Human eye

• The human eye is an organ that reacts to light and has several purposes. As a sense organ, the mammalian eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors.

• Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock.

Rods and cones

• A photoreceptor cell is a specialized type of neuron found in the retina that is capable of phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

• The two classic photoreceptor cells are rods and cones

Generator of VEP

• To understand the mechanism of VEPs, it is important to know the physiology of the visual path ways (in figure).

• The visual field is divided in to temporal and nasal fields. And an input from each field travels through separate pathways.

• Through the lens, the nasal visual field is projected to the temporal retina and temporal visual field is projected to the nasal retina.

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• Similarly the superior and inferior visual field are inverted. The superior visual field projects to inferior retina and the inferior visual field reflect to the superior retina.

• The output of rods and cones projects to bipolar cells and then to the ganglion cells. The axons of the ganglion cells from the retinal fibers that exit the eye ball as the optic nerve. The optic nerves are cranial nerve ii and both nerves from left and right eyes merge at the optic chiasm, where the input from nasal retina (tem visual field) of each eye crosses to the opposite hemisphere while the input from the temporal retina (nasal visual field) travels ipsilaterally, forming the optic tract after the chiasm.

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• As a consequence, the Lt optic tract contains fibers from the temporal retina of Lt eye and the nasal retina of Rt eye (Rt homonymous visual field), which reflect the Rt half of the visual field from each eye. Conversely, the Rt optic tract contains fibers from the temporal retina of Rt eye and the nasal retina of the Lt eye, reflecting the Lt half of the visual field from each eye (Lt homonymous visual field).

• The optic tract enters the lateral geniculate body in each hemisphere. After the synaptic connection at the lateral geniculate body, the optic nerve fibers spread widely in the temporal and parietal lobes forming optic radiations. The optic radiation from the inferior retina (reflecting to the superior visual field) swings toward the tip of the temporal lobe. This is known as the loop of Meyers.

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• The wide spread optic radiation converge as they approach the occipital cortex. The primary visual cortex, named Brodman area 17, is situated at the medial occipital cortex and occipital pole, which is also called the striate cortex. The central 3 degrees of the vision (macular vision) occupy 1/3 of the primary visual cortex. The peripheral visual field is represented in a deeper portion of the medial cortex.

Generation • The visual evoked potential (VEP) is primarily a relatively large,

positive polarity wave generated in the occipital cortex in response to visual stimulation.

• The optic nerve is the primary structure examined.

• VEP measures the conduction time of neuronal activity from the retina to the occipital cortex and is used clinically as a measure of the integrity and function of that pathway.

• Of primary interest is the latency of the positive wave at a midline occipital EEG electrode, usually at approx 100 ms after stimulation, called the P100.

Medial aspect of the occipital lobe showing primary visual cortex (area 17) and visual association areas (areas 18 and 19)