Psychophysiology and Electrophysiology of the Visual System

The organization of the human afferent visual system is complex and visual processing requires enormous computational challenges and energy consumption at each stage of the visual pathway. Once the eye receives visual information, the signal is relayed by the retina, optic nerve, chiasm, tracts, lateral geniculate nucleus, and optic radiations to the striate and extra-striate association cortical areas for final visual processing. In the retina, photoreceptors convert light photons to electrochemical signals that are relayed to retinal ganglion cells via intermediate neurons. Ganglion cell axons run through the optic nerve, and after their partial decussation at the level of the optic chiasm, they reach the lateral geniculate nucleus where corresponding inputs from each hemiretina overlap. A minority of optic nerve axons target subthalamic nuclei that mediate pupil light reflexes and circadian rhythms. Axons originating from the lateral geniculate nucleus relay visual information through the optic radiations to the striate cortex. Feedback mechanisms from higher cortical areas shape the neuronal responses in early visual areas, supporting coherent visual perception. Detailed knowledge of the anatomy of the afferent visual system, in combination with skilled examination, allows precise localization of neuropathological processes and guides effective diagnosis and management of neuro-ophthalmic disorders. Assessment of all stages of visual system processing is allowed by a set of psychophysical and electrophysiological tests that is crucial in the clinical practice. This chapter encompasses the psychophysical basis and electrophysiological correlates of vision in the normal and pathological human visual system. It is our hope that this approach will facilitate the understanding and the interest in visual system investigation and assessment.