Abstract
Transcranial magnetic stimulation (TMS) has become a popular method for studying the functional properties, connectivity and chronometry of brain regions associated with visual encoding. However comparatively little is known about the precise mechanisms by which TMS influences on-going visual processing, though studies suggest it may suppress the processing of the signals associated with a task and/or induce increased levels of internal noise. To investigate this issue single-pulse TMS was applied over left-hemisphere V1 in eight observers during a forced-choice, orientation-identification task (horizontal vs. vertical) using a Gabor target (2 c/deg, centred 6 deg in the right visual field). Stimulus contrast was set to each observer’s threshold, corresponding to 79% correct performance, measured in the absence of TMS. When TMS was applied over V1 performance decreased in all observers (~ 10% on average) compared to accuracy levels obtained during stimulation over a control site (Cz). Unexpectedly we found accuracy levels improved during V1 stimulation across a block of 200 trials in most (5/8) subjects, but remained stable during control site stimulation. Furthermore, no recovery was found when a brief, external, visual noise mask was used instead of a TMS pulse. These results show that the magnitude of TMS disruption can dissipate with repeated stimulation. To explore the potential mechanism underlying this recovery phenomenon we also measured the critical flicker fusion threshold (CFFT), using an LED driven by a square-wave temporal waveform of variable frequency, both prior to and following the same TMS protocol. For observers that previously exhibited TMS recovery, occipital simulation extended temporal integration periods by an average of 12% (by 3-8 ms). This suggests that the visual system can dynamically adapt to increased internal noise levels, by increasing the temporal interval over which visual stimuli are integrated, thus minimising the deleterious effects of TMS-induced cortical activity on sensory judgments.
Meeting abstract presented at VSS 2015