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TMS Over the Intraparietal Sulcus Induces Perceptual Fading

Institute of Cognitive Neuroscience and Department of Psychology, University College London, London, United Kingdom

Submitted 5 August 2008; accepted in final form 12 October 2008

During prolonged fixation, visual objects presented in the periphery of the visual field often fade from awareness. This phenomenon, known as the Troxler effect, has been largely attributed to adaptation of neurons responding to peripheral targets. Here, we hypothesized that perceptual disappearance might result from degeneration of feedback from attention-related cortical areas to early visual areas and that visual transients disrupt the feedback loop sustaining low-level signals and thereby trigger perceptual fading. We examined this hypothesis by briefly disrupting the functions of attention-related regions in the intraparietal sulcus (IPS) using transcranial magnetic stimulation (TMS). The hypothesis predicted that temporary disruption of IPS would trigger perceptual disappearances. We measured perceptual disappearance using a task in which participants were asked to discriminate the presence or absence (fading) of a peripheral green target immediately after a TMS pulse. On one half of the trials, the target remained on the screen until the end of a trial, and on the other half of the trials, it gradually faded. The results of this experiment show that brief disruption of the IPS with a single pulse TMS is sufficient to trigger perceptual disappearance. TMS over the IPS resulted in a fewer reports of continuous percepts (i.e., more fading) without changes in the perception of physically fading stimuli. Our control experiment shows the loss of sensitivity is not caused by suppression of microsaccades in response to the sound produced by TMS. This study supports the idea that conscious visual perception requires a coupling between the early visual areas representing sensory data and the parietal areas subserving spatial localization functions.

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