Fast Method for Precise Phosphene Threshold Identification in Research Using Transcranial Magnetic Stimulation Abrahamyan, A., Clifford C.W.G., and Harris.

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Presentation transcript:

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

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

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

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

Motor Threshold 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

Phosphene is an illusory 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

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

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

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

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

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

Efficiency of REPT vs MOBS Error bars indicate SD Procedure duration (sec) Participants 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

Limitation of the Method It only works with Magstim Rapid 2 stimulator

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