Visual information is relayed from the lateral geniculate body to the visual cortex via myelinated axons in the optic radiations. As described later, multiple retinotopic maps of the visual world are present within the cortex. The primary visual cortex is the main way station for incoming visual signals. Ultimately, however, visually responsive neurons within at least six parts of the occipital cortex and within the temporal and parietal lobes form separate visual areas, each with its own map of the retina.
A functional magnetic resonance image showing activation of the visual cortex in response to a patterned visual stimulus is displayed in Figure 15–18. When the left half of the visual field is stimulated with a visual pattern, the visual cortex on the right side is activated and vice versa.
The primary visual cortex receives its blood from the calcarine branch of the posterior cerebral artery. The remainder of the occipital lobe is supplied by other branches of this artery. The arterial supply can be (rarely) interrupted by emboli or by compression of the artery between the free edge of the tentorium and enlarging or herniating portions of the brain.
The primary visual cortex (also termed calcarine cortex, area 17, or V1) is located on the medial surface of the occipital lobe above and below the calcarine fissure (Fig 15–15). This cortical region is also called the striate cortex because, when viewed in histologic sections, it contains a light-colored horizontal stripe (corresponding to white matter–containing myelinated fibers) within lamina IV. When stained for the mitochondrial enzyme cytochrome oxidase, superficial layers (layers 2 and 3) of area 17 appear to be organized into enzyme-rich regions (termed blob regions on their discovery) and enzyme-poor interblob regions. Within the superficial layers of area 17, parvocellular inputs carrying color information tend to project to the blob regions, whereas inputs concerned with shape as well as color converge on the interblob regions. Magnocellular inputs, carrying information about motion, depth, and form, in contrast, project to deeper layers of the striate cortex.
Visual Association (Extrastriate) Cortex
Beyond the primary visual cortex, several other visual areas (area 18 and area 19) extend concentrically outside the primary visual cortex. These areas are also called the extrastriate cortex or the visual association cortex. Two separate retinotopic visual maps are located within area 18 (V2 and V3), and three retinotopic maps are located in area 19 (V3A, V4, and V5). V2 contains cytochrome-rich stripes, separated by cytochrome-poor interstripes. Continuing the theme of multiple, parallel visual information-processing pathways, magnocellular inputs relay within the thick stripe regions, whereas parvocellular inputs are processed in interstripe and thin stripe zones of V2.
The accurate examination of visual defects in a patient is of considerable importance in localizing lesions. Such lesions may be in the eye, retina, optic nerve, optic chiasm or tracts, or visual cortex.
Impaired vision in one eye is usually due to a disorder involving the eye, retina, or the optic nerve (Fig 15–16A).
Field defects can affect one or both visual fields. If the lesion is in the optic chiasm, optic tracts, or visual cortex, both eyes will show field defects.
A chiasmatic lesion (often owing to a pituitary tumor or a lesion around the sella turcica) can injure the decussating axons of retinal ganglion cells within the optic chiasm. These axons originate in the nasal halves of the two retinas. Thus, this type of lesion produces bitemporal hemianopsia, characterized by blindness in the lateral or temporal half of the visual field for each eye (Fig 15–16B).
Lesions behind the optic chiasm cause a field defect in the temporal field of one eye, together with a field defect in the nasal (medial) field of the other eye. The result is a homonymous hemianopsia in which the visual field defect is on the side opposite the lesion (Figs 15–16C, E, and 15–17).
Because Meyer's loop carries optic radiation fibers representing the upper part of the contralateral field, temporal lobe lesions can produce a visual field deficit involving the contralateral superior ("pie in the sky") quadrant. This visual field defect is called a superior quadrantanopsia (Fig 15–16D). An example is discussed in Clinical Illustration 15–1.
Typical lesions of the visual pathways. Their effects on the visual fields are shown on the right side of the illustration. A: Blindness in one eye. B: Bitemporal hemianopia. C: Homonymous hemianopia. D: Quadrantanopsia. E: Homonymous hemianopia.
Occipital hematoma (arrow) resulting from a bleeding arteriovenous malformation. This lesion produced homonymous hemianopia and headache. (Reproduced, with permission, from Riordan-Eva P, Whitcher JP: General Ophthalmology. 17th ed. McGraw-Hill, 2008.)
Activation of visual cortex as shown by functional magnetic resonance imaging (fMRI). A: An oblique axial anatomic MRI. The region showing increased activity in response to a full-field patterned stimulus has been assessed by fMRI (using a method known as echoplanar MRI) and is shown in white. B: Activation of the visual cortex on the left side in response to patterned visual stimulation of the right visual hemifield (black) and activation of the right-sided visual cortex in response to patterned stimulation of the left hemifield. The changes in signal intensity are the result of changes in flow, volume, and oxygenation of the blood in response to the stimuli. (Data from Masuoka LK, Anderson AW, Gore JC, McCarthy G, Novotny EJ: Activation of visual cortex in occipital lobe epilepsy using functional magnetic resonance imaging. Epilepsia 1994;35[Supp 8]:86.)
CLINICAL ILLUSTRATION 15–1
A 28-year-old physical education teacher, previously well, began to experience "spells," which began with a feeling of fear and epigastric discomfort that gradually moved upward. This was followed by a period of unresponsiveness in which the patient would make chewing movements with his mouth. Over the ensuing year, the patient had several generalized seizures. A computed tomography (CT) scan was read as normal, but an electroencephalogram showed epileptiform activity in the right temporal lobe. Temporal lobe epilepsy was diagnosed, and the patient was treated with anticonvulsants. The seizures stopped.
Three years later, the patient complained of "poor vision in his left eye" and of a left-sided headache, which was worse in the morning. An ophthalmologist found a homonymous quadrantanopsia ("pie in the sky" field deficit) in the left upper quadrant. Neurologic examination now revealed a Babinski response and increased deep tendon reflexes on the left side in addition to the homonymous quadrantanopsia. CT scan showed a mass lesion in the right anterior temporal lobe surrounded by edema.
The patient underwent surgery and an oligodendroglioma was found. After surgical removal, the patient's visual field deficit persisted. Nevertheless, he was able to return to work.
This case illustrates that patients may complain of visual loss in the right or left eye when, in fact, they have a homonymous hemianopia or quadrantanopsia on the corresponding side. In this patient, examination revealed a left-sided upper quadrantanopsia, which was caused by a slow-growing oligodendroglioma impinging on the optic radiation axons traveling in Meyer's loop. Recognition of the tumor at a relatively early stage facilitated its neurosurgical removal.
Examination of the visual fields is an important part of the workup of any patient with a suspected lesion in the brain. The visual pathway extends from the retina to the calcarine cortex in the occipital lobe. As outlined in Figure 15–16, lesions at a variety of sites along this pathway produce characteristic visual field defects. Recognition of these visual field abnormalities often provides crucial diagnostic information.
Abnormalities of pupillary size may be caused by lesions in the pathway for the pupillary light reflex (see Figs 8–9 and 15–14) or by the action of drugs that affect the balance between parasympathetic and sympathetic innervation of the eye (Table 15–1).
Argyll-Robertson pupils, usually caused by neurosyphilis, are small, sometimes unequal or irregular, pupils. The lesion is thought to be in the pretectal region, close to the Edinger–Westphal nucleus.
In Horner's syndrome, one pupil is small (miotic), and there are other signs of dysfunction of the sympathetic supply to the pupil and orbit (see Chapter 20 and Figs 20–6 and 20–7).
TABLE 15–1Local Effects of Drugs on the Eye.
Still another visual area, termed MT, is located on the posterior part of the superior temporal sulcus. This visual area receives and analyzes information about the location of visual stimuli but not their shape or color. The MT area does not provide information about what a stimulus is but does provide information about where it is located.
The primary visual cortex appears to contain six layers. It contains a line of myelinated fibers within lamina IV (the line of Gennari, or the external line of Baillarger; see Fig 15–19). The stellate cells of lamina IV receive input from the lateral geniculate nucleus, and the pyramidal cells of layer V project to the superior colliculus. Layer VI cells send a recurrent projection to the lateral geniculate nucleus.
Light micrograph of the primary visual cortex (calcarine cortex) on each side of the calcarine fissure.
As noted earlier, there is an orderly mapping (again termed retinotopic) of the visual world onto multiple parts of the visual cortex. The projection of the macular part of the retina is magnified within these maps, a design feature that probably provides increased sensitivity to visual detail in the central part of the visual field.
As visual information is relayed from cell to cell in the primary visual cortex, it is processed in increasingly complex ways (Fig 15–20). Simple cells in the visual cortex have receptive fields that contain an "on" or "off" center, shaped like a rectangle with a specific orientation, flanked by complementary zones. Simple cells usually respond to stimuli at one particular location. For example, an "on"-center simple cell may respond best to a bar, precisely oriented at 458, flanked by a larger "off" area, at a particular location. If the bar is rotated slightly or moved, the response of the cell will be diminished. Thus, these cells respond to lines, at specific orientations, located in particular regions within the visual world.
Receptive fields of cells in visual pathways. Left: Ganglion cells, lateral geniculate cells, and cells in layer IV of cortical area 17 have circular fields with an excitatory center and an inhibitory surround or an inhibitory center and an excitatory surround. There is no preferred orientation of a linear stimulus. Center: Simple cells respond best to a linear stimulus with a particular orientation in a specific part of the cell's receptive field. Right: Complex cells respond to linear stimuli with a particular orientation, but they are less selective in terms of location in the receptive field. They often respond maximally when the stimulus is moved laterally, as indicated by the arrow. (Modified from Hubel DH: The visual field cortex of normal and deprived monkeys. Am Sci 1979;67:532; and Ganong WF: Review of Medical Physiology. 19th ed. Appleton & Lange, 1999.)
Complex cells in the visual cortex have receptive fields that are usually larger than those of simple cells (see Fig 15–20). These cells respond to lines or edges with a specific orientation (eg, 608) but are excited whenever these lines are present anywhere within the visual field regardless of their location. Some complex cells are especially sensitive to movement of these specifically oriented edges or lines.
D. Hubel and T. Wiesel, who received the Nobel Prize for their analysis of the visual cortex, suggested that the receptive fields of simple cells in the visual cortex could be built up from the simpler fields of visual neurons in the lateral geniculate. The pattern of convergence of geniculate neurons onto visual cortical cells supports this hypothesis. Similarly, by projecting onto a complex cell in the visual cortex, a set of simple cells with appropriate receptive fields can create a higher-level response that recognizes lines and edges at a particular orientation at a variety of positions.
The visual cortex contains vertical orientation columns, each about 1 mm in diameter. Each column contains simple cells whose receptive fields have almost identical retinal positions and orientations. Complex cells within these columns appear to process information so as to generalize by recognizing the appropriate orientation regardless of the location of the stimulus.
About one half of the complex cells in the visual cortex receive inputs from both eyes. The inputs are similar for the two eyes in terms of the preferred orientation and location of the stimulus, but there is usually a preference for one eye. These cells are referred to as showing ocular dominance, and they are organized into another overlapping series of ocular dominance columns, each 0.8 mm in diameter. The ocular dominance columns receiving input from one eye alternate with columns receiving input from the other (Fig 15–21).
Reconstruction of ocular dominance columns in a subdivision of layer IV of a portion of the right visual cortex of a rhesus monkey. Dark stripes represent one eye; light stripes represent the other. (Reproduced, with permission, from LeVay S, Hubel DH, Wiesel TN: The pattern of ocular dominance columns in macaque visual cortex revealed by a reduced silver stain. J Comp Neurol 1975;159:559.)
A 50-year-old woman experienced a loss of consciousness 3 months before admission. Her husband described the incident as an epileptiform attack. More recently, her family thought that her memory was failing, and the patient noted that her right hand had begun to feel heavy. Two weeks earlier, the patient began to have a constant frontal headache. She felt her eyeglasses needed changing, and the ophthalmologist referred her to the neurologic service. While giving her history, the patient appeared distractible, displayed impaired memory, and made inappropriate jokes about her health.
Neurologic examination showed that olfaction was totally lost on the left side but normal on the right. The right optic papilla was congested and edematous, and the left optic disk was abnormally pale. Visual acuity was normal in the right eye but impaired in the left. The muscles of facial expression were slightly weaker on the right side than on the left. Deep tendon reflexes on the right side of the body were brisker than those on the left, and there was a Babinski reflex on the right. The remainder of the findings were within normal limits.
Where is the lesion? What is the differential diagnosis? Would imaging be useful? What is the most likely diagnosis?
Cases are discussed further in Chapter 25.
BOX 15–1 Essentials for the Clinical Neuroanatomist After reading and digesting this chapter, you should know and understand:
Anatomy and physiology of the retina
Roles of retinal rods and cones in vision
Anatomy of the visual pathways (Fig 15–14)
Monocular and binocular visual fields
Optic nerve and optic chiasm
The visual cortex (primary, association); simple and complex cells; columnar architecture
Clinical presentation of lesions along the visual pathways (Fig 15–16)