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

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

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
EMG

Motor Threshold is measured by applying TMS to the
primary motor cortex

And measuring motor evoked potential that occurs on
certain proportion of trials
Motor Threshold


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
Abbreviated method of
constant stimuli (MOCS) –
slow and imprecise

Modified Binary Search
(MOBS) – fast but still not
so accurate
Proportions

Stimulus Intensity
Existing Methods of Measuring
Phosphene Thresholds
+


=
Matlab library allows full control of the TMS unit
No need to use the dial to adjust the pulse intensity
Matlab Library to Control TMS unit
Proportions
Stimulus Intensity


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)
Proposed Method for Measuring
Phosphene Thresholds
Proportions

REPT vs Method of
Constant Stimuli
(MOCS)

MOBS vs Method of
Constant Stimuli
(MOCS)
Stimulus Intensity
Comparing REPT with MOBS using
Method of Constant Stimuli
REPT vs MOCS
80
Phosphene Threshold
70

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
60
50
40
REPT
30
MOCS
20
10
0
JH
MR
AA
Participants
HM
MOBS vs MOCS
80
Phosphene Threshold
70
60
50
40
MOBS
30
MOCS
20
10
0
JH
MR
AA
Participants
HM
Error bars indicate SEM
Accuracy of Threshold Estimates
250
225

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)
200
175
150
125
REPT
100
MOBS
75
50
25
0
JH
MR
AA
Participants
Error bars indicate SD
HM
Efficiency of REPT vs MOBS

It only works with
Magstim Rapid2
stimulator
Limitation of the Method

REPT is
◦ Accurate
◦ Rapid
◦ Predictable

REPT is user friendly

Will soon be available for free download
Summary and Conclusion