrupts global motion perception. The temporal resolution of TMS is considered to be one of its main advantages as each pulse has a duration of less than 1 ms. Despite this impressive temporal resolution, however, the critical period(s) during which TMS of area V5/MT disrupts performance on motion-based tasks is still far from clear.
To resolve this issue, the influence of TMS on direction discrimination was measured for translational global motion stimuli and components of optic flow (rotational and radial global motion). The results of these experiments provide evidence that there are two critical periods during which delivery of TMS over V5/MT disrupts performance on global motion tasks: an early temporal window centred at 64 ms prior to and a late temporal window centred at 146 ms post global motion onset. Importantly, the early period cannot be explained by a TMS-induced muscular artefact. The onset of the late temporal window was contrast-dependent, consistent with longer neural activation latencies associated with lower contrasts. The theoretical relevance of the two epochs is discussed in relation to feedforward and feedback pathways known to exist in the human visual system, and the first quantitative model of the effects of TMS on global motion processing is presented.
A second issue is that the precise mechanism behind TMS disruption of visual perception is largely unknown. For example, one view is that the “virtual lesion” paradigm reduces the effective signal strength, which can be likened to a reduction in perceived target visibility. Alternatively, other evidence suggests that TMS induces neural noise, thereby reducing the signal-to-noise ratio, which results in an overall increase in threshold.
TMS was delivered over the primary visual cortex (area V1) to determine whether its influence on orientation discrimination could be characterised as a reduction in the visual signal strength, or an increase in TMS-induced noise. It was found that TMS produced a uniform reduction in perceived stimulus visibility for all observers. In addition, an overall increase in threshold (JND) was also observed for some observers, but this effect disappeared when TMS intensity was reduced. Importantly, susceptibility to TMS, defined as an overall increase in JND, was not dependent on observers’ phosphene thresholds. It is concluded that single-pulse TMS can both reduce signal strength (perceived visibility) and induce task-specific noise, but these effects are separable, dependent on TMS intensity and individual susceptibility.
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