Transcranial magnetic stimulation and vision.

The challenge in visual neuroscience is to characterize the neuronal properties and functional significance of the numerous regions of the visual cortex, and to understand how they interact during the processing of visual information. The strength of transcranial magnetic stimulation (TMS) in this endeavor is its ability to assess the necessity of visual cortical areas in perceptual functions and to trace the corticocortical interactions that underlie them. Most of the early studies in this field were carried out using the so-called "virtual lesion" approach, in which the impact of TMS was thought to be akin to inducing a brain lesion in the stimulated area. This approach established causal links between specific visual areas and perceptual functions such as motion perception, object processing, and visual awareness. Recently, the view of TMS as a tool for inducing "virtual lesion" has been challenged by a number of experimental findings, giving rise to the conceptualization of TMS effects as a state-dependent interaction between the initial state of the stimulated area and the parameters of the TMS pulse. This state dependency is the basis of paradigms aiming selectively to target specific neuronal representations and thus reveal neuronal tuning properties, a major challenge in the understanding of the cortical visual system.