Y P O C T O N Phosphenes O D Intensive Course in E - - PowerPoint PPT Presentation

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Nov 01, 2023 439 likes •544 views

Y P O C T O N Phosphenes O D Intensive Course in E Transcranial Magnetic Stimulation S A E October 2016 L P Y P Phosphene: definition O C From Greek phs light + phainein to show T Brief visual percept

SLIDE 1

Phosphenes

Intensive Course in Transcranial Magnetic Stimulation October 2016

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

SLIDE 2

Phosphene: definition

From Greek phōs ‘light’ + phainein ‘to show’
Brief visual percept “flash of light”
Not caused by phototransduction
But interpreted as visual stimuli

Modified from Bear, Connors, Paradiso, 1996

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

SLIDE 3

Phosphene: causes

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)

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

SLIDE 4

Characteristics of TMS phosphenes

Subjective experience
Retinotopically organized
Simple geometric shapes
Appear as light (in

darkness or eyes closed)

Appear as blurring or

scotoma (eyes open)

From Marg & Rudiak, 1994

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

SLIDE 5

Locating Phosphene Hotspot

Find the inion
Mark point 2cm dorsal and

2cm lateral

Orient coil with handle

pointing laterally

– Optimal current direction is lateral to medial (in brain)

Phosphene should be

reported in contralateral visual field

Optimal current direction: lateral to medial in brain

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

SLIDE 6

Assess Phosphene Threshold

Follow same procedure as motor threshold

– Begin subthrehsold (e.g., 30% MSO) – Increase in steps of 10 until phosphene reported – Decrease in steps of 1-2 until occurrence < 50% – Count only unambiguous responses (“yes” or “no”), redo “maybe”

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

SLIDE 7

Considerations

Can be easier if subject is blindfolded
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

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

SLIDE 8

No significant correlation between motor and phosphene thresholds

From Gerwig et al., 2003, J Neurol Sci r28=0.29, p>0.1

0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.25 0.35 0.45 0.55 0.65 0.75

Phosphene Threshold (% MSO) Motor Threshold (% of MSO)

Correlation (PT vs MT)

From Fried et al., 2011, PLoS1 r16=0.27, p>0.2

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

SLIDE 9

Neural basis of phosphene awareness

From Taylor et al., 2010, Human Brain Mapping