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Abdlsaed Al Abdlseaed, Daphne L. McCulloch, Ruth Hamilton; Characterisation Of Short Latency Visual Evoked Potentials (VEPs). Invest. Ophthalmol. Vis. Sci. 2011;52(14):270.
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Short latency VEPs (<60 ms), thought to represent sub-cortical visual activity, may contribute to localisation of lesions affecting the anterior and/or posterior visual pathways. The existence, source and optimal recording for short latency VEPs has been debated. We aimed to investigate the optimal acquisition and stimulation parameters for these responses.
Transient averaged VEPs were recorded monocularly and binocularly in eleven healthy adults with simultaneous ERG recorded from skin electrodes. The acquisition parameters altered to find a robust protocol for capturing short latency VEPs were: electrode montage (Oz-Cz, T4½-Cz and Cz-neck) and amplifier bandpass including lowpass settings (1000, 800, 400 and 300 Hz) and highpass settings (10, 20, 40, 80 and 100 Hz). Recording epochs were 100, 150 and 250 ms; sampling frequency was 2000 Hz. At least 800 epochs were recorded as two sub-averages to judge reproducibility. For xenon flash stimulation, we investigated three stimulus frequencies (3.30, 6.06, and 9.01 Hz) and seven luminance levels (of 1.1, 2.7, 3.0, 6.8, 10.0, 17.1 and 43.0 cd s/m2). Pattern stimuli were high contrast checkerboards including: black & white checks (120’, 60’ and 15’) reversing at 3.03 and 8.03 reversals/s; red & green checks (24’, 8.03 reversals/s) and black & white onset-offset checks (60’ and 15’).
For xenon flash, a consistent tri-phasic short-latency waveform with a positive, negative, and positive at 35-50 ms was recorded at T4½-Cz and Cz-neck. ERGs and cortical VEPs were evident but differences in peak latencies and luminance-response functions suggested different origins. No pattern stimuli elicited a reproducible short latency waveform and there was no detectable auditory response from the flash stimulator. A stimulation rate of 6.06 Hz, recording epoch of 150 ms and bandpass of 40-400 Hz or 80-400 Hz were optimal for viewing the response. Flash luminance of 10.0, 17.1 and 43.0 cd s/m2 were each effective. By inspection, the short latency VEP waveform was larger for binocular viewing.
A short latency VEP waveform is reliably recordable to full field flashes of 10 cd s/m2 or stronger in healthy adults, and appears to be independent of the ERG and of the auditory evoked potential. A short-latency tri-phasic waveform is well visualised when recorded over Cz using a bandpass of 40-400 or 80-400 Hz. This may provide a tool for the exploration of sub-cortically-mediated vision.
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