State-Dependency Effects on TMS: A Look at Motive Phosphene Behavior

Summary

In this article, we examine the effects of visually relevant state dependency on TMS induced motive phosphenic presentations.

Abstract

Transcranial magnetic stimulation (TMS) is a non-invasive neurostimulatory and neuromodulatory technique that can transiently or lastingly modulate cortical excitability (either increasing or decreasing it) via the application of localized magnetic field pulses.^1,2 Within the field of TMS, the term state dependency refers to the initial, baseline condition of the particular neural region targeted for stimulation. As can be inferred, the effects of TMS can (and do) vary according to this primary susceptibility and responsiveness of the targeted cortical area.^3,4,5

In this experiment, we will examine this concept of state dependency through the elicitation and subjective experience of motive phosphenes. Phosphenes are visually perceived flashes of small lights triggered by electromagnetic pulses to the visual cortex. These small lights can assume varied characteristics depending upon which type of visual cortex is being stimulated. In this particular study, we will be targeting motive phosphenes as elicited through the stimulation of V1/V2 and the V5/MT+ complex visual regions.⁶

Protocol

1) Preparation

1. To begin, seat the subject in a comfortable chair in front of a computer screen.
2. Use a tape measure to ensure the distance between the screen and the subject's nasion is 60 cm.
3. Finally, place a light-blocking mask over the subject's eyes.

2) Determining Phosphene Threshold over V1/V2

1. Knowing the subject's phosphene threshold will be important when exploring phosphenic behavior later in this protocol. To determine this threshold, first set the TMS machine to 70% power.
2. Hold the coil so the top of the figure-8 faces cranial midline.
3. Begin single-pulse s....

Discussion

This experiment gets at the heart of state dependency. Neurons in V1/V2 are thought to correspond with simple, directional movement.^7,8 Therefore, adaptation to the uni-directional motive stimuli increased the excitability of neurons registering this movement - as such, they should be the first to react to the TMS pulse. The V5/MT+ complex also contains neurons tuned to simple translational motion, therefore the phosphenes induced from this region should also be affected by the adapting stimulus. However, th.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Light Blocking Eye Mask
Ear Plugs
Swim Cap
Marker
Tape Measure
Blank Graph Paper
Stimuli Developed & Presented on Computer using Adobe Photoshop
Any Single Pulse Capable TMS Device
Any Figure-of-Eight Coil