RETeval Complete Option
RET
eval
Device User Manual 52
square wave (on-off) stimuli can be specified. Using the on-off stimulus specification, one can,
for example, experiment with variable-duration flashes. The RET
eval
sinusoidal stimulus has
been carefully constructed to minimize the harmonic distortion (< 1% per harmonic), so that any
harmonics in the response are attributable to nonlinearities in the visual system. The dominant
wavelength and brightness range for each LED is shown in the specification table on Page 86.
Luminance is specified in photopic units. The effective luminance for rods (scotopic units) is
different as the spectral sensitivity between rods and cones differ. For the RET
eval
LEDs, the
ratio of scotopic to photopic sensitivity is 0.032, 2.3, and 16 for red, green, and blue respectively.
As an example, rods are 16 times more sensitive to blue light than cones. For white light (CIE
0.33, 0.33), rods are 3.0 times more sensitive than cones.
Analysis
The sampling rate can be selected to have a period of 2048 µs (~500 Hz), 1024 µs (~1 kHz), 512
µs (~2 kHz, default), or 256 µs (~4 kHz). Flicker tests can specify the number of harmonics to
analyze, up to 32 harmonics. Flash tests can specify filtering used. The high-pass filter frequency
cutoff (3 dB) point can be specified. The low-pass filtering can be selected between wavelet
denoising and a 0 phase low pass filter. The low pass filter frequencies can be selected among
25, 50, 61, 75, 100, 125, 150 Hz for the ~500 Hz sampling rate; 50, 61, 75, 100, 122, 150, 200, 250,
300 Hz for the ~1 kHz sampling rate; 61, 100, 122, 150, 200, 244, 300, 400, 500, 600 Hz for the
~2 kHz sampling rate; and 61, 122, 200, 244, 300, 400, 488, 600, 800, 1000, 1200 Hz for the ~4 kHz
sampling rate. The low-pass filter frequencies specify the edge of the pass band of the filter.
Pupil measurements can be collected regardless of the stimulus selected.
Any stimulus can be post-processed for oscillatory potential analysis.
Any stimulus can be post-processed for a- and b- wave cursors, and PhNR cursor analysis.
Reference data
Reference data depends on the stimulus, electrode, and analysis used. If there is a match
between a test step and the reference data on the device, the relevant reference data will be
presented automatically. Reference data can also be explicitly disabled in a custom protocol.
Language translations
Custom protocols can be written in any language; however, they cannot be automatically
translated into other languages.
Performing a VEP test
There is an ISCEV standard for performing flash VEPs (Odom et al. 2010). Place the electrodes as
described below on the head and stimulate each eye in a similar manner to an ERG test. Perform
replicates so that the aspects of the waveforms resulting from the light stimulation can be more
easily identified.
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