How the Crossed Visual Pathway Reaches Earlier than the Uncrossed One Does

Abstract

Abstract: : Purpose: Hood, et al. (IOVS, 2000) discovered monocular recorded multifocal visual evoked potentials (mfVEPs) from both eyes of the same subject are nearly identical, and also the latency difference at each location seem to reflect a small difference in the time it takes signals. We extract the spatial distribution of the difference in time in the interocular comparison of mfVEPs between the both eyes. Methods: Three normal volunteers served as subjects. MfVEPs were recorded with the VERIS Version 4.0 (EDI, San Mateo, CA). A dartboard pattern consisting of 60 stimulus sectors stimulated the central 40 ° of the visual field. Total net recording time was 116'30" for each eye. The difference in timing between two eyes for each subject was extracted with a scalar product method at each stimulus sector and plotted. Results: The left eye leads in several ms in the left visual field and delays symmetrically in the right field. That discrepancy has horizontal gradient approximately up to 6 ms and, therefore, small in sectors close to the nasotemporal division. It is also relatively equable across the eccentricity in the visual field except at the very center region. Conclusions: The interocular timing comparison of mfVEPs illustrates the crossed visual pathway reaches earlier than the uncrossed one does. Anatomical differences between the crossed and uncrossed pathways should contribute to the difference in timing. Whether the fiber is crossed or uncrossed itself does not decide the timing because the topography of the gap did not draw any demarcation line on the nasotemporal division in the visual field. As Hood, et al. suggested, the unmyelinated intraretinal conduction from the ganglion cell lying to the optic disc is probably the most responsible. The relative equableness of the gap across the eccentricity may be explained by the fiber diameter that increases with the eccentricity of the originating ganglion cell and is the smallest in the papillo-macular bundle. Even though the nasal region of retina has the disadvantage of having the blind spot, it may transmit visual events more quickly and, therefore, be favorable for handling the broader lateral visual field.

Keywords: visual fields • binocular vision/stereopsis • visual cortex