Electrophysiological assessment of the human visual system

In humans, visual information is processed simultaneously via multiple parallel channels. Condensed and parallel signals from the retina arrive in the primary visual cortex via the lateral geniculate nucleus. These signals then remain segregated until the higher levels of visual cortical processing through at least two separate but interacting parallel pathways; the ventral and dorsal streams. The former projects to the inferior temporal cortex for processing form and color, because it can detect visual stimuli with high spatial frequency and color. In contrast, the latter connects to the parietal cortex for detecting motion, because it responds to high temporal frequency stimuli. Based on these distinct physiological characteristics, we hypothesized that manipulating visual stimulus parameters would enable us to evaluate the different levels of each stream. So far, we have developed several techniques to record visual evoked potentials (VEPs) and event-related potentials (ERPs) with optimal stimuli. In this review, we first summarize current concepts of the major parallel visual pathways. Second, we describe the relationship between the parallel visual pathways and higher visual system dysfunction. Third, we introduce VEP and ERP techniques that can assess the function of each stream and region of visual cortex. Finally, we address the clinical applications of VEP and ERP recording techniques for several neurological disorders involving specific visual dysfunction.