Neural correlates of phosphene perception.

Phosphenes are elementary light perceptions which can be elicited by transcranial magnetic stimulation (TMS) over the occipital cortex (Meyer et al., 1991; Marg and Rudiak, 1994). Phosphene threshold, which is the minimum magnetic stimulation intensity capable of eliciting phosphenes has been used as a measure of the excitability of the visual cortex (Afra et al., 1998; Aurora et al., 1998). In addition, phosphene perception has been utilized to assess short-term plasticity of the visual cortex and its underlying mechanisms (Boroojerdi et al., 2000a, b, 2001). Single pulse focal TMS does not elicit phosphenes in all normal subjects. The percentage of investigated subjects perceiving phosphenes varies across studies. Our results, along with those of Meyer et al. (1991) indicate that about two-thirds of the subjects tested with TMS report phosphenes. The mechanism underlying the absence of phosphene perception in the remaining third is still unknown. Furthermore, the neural correlates underlying phosphene perception are not well understood. Studies investigating the cortical site of phosphene perception using different TMS coil positions are yet to determine the exact site in the visual system (Kammer, 1999; Kammer et al., 2001). Although it is widely agreed that phosphenes are evoked within the central visual system, the exact mechanism of phosphene perception could not be characterised, mainly due to methodological limitations. The present study adopted a new approach to further characterise the neural correlates of phosphene perception. Whereas, previous studies investigated only those subjects who reported phosphene perception, we sought to investigate differences in visual cortex excitability between subjects with, and without, phosphene perception. We addressed this issue by combining the TMS data with functional magnetic resonance imaging (fMRI) measurements of the activated visual network in response to a standard checkerboard stimulus. Previous studies locating the primary motor area of the hand using TMS mapping and fMRI activation found a high coherence between the two methods (Krings et al., 1997, 2001a, b; Boroojerdi et al., 1999), and TMS and fMRI can be seen as complimentary approaches in the evaluation of cortical 1 2 3 4 5 6 7 8 9 1011 1 2 3 4 5 6 7 8 9 2011 1 2 3 4 5 6 7 8 9 3011 1 2 3 4 5 6 7 8 9 4011 1 2 3 4 5 Transcranial Magnetic Stimulation and Transcranial Direct Current Stimulation (Supplements to Clinical Neurophysiology, Vol. 56) Editors: W. Paulus, F. Tergau, M.A. Nitsche, J.C. Rothwell, U. Ziemann, M. Hallett © 2003 Elsevier Science B.V. All rights reserved 305