Abstract
Purpose :
Previously, we developed and validated a stereo letter test that measures depth perception as a function of spatial frequency by varying the letter size. The test was easy to administer, fast, and the behavioral results were comparable to gratings oscillating in depth, which have been conventionally used in other studies. However, letters are cognitively meaningful stimuli and may be processed differently. Here, we assess the electrophysiological signals of 3D letters and gratings.
Methods :
We used a Biosemi 32 electrode EEG system to record visual event-related potentials while the subjects (n=6 with normal stereopsis) looked at stereo targets embedded in a dynamic Random Dot Stereogram (dRDS). Unlike flashed static RDS, dRDS have no monocular cues or motion artifacts. The dichoptic targets, which consisted of Sloan letters and proportionate square gratings at different orientations, were presented with a PROPixx projector and passive polarized glasses. Disparity profiles of crossed and uncrossed at 3 and 30 arc min were tested. The target appeared for 500ms, followed by interstimulus intervals between 1500-2500ms. The target size was 6.8°, corresponding to 0.4cpd gratings. The data were processed offline using EEGLAB software and referenced to the average.
Results :
Similar to gratings, stereo letters also evoked a negative component between ~150ms from target onset to ~500ms after offset in the occipital and parietal area. Consistent with the literature, the negative component had a larger amplitude with a larger disparity (~6mV for 3’ vs. ~12mV for 30’ in O1, Oz, and O2). The peak implicit time was also shorter with higher disparity. The amplitudes were similar between crossed and uncrossed disparity but crossed disparity had shorter peak implicit time (~300ms vs. ~400ms). The negative component was similar between them, except for the early phase before the peak. While there was no difference between letters and gratings in the occipital region, there were differences in the parietal region, with a slight hemispheric asymmetry with letters.
Conclusions :
In conclusion, stereo letters and gratings embedded in dRDS produce robust neural signals in the occipital area. The difference between letters and gratings likely indicates the recruitment of higher cognitive mechanisms. The latency difference in crossed versus uncrossed may reflect the general superiority in processing crossed disparity.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.