Multifocal VEP Responses to Pattern–Reversal, Onset, Offset, and Sparse Pulse Stimuli

Abstract

Abstract: : Purpose: Hood and colleagues (see PRER 2003) have proposed that diagnostic performance of the multifocal VEP (mfVEP) will be related to its signal–to–noise ratio (SNR). James (IOVS 2003) suggested that pattern pulse stimulation provides a nearly two–fold increase in mfVEP SNR. Our purpose was to compare standard mfVEP pattern–reversal responses with those produced by pattern–onset, –offset and pulsed stimuli. Methods: Three–channel mfVEP recordings were obtained from 5 normal subjects using VERIS and a standard 4–electrode array. The stimulus was presented on a monitor refreshed at 75 Hz; geometry was constant (Dart Board 60 With Patterns), had a total diameter of 44^o, mean luminance of 100 cd/m² and contrast >98%. Signals were filtered 3–100 Hz and sampled at 1200 Hz. The standard reversal stimulus had 2¹⁵ m–sequence steps that required 7.5 min to complete. Pattern–onset and –offset responses were evaluated using sequences that all had 32 frames per m–step and 2¹⁰ total steps (7.5 min); but the duration of the contrast step varied so that it was either 1, 2, 4, 8, 12, or 16 of the 32 frames. The same series was also inverted so that adapting contrast was high and the stimulus step began with a contrast decrement. The effect of temporal sparseness was studied with positive contrast pulses (always 2–frames in duration) within either 16, 20, 24, 28 or 32 frames per m–step (all had 2¹¹ total steps, so the 2–in–16 required 7.5 min to complete). There were two runs of each stimulus for each subject (completed in random order over 6 days). SNR was calculated by two methods for the vertical channel and the "best" of 6 channels (3 recorded, 3 derived). Results: Comparable results were obtained in all 5 subjects. The standard mfVEP reversal response was virtually identical to the onset response throughout the entire field, but ∼3.5 times smaller. Onset responses were larger than offset, especially in the lower hemifield, irrespective of adapting contrast level. Pulse responses were 4 times larger than standard reversal responses throughout the field, but the SNR was only improved by 10 ± 21% (n.s.). Conclusions: Temporally sparse pattern pulse stimuli improve SNR by only 10% over the standard reversal mfVEP stimulus. The mfVEP response depends on the polarity of the contrast step, irrespective of the state of contrast adaptation, and this effect is markedly stronger in the inferior hemifield.

Keywords: electrophysiology: clinical • visual fields • visual cortex