In migraine with aura, the visual phenomena usually last 10–30 minutes and are followed by a typical headache (see Chapter 12). The hallucinations are binocular. Besides the classic fortification spectra (teichopsia), patients may experience “Alice in Wonderland effect” (micropsia/macropsia), formed or unformed images, or visual distortion. Common descriptions include heat waves, cracked glass, kaleidoscopic vision, or fragmented vision. Patients may also experience the visual phenomena without a headache (acephalgic migraine).

Persistent positive visual phenomena

This form of visual phenomena is often described as TV snow, TV static, dots (black-and-white or colored), or a rainlike pattern affecting the entire visual fields. Such visual phenomena usually last for months to years, and while they can be bothersome for the patient, they rarely interfere with visual function. They are commonly, but not exclusively, noted in patients with a personal or family history of migraine. Prophylactic treatment with migraine medication, however, is often unsuccessful.

Liu GT, Schatz NJ, Galetta SL, Volpe NJ, Skobieranda F, Kosmorsky GS. Persistent positive visual phenomena in migraine. Neurology. 1995;45(4):664–668.

Sleep-associated visual hallucinations

Hypnagogic hallucinations are vivid perceptual experiences occurring at sleep onset, whereas hypnopompic hallucinations are similar experiences that occur during awakening. Both phenomena are frequently associated with sleep disorders such as insomnia and daytime sleepiness and may be suggestive of narcolepsy.

Charles Bonnet syndrome

Visually impaired patients with preserved cognitive status may experience visual hallucinations, a condition known as Charles Bonnet syndrome. Patients may have formed or unformed hallucinations that are persistent or come and go abruptly. Hallucinations may be elementary or highly organized and complex. Patients with this syndrome have a clear sensorium and are aware that the visions are not real. If the cause of the vision loss is known and no homonymous visual field defect is present, neuroimaging is not necessary. Common underlying conditions for this syndrome include age-related macular degeneration, glaucoma, diabetic retinopathy, and cerebral infarction. Medical treatment is often disappointing.

Menon GJ, Rahman I, Menon SJ, Dutton GN. Complex visual hallucinations in the visually impaired: the Charles Bonnet syndrome. Surv Ophthalmol. 2003;48(1):58–72.

The Patient With Disorders of Higher Cortical Function

The visual information that reaches the occipital striate cortex (area V1) represents the beginning of the process of “seeing.” This information must be processed by the associative cortical visual areas for visual awareness to occur (see Chapter 1). Visual information that reaches the primary occipital cortex is projected through 2 pathways (see Chapter 1, Fig 1-22): a ventral occipitotemporal pathway and a dorsal occipitoparietal pathway. The ventral pathway is involved with processing the physical attributes of an image (the “what”) such as color, shape, and pattern. The dorsal pathway is important for visuospatial analysis (the “where,” or the localization of items in space) and for guiding

movements toward items of interest. In addition, interconnecting pathways are crucial to the transfer of information from the primary cortex to the associative areas (areas V2–V5).

In general, cortical syndromes due to abnormalities in visual processing can result through 2 mechanisms: (1) specific cortical areas responsible for processing information may be damaged, or

(2) the flow of information between such areas may be interrupted (disconnection syndromes). The disorders of higher cortical visual function may be further subdivided into disorders of object recognition, visual–spatial relationships, and awareness of visual deficit (Table 6-2).

Table 6-2

Girkin CA, Miller NR. Central disorders of vision in humans. Surv Ophthalmol. 2001;45(5):379–405.

Disorders of Recognition

Object agnosia

Interruption of signal flow from the occipital lobe to the area of the temporal lobe involved in object identification results in an inability to recognize objects (eg, a pen, bottle, or car) termed object agnosia, a form of visual–visual disconnection. The condition often results from a bilateral occipitotemporal (ventral pathway) dysfunction affecting the inferior longitudinal fasciculi. Patients can identify objects by touch or by description but not by sight.

Prosopagnosia

Prosopagnosia, the inability to recognize familiar faces, is a more specific form of visual–visual disconnection. These patients usually have difficulty with other visual memory tasks. The condition usually occurs with bilateral occipital lobe damage but may also occur with right inferior occipital lobe damage. Accompanying superior homonymous visual field defects are common. It is thought that this form of agnosia is the reason patients with advanced Alzheimer disease do not recognize their relatives.

Barton JJ, Cherkasova MV. Impaired spatial coding within objects but not between objects in prosopagnosia. Neurology. 2005;65(2):270–274.

Akinetopsia

Patients with pathology affecting the dorsal pathway (area V5; also known as MT, medial temporal area) may experience akinetopsia, loss of the perception of visual motion, but still be able to perceive form, texture, and color.

Alexia without agraphia

The interruption of visual information between the occipital lobe and the dominant angular gyrus causes visual–verbal disconnection. During the act of reading, visual information from the left visual field is received in the right occipital lobe and is transferred to the left side of the brain through the corpus callosum, where the information is relayed anteriorly to the angular gyrus of the parietal lobe for comprehension. However, the information from the left visual field cannot cross from the right to the left occipital lobes if the splenium of the corpus callosum is damaged (Fig 6-1). Typically, there is also damage to the left occipital lobe, and this combination results in alexia without agraphia (ie, the patient cannot read but can write). This condition is usually due to infarction of the left occipital lobe and fibers crossing in the splenium of the corpus callosum. However, because the structures anterior to the splenium are intact, these patients can produce language and write. Interestingly, they cannot read what they have just written! If the left angular gyrus itself is damaged, then both reading and writing will be affected (alexia with agraphia). Such patients also often have acalculia, right–left confusion, and finger agnosia (Gerstmann syndrome).

Figure 6-1 Alexia without agraphia. The diagram depicts the flow of information (arrows) from the right occipital lobe through the splenium of the corpus callosum to the angular gyrus. A lesion (bright-colored polygon) in the left occipital lobe

obstructs this flow. (Courtesy of Eric Eggenb erger, DO.)

Biran I, Coslett HB. Visual agnosia. Curr Neurol Neurosci Rep. 2003;3(6):508–512.

Cerebral achromatopsia

Color discrimination may be abnormal with bilateral inferior occipitotemporal lobe lesions (lingual and fusiform gyrus; see Chapter 1). Affected patients cannot match colors or order them in a series