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Abrahamyan, A., Clifford C.W.G., and Harris J.A.

Fast Method for Precise Phosphene Threshold Identification in Research Using Transcranial Magnetic Stimulation. Abrahamyan, A., Clifford C.W.G., and Harris J.A. School of Psychology, University of Sydney. Non-invasive method for brain stimulation Stimulates through intact skull

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Abrahamyan, A., Clifford C.W.G., and Harris J.A.

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  1. Fast Method for Precise Phosphene Threshold Identification in Research Using Transcranial Magnetic Stimulation Abrahamyan, A., Clifford C.W.G., and Harris J.A. School of Psychology, University of Sydney

  2. Non-invasive method for brain stimulation • Stimulates through intact skull • Can briefly and reversibly interfere with function of a target brain area What is TMS

  3. Current passes through the coil and induces magnetic field • Magnetic field induces electric current in the brain tissue • Induced electric current activates nearby neurons How Does TMS Work

  4. Level of TMS intensity requires calibration for each subject • Calibration ensures that TMS evokes comparable neural and behavioural responses between participants Calibration of TMS Intensity

  5. Motor Threshold is measured by applying TMS to the primary motor cortex • And measuring motor evoked potential that occurs on certain proportion of trials EMG Motor Threshold

  6. Phosphene is anillusory percept of flashes of light • TMS to the occipital lobe can induce phosphenes • Phosphene threshold is the intensity of TMS that induces phopshenes on an arbitrary number of trials (usually 50%) Phosphenes and Phosphene Threshold

  7. Abbreviated method of constant stimuli (MOCS) – slow and imprecise Proportions Stimulus Intensity • Modified Binary Search (MOBS) – fast but still not so accurate Existing Methods of Measuring Phosphene Thresholds

  8. Matlab library allows full control of the TMS unit • No need to use the dial to adjust the pulse intensity Researcher + = Matlab Library to Control TMS unit

  9. The method combines Matlab library with an adaptivestaircase • The staircase places the next trial in a way to gain maximum information when the trial is complete (Kontsevich & Tyler, 1999) Proportions • Method’s name: Rapid Estimation of Phosphene Threshold (REPT) Stimulus Intensity Proposed Method for Measuring Phosphene Thresholds

  10. REPT vs Method of Constant Stimuli (MOCS) • MOBS vs Method of Constant Stimuli (MOCS) Proportions Stimulus Intensity Comparing REPT with MOBS using Method of Constant Stimuli

  11. REPT threshold: average of 4 threshold measures • REPT thresholds are consistent with method of constant stimuli • Phosphene Threshold • Participants • MOBS threshold: average of 4 threshold measures • Thresholds for two subjects are not consistent with method of constant stimuli • Phosphene Threshold Accuracy of Threshold Estimates • Error bars indicate SEM • Participants

  12. Mean of 4 measures of threshold for each method • REPT takes around a minute • REPT is 1.5 to 2.5 times faster • REPT is predictable whereas MOBS is not • Procedure duration (sec) • Participants • Error bars indicate SD Efficiency of REPT vs MOBS

  13. It only works with Magstim Rapid2 stimulator Limitation of the Method

  14. REPT is • Accurate • Rapid • Predictable • REPT is user friendly • Will soon be available for free download Summary and Conclusion

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