The process of “seeing” begins when visual information reaches the occipital striate cortex (area V1). For visual awareness to occur, this information must be processed by the associative cortical areas (see Chapter 1). Visual information is then projected through a ventral occipitotemporal pathway and a dorsal occipitoparietal pathway (see Chapter 1, Fig 1-22). The ventral pathway helps process the physical attributes of an image (the “what”) such as color, shape, and pattern. The dorsal pathway is important for visuospatial analysis (the “where”) and for guiding movements toward items of interest. In addition, interconnecting pathways are crucial for transferring information from the primary cortex to the associative areas (V2–V5).
Girkin CA, Miller NR. Central disorders of vision in humans. Surv Ophthalmol. 2001;45(5): 379–405.
Disorders of Recognition
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 termed object agnosia. The condition, which is a form of visual–visual disconnection, often results from a bilateral ventral pathway dysfunction that affects occipitotemporal projections. Patients with object agnosia can identify objects by touch or description but not by sight.
Prosopagnosia, the inability to recognize familiar faces, is a more specific form of visual– visual disconnection. It is more frequently congenital, with acquired cases resulting predominantly from stroke. Patients usually have difficulty performing other visual memory tasks. The condition usually occurs with bilateral inferior occipitotemporal lobe damage but may also occur with right inferior occipital lobe damage. Accompanying superior homonymous visual field defects are common.
Schmidt D. Neuro-ophthalmological findings in patients with acquired prosopagnosia. Graefes Arch Clin Exp Ophthalmol. 2015;253(3):333–334.
Patients with pathology affecting the dorsal pathway (area V5; also known as MT, medial temporal area) may experience akinetopsia, which is the loss of the perception of visual motion, but still retain perception of 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, the right occipital lobe receives visual information from the left visual field. This information is transferred to the left side of the brain through the corpus callosum and then relayed anteriorly to the angular gyrus of the parietal lobe for comprehension. If the splenium of the corpus callosum is damaged, the information from the left visual field (right occipital lobe) cannot cross to the opposite hemisphere (Fig 6-1). Typically, the left occipital lobe is also damaged and no visual input reaches the angular gyrus, resulting in alexia (ie, the patient cannot read); however, because the structures anterior to the splenium are intact, the patient can produce language and write (alexia without agraphia). These patients cannot read what they have just written! This condition is usually due to infarction. If the left angular gyrus 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).
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 according to hue. Bilateral occipital ventromedial cortex damage may cause complete achromatopsia. Unilateral damage may cause only hemiachromatopsia. Accompanying superior homonymous visual field defects are often present.
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 Eggenberger, DO.)
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.