Visual Acuity. Test visual acuity for each eye separately (by covering one eye at a time) using an eye chart.
Color Vision. Test each eye separately for ability to distinguish colors. Test for red desaturation, a sign of subtle asymmetry in optic nerve function seen, for example, in optic neuritis (see Neuroanatomy through Clinical Cases, Key Clinical Concepts 11.4), by asking the patient to cover each eye alternately while looking at a red object and report any relative dullness of the color in one eye.
Visual Fields. Test visual fields for each eye by asking the patient to fixate straight ahead and to report when a finger can be seen moving in each quadrant. Alternatively, ask the patient to report how many fingers are being shown in each quadrant. More precise mapping of visual fields can be done in the laboratory for patients who will be followed over time (see Neuroanatomy Through Clinical Cases, Key Clinical Concepts 11.2).
Visual Extinction. Test for visual extinction on double simultaneous stimulation by asking the patients how many fingers they see when fingers are presented to both sides at the same time. In visual extinction, a form of hemineglect , patients do not report seeing the fingers on the affected (usually left) side of the visual field, although they can see fingers when they are presented to that side alone.
In comatose or uncooperative patients, visual fields can be tested roughly using blink-to-threat, in which the examiner's fingers are moved rapidly towards the patient's eyes from each quadrant to see if a blink occurs.
Damage anywhere in the visual pathway from the eye to the visual cortex can cause specific deficits in the visual fields of one or both eyes (see Neuroanatomy through Clinical Cases, Figure 11.15). Importantly, some visual information from each eye crosses to the opposite side at the optic chiasm. Therefore, lesions in front of the optic chiasm (eye, optic nerve) cause visual deficits in one eye, while lesions behind the optic chiasm (optic tract, thalamus, white matter, visual cortex) cause visual field deficits that are similar for both eyes.
Visual hemineglect or extinction is usually caused by contralateral parietal lesions, and less often by frontal or thalamic lesions. Neglect is usually more robust in lesions of the right hemisphere.