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Y P Berenson-Allen Center for Noninvasive Brain Stimulation O Department of Neurology Beth Israel Deaconess Medical Center C Harvard Medical School T O N Phosphenes O D Franziska Plessow, Ph.D. E S A E L Intensive Course in

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Intensive Course in Transcranial Magnetic Stimulation, 02/28/2017

Phosphenes

Franziska Plessow, Ph.D.

Berenson-Allen Center for Noninvasive Brain Stimulation Department of Neurology Beth Israel Deaconess Medical Center Harvard Medical School

P L E A S E D O N O T C O P Y

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What are the learning objectives of this session?

1. What are phosphenes?
2. What characterizes TMS-induced phosphenes?
3. When should you consider determining phosphene

threshold?

4. What are the key steps for determining phosphene

threshold?

Phosphenes

P L E A S E D O N O T C O P Y

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What are phosphenes?

§ From Greek phōs ‘light’ + phainein ‘to show’ § Brief visually perceived flashes of light triggered by activation of neurons within V1 § Not caused by phototransduction but interpreted as visual stimuli

Modified from Bear, Connors, & Paradiso, 1996

Phosphenes

P L E A S E D O N O T C O P Y

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How can phosphenes be elicited?

§ Mechanical stimulation

Rubbing your eyes
“Seeing stars” from a blow to the head
Pituitary tumor encroaching on optic chiasm

§ Electrical stimulation

Transcranial alternating current stimulation (tACS)
Transcranial magnetic stimulation (TMS)

Phosphenes

P L E A S E D O N O T C O P Y

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What characterizes TMS-induced phosphenes? § Subjective experience § Retinotopically organized § Simple geometric shapes § Appear as light (in darkness or eyes closed) § Appear as blurring or scotoma (eyes open) § Vary depending on level of visual cortex stimulated

From Marg & Rudiak, 1994. Optom Vis Sci

Phosphenes

P L E A S E D O N O T C O P Y

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When should you consider determining phosphene threshold? § To map the functional organization of the visual cortex § To determine state-dependent stimulation effects § To explore cortico-cortical interactions underlying visual attention and awareness § To assess the relative excitability of the visual cortex to determine procedural stimulation intensity § To record a secondary index of cortical excitability in non- motor regions

Phosphenes

P L E A S E D O N O T C O P Y

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Relationship between motor and phosphene threshold

From Gerwig et al., 2003. J Neurol Sci r28=0.29, p>0.1 0.25 0.35 0.45 0.55 0.65 0.75 0.25 0.45 0.65 Phosphene Threshold (% MSO) Motor Threshold (% of MSO)

Correlation (PT vs MT)

From Fried et al., 2011. PLoS One r16=0.27, p>0.2

Phosphenes

P L E A S E D O N O T C O P Y

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Neural basis of phosphene awareness

From Taylor et al., 2010. Hum Brain Mapp

Phosphenes

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What are the key steps for determining phosphene threshold?

1. Choosing a stimulation target (typically V1)
2. Setting up and ensuring safety
3. Locating the phosphene hotspot (adjusting location)
4. Assessing the phosphene threshold (adjusting intensity)

Phosphenes

P L E A S E D O N O T C O P Y

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Setting up and ensuring safety

Phosphenes

P L E A S E D O N O T C O P Y

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Locating the phosphene hotspot

1. Find the inion
2. Mark point 2 cm superior and 2

cm lateral

3. Orient coil with handle pointing

laterally:

Optimal current direction is lateral to medial in the brain

4. Phosphene should be reported

in the contralateral visual field

Phosphenes

P L E A S E D O N O T C O P Y

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Locating the phosphene hotspot

1. Set intensity to 30% MOS and deliver 3 pulses (6-10 s

apart)

2. Go up in steps of 10% until phosphene is reported
3. Deliver several pulses to ensure a consistent response

(phosphene) is evident (suprathreshold)

4. Construct and test a 5 x 5 cm grid around marked spot
Deliver 3 pulses at each location
Record phosphene incidence and vividness
5. The location that elicits the most consistent, vivid, and

unambiguous phosphene is the phosphene hotspot

Phosphenes

P L E A S E D O N O T C O P Y

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Assessing phosphene threshold

1. Set intensity to 30% MOS and deliver 3 pulses (6-10 s

apart)

2. Go up in steps of 10% until phosphene is reported
3. Deliver several pulses to ensure a consistent response

(phosphene) is evident (suprathreshold)

4. Record responses to 6 pulses
5. Progressively lower intensity (1-2%) until occurrence <50%
6. The lowest intensity that elicits a consistent, vivid, and

unambiguous phosphene in ≥3/6 pulses is your phosphene threshold

Phosphenes

P L E A S E D O N O T C O P Y

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Further considerations

§ Count only unambiguous responses (“yes” or “no”), redo “maybe” § Can be easier if subject is blindfolded (+ waiting time) § Have subject focus on center of visual field:

Directing attention to a part of the visual field will decrease threshold to relative to rest of visual field

Phosphenes

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Thank you for your active participation and good luck for the rest of the week! For further questions, contact me at any time: Franziska Plessow fplessow@mgh.harvard.edu