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Neurophysiological Evaluation of Visual Cortex Excitability Using Image-guided Transcranial Magnetic Stimulation

¹ Instituto de Bioingeniería Universidad Miguel Hernandez San Juan de Alicante Spain

Commercial Relationships:  E. Fernandez, None; A. Alfaro, None; J. Tormos, None; R. Climent, None; O. Martínez, None; M. Martínez, None; A. Pascual-Leone, None.
Grant Identification: Support: Fundación ONCE, MAT2000-1049, FISS 01/0674, QLK6-CT-2001-00279

Abstract

Purpose: The present study was designed to systematically map the visual sensations induced by focal and non invasive stimulation of the human occipital cortex using transcranial magnetic stimulation (TMS). We have tried to address the following questions: (i) Is it possible to induce visual perceptions by TMS pulses to the occipital cortex of sighted and blind subjects? (ii) Is there variability in the properties of visual perceptions as a function of the scalp position stimulated? (iii) Does the orientation of the coil influence the size, intensity, location, or other characteristics of the perceived phosphenes? (iv) Can TMS mapping be used for non-invasive investigation of the functional organization of the human visual cortex?
Methods: TMS was applied with a figure of eight coil to 28 positions arranged in a 2x2 cm grid over the occipital area. A digitizing tablet connected to a PC computer running customized software, and audio and video recording were used for detailed and accurate data collection and analysis of evoked visual perceptions. Phosphenes were analyzed in regards to subjective size, intensity, characteristics, and position in visual field. A frameless image-guided neuronavigational device was used to describe the position of the actual sites of the stimulation coils relative to the cortical surface.
Results: Our results show that TMS is able to elicit visual perceptions in almost all sighted subjects and in some proportion of blind subjects. Many subjects perceived structured phosphenes, as well as chromatic and kinetic phosphenes. Evoked perceptions were topographically organized. Despite minor inter-individual variations, the mapping results was reproducible and showed good congruence among different subjects.
Conclusion: We conclude that TMS mapping has potential to improve our understanding of the physiologic organization and plastic changes in the human visual system and to establish the degree of remaining functional visual cortex in blind subjects. Such a non-invasive method could be used for the selection of suitable subjects for a cortical visual prosthesis.

Keywords: 621 visual cortex • 625 visual impairment: neuro-ophthalmological disease • 432 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)

© 2002, The Association for Research in Vision and Ophthalmology, Inc., all rights reserved. For permission to reproduce any part of this abstract, contact the ARVO Office at arvo{at}arvo.org.