We used a novel binocular VEP setup for dichoptic mfVEP testing. Previous reports by Arvind et al.
15,16 described the use of virtual-reality goggles. However, inability to monitor fixation during recording made the technology difficult to implement. The new setup consisted of two mounted LCD screens (response time 2 ms; Flatron L1954TQS monitor; LG, Englewood Cliffs, NJ) on each side of the subject reflected through centrally located semitransparent mirrors to project a stimulus of 0° to 24° of eccentricity simultaneously to each eye. Mounted behind the mirrors were two infrared cameras, which continually monitored pupil position. Four infrared light-emitting diodes were placed around a lens holder to illuminate the eyes (
Fig. 1).
Stimuli were presented simultaneously (dichoptically) to both eyes, as described previously.
15 Briefly, the display to each eye consisted of a cortically scaled dartboard (
Fig. 1A) with 56 segments arranged in five concentric rings (1°–2.5°, 2.5°–5°, 5°–10°, 10°–16°, and 16°–24°) and a central fixation target extending up to 0.5°. The stimulus in any segment consisted of a 4 × 4 blue-on-yellow (BonY)
21 check pattern. Segment size was scaled according to the cortical magnification factor.
22 Corresponding to the size of the segments, the size of the individual checks also increased with eccentricity. The luminance of the blue check was 40 cd/m
2, and the luminance of the yellow background was 125 cd/m
2. A central fixation target was provided that consisted of rotating and slowly changing letters. These features—the ring arrangement and the fixation target—were identical for both eyes, which helped to fuse the images. The patient, therefore, perceived a single binocular image of the dartboard stimulus.
Pseudorandom binary sequences (PRBSs) were used to drive the stimuli at each test location, so that the presentation at each location was random and independent of other locations. Each binary sequence had a 50% probability of being 1 or 0 at any point of time. Element 1 was represented by two consecutive states:
pattern on, lasting two frames of the screen (33.3 ms), when the stimulus pattern was displayed, and
pattern off, lasting 16 frames (266.4 ms), when the whole segment was diffusely illuminated with an intensity of the mean luminance. Element 0 consisted of the pattern-off state for 18 frames. The average rate of presentation at each segment was 1.66 times/s. The presentation of the stimulus to the corresponding segment of the second eye was always shifted by nine frames; therefore, the minimum separation between stimuli to corresponding areas of the visual fields of both eyes was seven frames (116.7 ms). Three runs were recorded, each lasting 139 seconds. The technique is described in detail elsewhere.
15,16 Monocular recordings were performed with the same setup with one eye covered.