Asymmetry of visual sensory mechanisms: electrophysiological, structural, and psychophysical evidences.

Psychophysical visual field asymmetries are widely documented and have been attributed to anatomical anisotropies both at the retinal and cortical levels. This debate on whether such differences originate within the retina itself or are due to higher visual processing may be illuminated if concomitant anatomical, physiological, and psychophysical measures are taken in the same individuals. In the current study, we have focused on the study of objective functional and structural asymmetries at the retinal level and examined their putative correlation with visual performance asymmetries. Forty healthy participants (80 eyes; 13 male and 27 female subjects) were included in this study. Objective functional/structural asymmetries were probed using the multifocal electroretinogram (mfERG) technique and optical coherence tomography (OCT), respectively. A nasal/temporal pattern of asymmetry (nasal visual hemifield disadvantage) was found for all methods (retinal thickness, contrast sensitivity, and mfERG P1 amplitude). Furthermore, superior/inferior asymmetries could be documented only with psychophysics and structural measures. These patterns likely arise at different levels of the retina as inferred by partly independent correlation patterns. We conclude that patterns of structural/functional asymmetries arise at different levels of visual processing with a strong retinal contribution.