RETeval Complete Option
RET
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Device User Manual 51
VEP protocols
Flash VEP protocols flash light in the eye and measure the visual system’s response on the back
of the head (Odom et al. 2010). There are two flash VEP protocols: a 3 cd·s/m² @ 1 Hz protocol
and a 24 Td·s @ 1 Hz. The two protocols are equivalent when the pupil diameter is 3.2 mm
(8 mm2 area). Both use 64 flashes to average the response.
The analysis uses the same processing as the ISCEV protocols, with the following exceptions: The
0-phase high pass filter is set to 1 Hz to reduce electrode drift. Outlier replicates whose
amplitudes exceed 100 µV are removed. The wavelet-based denoising is configured to remove
additional high frequency components as the expected VEP response does not contain high
frequencies.
Flash VEP measurements depend on the response from the retina being transmitted through the
optic nerve to the occipital cortex and therefore can be used as an indicator of visual function.
Flash VEP measurements are highly variable among individuals, but are fairly repeatable for one
individual. Running replicates, which is an option in these tests, can help distinguish the evoked
response from other biological signals.
See Performing a VEP test on page 52 for details on how to do a flash VEP.
Custom protocols
If there is a protocol that you would like to run that is not built-in, the RET
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device has support
for extending the number of options through custom protocols. Custom protocols can be placed
in the Protocols folder on the device, and then can be selected through the User Interface in a
manner similar to selecting a built-in protocol. The built-in protocols can be viewed on the device
in the folder EMR/built-in protocols, which can be a starting point for creating your own custom
protocols. Protocols are written in the full-featured Lua programming language. Contact LKC
(email: support@lkc.com) if you would like assistance in making a custom protocol.
Examples of what can be done with custom protocols follows.
Multiple test steps
Custom protocols can have multiple test steps. These test steps can have the same or different
stimulation and analysis settings. They can be performed in a pre-specified or randomized order.
Randomization may be useful to eliminate time being a confounding variable. The device can
pause between test steps, enabling a review of the data and possible replication of the trial, or
the device can proceed between steps as fast as possible (without operator review).
Stimulus
The stimulus can compensate for pupil size (Trolands) or not. When compensating for pupil size,
one can also choose whether or not to compensate for the Stiles-Crawford effect. The stimulus
color can be expressed in CIE 1931 (x,y) chromaticity or in brightness for each color LED
separately (red, green, blue). Flash energy and background luminance can be specified.
Alternatively, extended-duration stimuli, such as ramps (step on and step off), sinusoids, and
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