A Novel Approach for Documenting Phosphenes Induced by Transcranial Magnetic Stimulation

Summary

Phosphenes are transient percepts of light that can be induced by applying Transcranial Magnetic Stimulation (TMS) to visually sensitive regions of cortex. We demonstrate a standard protocol for determining the phosphene threshold value and introduce a novel method for quantifying and analyzing perceived phosphenes.

Abstract

Stimulation of the human visual cortex produces a transient perception of light, known as a phosphene. Phosphenes are induced by invasive electrical stimulation of the occipital cortex, but also by non-invasive Transcranial Magnetic Stimulation (TMS)¹ of the same cortical regions. The intensity at which a phosphene is induced (phosphene threshold) is a well established measure of visual cortical excitability and is used to study cortico-cortical interactions, functional organization ², susceptibility to pathology ^3,4 and visual processing ^5-7. Phosphenes are typically defined by three characteristics: they are observed in the visual hemifield contralateral to stimulation; they are induced when the subject s eyes are open or closed, and their spatial location changes with the direction of gaze ². Various methods have been used to document phosphenes, but a standardized methodology is lacking. We demonstrate a reliable procedure to obtain phosphene threshold values and introduce a novel system for the documentation and analysis of phosphenes. We developed the Laser Tracking and Painting system (LTaP), a low cost, easily built and operated system that records the location and size of perceived phosphenes in real-time. The LTaP system provides a stable and customizable environment for quantification and analysis of phosphenes.

Protocol

1. Inducing a Phosphene with Stimulation to the Right Occipital Lobe

1. In a dimly lit room, seat the participant in a comfortable position. Have the participant put on a swim cap, provide them with hearing protection, and adjust the cap and ear plugs for comfort. To locate and mark the initial point of stimulation, first identify the inion, and then move ~2cm rostrally and ~2cm laterally (figure 1). Alternatively, a frameless stereotaxic system can be used to position the TMS coil over the targeted cortical region.
2. Using a standard figure-eight (double) TMS coil (70mm), place the center of the coil directly over the location det....

Discussion

In this video, we have demonstrated a standard protocol for obtaining a phosphene threshold value and introduced a novel method for recording phosphene size and visual field location. Phosphenes were elicited by delivering individual pulses of TMS to the right occipital cortex adjacent to the occipital pole. Phosphene threshold values provide a valuable indication of relative cortical excitability; and have been used both as an outcome measure and as a means of determining the appropriate intensity of TMS in variety of s.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
TMS	Magstim	Magstim 200
Webcam	Logitech	QuickCam Orbit MP
Projector	Epson	Epson PowerLite 7900p
Projection Screen	DA LITE	Da-View fast-fold deluxe screen
Laser Pointer	Generic