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Y P Berenson-Allen Center for Noninvasive Brain Stimulation O - PowerPoint PPT Presentation

Mar 09, 2024 •381 likes •546 views

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

  1. 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 Transcranial Magnetic Stimulation, P 02/28/2017

  2. Y P What are the learning objectives of this session? O C 1. What are phosphenes? T 2. What characterizes TMS-induced phosphenes? O N 3. When should you consider determining phosphene threshold? O 4. What are the key steps for determining phosphene D threshold? E S A E L P Phosphenes 2

  3. Y P What are phosphenes? O C § From Greek ph ō s ‘light’ + phainein ‘to show’ T § Brief visually perceived flashes of light triggered by O activation of neurons within V1 N § Not caused by phototransduction but interpreted as visual O stimuli D E S A E L P Modified from Bear, Connors, & Paradiso, 1996 Phosphenes 3

  4. Y P How can phosphenes be elicited? O C § Mechanical stimulation T - Rubbing your eyes O - “Seeing stars” from a blow to the head N - Pituitary tumor encroaching on optic chiasm O § Electrical stimulation D - Transcranial alternating current stimulation (tACS) - Transcranial magnetic stimulation (TMS) E S A E L P Phosphenes 4

  5. Y P What characterizes TMS-induced phosphenes? O C § Subjective experience T § Retinotopically organized O N § Simple geometric shapes § Appear as light (in darkness or O eyes closed) D § Appear as blurring or scotoma E (eyes open) S § Vary depending on level of A visual cortex stimulated E L P From Marg & Rudiak, 1994. Optom Vis Sci Phosphenes 5

  6. Y P When should you consider determining phosphene threshold? O C § To map the functional organization of the visual cortex T § To determine state-dependent stimulation effects O N § To explore cortico-cortical interactions underlying visual attention and awareness O § To assess the relative excitability of the visual cortex to D determine procedural stimulation intensity E § To record a secondary index of cortical excitability in non- S motor regions A E L P Phosphenes 6

  7. Y P Relationship between motor and phosphene threshold O C T O 0.75 Correlation (PT vs MT) N Phosphene Threshold (% MSO) 0.65 0.55 O 0.45 D 0.35 E r 28 =0.29, p>0.1 r 16 =0.27, p>0.2 0.25 S 0.25 0.45 0.65 Motor Threshold (% of MSO) From Gerwig et al., 2003. J Neurol Sci A From Fried et al., 2011. PLoS One E L P Phosphenes 7

  8. Y P Neural basis of phosphene awareness O C T O N O D E S A E L P From Taylor et al., 2010. Hum Brain Mapp Phosphenes 8

  9. Y P What are the key steps for determining phosphene threshold? O C 1. Choosing a stimulation target (typically V1) T 2. Setting up and ensuring safety O N 3. Locating the phosphene hotspot (adjusting location) 4. Assessing the phosphene threshold (adjusting intensity) O D E S A E L P Phosphenes 9

  10. Y P Setting up and ensuring safety O C T O N O D E S A E L P Phosphenes 10

  11. Y P Locating the phosphene hotspot O C 1. Find the inion T 2. Mark point 2 cm superior and 2 O cm lateral N 3. Orient coil with handle pointing O laterally: D Optimal current direction is lateral to medial in the brain E 4. Phosphene should be reported S in the contralateral visual field A E L P Phosphenes 11

  12. Y P Locating the phosphene hotspot O C 1. Set intensity to 30% MOS and deliver 3 pulses (6-10 s T apart) O 2. Go up in steps of 10% until phosphene is reported N 3. Deliver several pulses to ensure a consistent response O (phosphene) is evident (suprathreshold) D 4. Construct and test a 5 x 5 cm grid around marked spot E - Deliver 3 pulses at each location S - Record phosphene incidence and vividness A 5. The location that elicits the most consistent, vivid, and E unambiguous phosphene is the phosphene hotspot L P Phosphenes 12

  13. Y P Assessing phosphene threshold O C 1. Set intensity to 30% MOS and deliver 3 pulses (6-10 s T apart) O 2. Go up in steps of 10% until phosphene is reported N 3. Deliver several pulses to ensure a consistent response O (phosphene) is evident (suprathreshold) D 4. Record responses to 6 pulses E 5. Progressively lower intensity (1- 2 %) until occurrence <50% S 6. The lowest intensity that elicits a consistent, vivid, and A unambiguous phosphene in ≥ 3/6 pulses is your E phosphene threshold L P Phosphenes 13

  14. Y P Further considerations O C § Count only unambiguous responses (“yes” or “no”), T redo “maybe” O § Can be easier if subject is blindfolded (+ waiting time) N § Have subject focus on center of visual field: O Directing attention to a part of the visual field will decrease threshold to D relative to rest of visual field E S A E L P Phosphenes 14

  15. Y P O C T Thank you for your active participation O and good luck for the rest of the week! N O For further questions, contact me at any time: D Franziska Plessow fplessow@mgh.harvard.edu E S ? A E L ? P Phosphenes 15

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