Ординатура / Офтальмология / Английские материалы / Neuro-Ophthalmology_Kidd, Newman, Biousse_2008

by other cognitive impairments. There are two types—apperceptive and associative.

The fusiform face area lies anterior to V4 in the fusiform gyrus and is involved in face recognition along with a complex neural network. Damage to this area leads to prosopagnosia—a failure of face recognition.

Balint’s syndrome is a triad of visual deficits occurring in patients with bilateral occipitoparietal lesions. The deficits are simultanagnosia, ocular apraxia, and optic ataxia.

Several positive visual phenomena occur as a result of destructive or irritative lesions of the visual system. Palinopsia refers to the perseveration of the visual image in time; cerebral polyopia refers to two or more copies of a visualized object seen simultaneously.

Visual illusions occur when an object of interest is perceived differently from reality, usually in size, shape, and spatial orientation.

Visual hallucinations are visual percepts without real external stimuli.

This chapter discusses disorders of vision related to damage of the primary visual cortex: hemianopic visual loss or, if bilateral, cortical blindness but in particular the visual disorders, such as achromatopsia, akinetopsia, and prosopagnosia, resulting from damage to the extrastriate visual cortex and other visually related temporoparietal areas. These disorders, or more properly termed “syndromes,” are often referred to as “higher visual disorders” or “cortical visual disorders,” although the latter term fails to recognize that sometimes they originate from damage to the white matter that connects the relevant areas together. Despite the fact that many of these disorders have been described for more than 100 years, the challenge to interpret their clinical aspects and their pathophysiology in the context of the rapid advances in visual neuroscience still remains.

Anatomy and Physiology of Vision

It is well recognized that the visual brain not only consists of the primary visual pathway from the retina to the primary visual cortex (Brodmann area 17, area V1, or striate cortex) but in addition many other areas are involved in vision, especially in the visual association cortex in the occipital lobe (Brodmann areas 18, 19), as well as areas of the parietal and temporal lobes. The occipital cortex contains an array of interconnected visual areas, many of which show evidence of a retinotopic map, and are relatively specialized for analyzing and categorizing different stimulus features, such as color, shape, and motion. Because these areas are closely located to each other anatomically, several of them may be damaged by the same pathology, such as cerebral infarction, so it is not uncommon to see combinations of visual dysfunctions that can give rise to a very complex clinical picture (Fig. 14–1).

The neurophysiology of early vision is rather linear and better understood than in the rest of the visual brain. The visual information transmitted to the

D

V

Figure 14–1 An outline drawing of the lateral surface of the human brain showing the pathway of the two visual processing streams—the dorsal stream (D) projecting into the parietal lobe, which is involved with spatial perception and visuomotor performance (the WHERE stream) and the ventral stream (V) projecting into the temporal lobe, which is involved in object discrimination and recognition (the WHAT stream).

visual cortex appears to be segregated at an early stage into two streams, the parvocellular and magnocellular geniculostriate inputs. The former is characterized by color opponency and slow-conducting axons, whereas the latter is defined by large, fast-conducting axons that convey information about transient visual signals. Geniculostriate fibers input to the striate cortex (V1) where early vision is processed. Projections from V1 pass to the visual association areas (V2–8).

The striate cortex (V1) is located around the calcarine sulcus and receives input from the lateral geniculate nucleus (LGN), which first projects to neurons in layer 4c. These neurons in the striate cortex, especially those in layer 2/3, 4B, and 5, are sensitive to the specific orientation and illumination of visual stimuli within their receptive field.1–7 Complex cells receive inputs from simple cells, have a larger receptive field, and respond to a specifically orientated stimulus. Neurons of common orientation for a particular region of the visual field are contained within a vertical column, and there are sets of such orientation columns (hypercolumns), which cover the full 360 degrees of orientations for each region of the visual field. Retinotopic organization is, therefore, still well maintained in area V1 (and also in V2), although there is significant magnification of the foveal projection, which results in much of the striate cortex representing the central 10 degrees of the visual field. A further role for area V1 is to combine visual formation from the two eyes, providing the substrate for binocular vision.

Although a prevailing view has been that the visual brain is organized on the basis of a hierarchical model whereby visual inputs travel through the association cortex toward a more complex and integrated visual image, this appears to be only partly true. Indeed there is now considerable evidence for a nonhierarchical parallel model of visual processing because the visual association areas around V1 are operationally divided into two pathways (streams): the dorsal and ventral processing streams (Fig. 14–1), a concept that is helpful in interpreting cortical visual disorders. It has been argued that the ventral stream through the temporal lobe plays a role in interpreting “what” the object is in terms of

The main complaint of patients with cortical visual disorders is usually rather nonspecific or they may not complain at all because, for example, they have visual neglect or fail to recognize the deficits (anosognosia). It must always be remembered that these patients may have coincidental language and memory disorders. For example, inability to describe seen color could be the result of cerebral achromatopsia (an inability to see color), color anomia (an inability to name color as part of expressive dysphasia), or amnesia (memory impairment). Comprehensive and extensive exploration of memory and language capabilities is sometimes required to isolate the exact pattern of functional deficits.

Negative Visual Disorders

V1 LESIONS

Cortical blindness occurs as a result of bilateral occipital lobe lesions involving either the optic radiations or V1. The patient is totally blind, but there is preservation of pupillary light reflexes because of sparing of the direct pathway from the optic tract to the Edinger-Westphal pupillomotor nucleus in the midbrain. The usual cause for the sudden onset of cortical blindness is occlusion of the posterior arterial circulation. Other causes include tumors,10 cerebral hemorrhage,11,12 venous infarcts, cardiac bypass surgery, cardiopulmonary arrest,13,14 demyelination,15,16 uncal herniation, systemic lupus erythematosus,17 and the Heidenhain variant of Creutzfeldt-Jacob disease.18 Transient cortical blindness

can occur as a result of cerebral or coronary angiography,19 migraine, traumatic brain injury,20–22 seizures,23 myelography,24 and drugs such as cyclosporine.25

Some recovery of vision usually occurs, which is most likely in younger patients with hypoxic insults and least likely in patients older than 40 years with stroke and vascular risk factors. In general, motion detection seems to be most tolerant to damage, whereas color perception and spatial sensitivity are the most vulnerable. As a consequence, Riddoch’s phenomenon26 (statokinetic dissociation) may be seen on rare occasions during recovery in which kinetic perimetry reveals significantly larger visual fields than the static technique.

Some patients with cortical blindness are unaware of their deficit and actively deny it. This syndrome is called Anton’s syndrome27 or visual anosognosia. These patients may try and carry on normally, inevitably bumping into objects, and may confabulate about what it is they are “seeing.” These patients usually have a bilateral occipital lesion that extends anteriorly to involve the medial temporal or visual association areas.

Blindsight is a term used to describe a phenomenon in which a patient with a V1 lesion fails consciously to perceive any visual information in their defective visual field, yet can still use some of the information presented. For instance, some patients can make accurate saccades or point to visual targets presented tachistoscopically in their blind hemifield using a forced choice paradigm, despite the fact that they believe that they are merely guessing.28 These patients tend to be able to discriminate brightness and movement and sometimes gross size and orientation. Some, when presented with pictures in the blind field, are able to guess the correct answers on multiple-choice questions at well above chance while not able to describe anything. It is believed that this phenomenon

represents activation of accessory visual pathways, either via the superior colliculus and pulvinar or directly from the LGN to V5 as demonstrated in monkey28; these pathways bypass the damaged area V1 and provide visual information for higher cortical areas. However, this does suggest that area V1 is important for the conscious awareness of visual stimuli.

DISORDERS OF SPECIFIC VISUAL ATTRIBUTES

Color—Cerebral Achromatopsia

Color vision is an important visual attribute that enhances object identification and recognition and form perception in low level illumination. Because objects are not “colored” and are not the different wavelengths of light that are reflected, color perception must be the result of sensations evoked in the visual brain. To make the task even more complex the wavelengths reflected from a particular object differ depending on the background illumination, yet we perceive the object as having the same color—a phenomenon known as color constancy. The perception of color, therefore, depends on a series of neural computations stemming from the trichromatic signals produced by the retinal cones.

Although early electrophysiologic single cell recordings from the extrastriate cortex revealed one region V4, which appeared to predominantly contain color selective neurons, ablation of this area in the monkey does not abolish color

vision but has a greater effect on form vision. However, functional imaging stud- ies29–31 and autopsy and structural scanning32 in patients with focal lesions

have indicated that the human homologue for V4, the “color area,” is located in the fusiform gyrus on the ventromedial region of the occipital lobe33 (Fig. 14–2). Cerebral achromatopsia describes an acquired loss of color perception, with intact or relatively intact visual acuity, caused by damage to the visual cortex in an alert patient without any language disturbance. It was first described in the late 1800s,34 well before the physiology of vision was understood, and is usually associated with damage in the “color center.” Patients with achromatopsia usually describe the affected area as having a “loss of color,” “the colors look dull,” or “everything looks a muddy grey.” Although complete achromatopsia from bilateral lesions occurs most often, this phenomenon may affect only half of the visual field (hemiachromatopsia) or only one quadrant of the field if the

lesion is limited to one side, and it may be accompanied by visual object agnosia or alexia.32

The most common etiology of cerebral achromatopsia is an embolic stroke in the territory of the PCA,35–42 affecting the ventral occipitotemporal area. Other conditions such as carbon monoxide poisoning43 and cerebral metastases44 have also been reported as causing cerebral achromatopsia. Hemiachromatopsia is usually, but not exclusively, associated with a superior quadrantanopia ipsilateral to the hemiachromatopsic field but any type of visual field loss can be seen (for a review see Bouvier and Engel45). In addition, prosopagnosia—an inability to recognize faces—seems to be commonly associated with achromatopsia, not surprising in view of the close proximity of the area specialized for face processing to V4.

The effect of cerebral achromatopsia on activities of daily living is usually minimal because of the relatively unaffected visual acuity in the achromatopsic field. However, in one report there were devastating effects to an artist who

V5

A

FFA

V4

B

Figure 14–2 An outline drawing of the lateral (A) and the medial (B) surfaces of the human brain showing some of the specialized visual areas in the extrastriate region around the primary visual area, striate cortex (V1): V4, the color area; V5, the motion area; FFA, the facial fusiform area.

encountered significant difficulty in painting with colors as a result of cerebral achromatopsia after a traumatic brain injury.46 He subsequently adapted to the deficit by painting in black and white, which fortunately was productive and successful.

Some patients are unable to name colors or point to a color when given its name—a condition called color anomia (also called color agnosia and colornaming defect)—yet have normal color vision when assessed with color-matching tests and pseudoisochromatic plates (both of which are abnormally performed in cerebral achromatopsia) and are not aphasic. It is usually associated with a rightsided homonymous hemianopia and pure alexia. The lesion responsible for this condition is located in the left mesial occipitotemporal region, inferior to the splenium. Rarely, disorders of color association have been reported, when in the context of aphasia, patients are unable to color in correctly drawings of common objects.

Motion—Cerebral Akinetopsia

Akinetopsia is the term used to describe loss of motion perception as a consequence of an acquired cerebral lesion. It was first reported in man by Zihl et al,47 after the discovery of an extrastriate region (area V5, also known as

the middle temporal area [MT]) in monkeys containing motion selective neurons, which suggested that a disorder of motion perception may occur in humans with damage to homologous areas. MT neurons are selective for detecting direction and speed33 but not for shape or color. Monkeys with lesions in area MT exhibit impaired perception of motion. Functional imaging studies in humans have located V5 to the posterior continuation of the middle temporal gyrus at the lateral occipitotemporal junction.8 In general, overt akinetopsia results from bilateral lesions to these regions, although subclinical deficits may be seen with unilateral lesions. This condition is considered to be very rare because only two clearly defined cases have been described. Both patients, LM47 and AF,48 have been well described and tested extensively, although similar symptoms have been described in two patients with transient akinetopsia resulting from nefazodone (a selective serotonin reuptake inhibitor) toxicity.49 LM and AF both suffered bilateral cerebral hemisphere damage because of cerebrovascular disease involving area V5 at the lateral occipitotemporal junction. Akinetopsics describe the deficit as an inability to appreciate the smooth movement of a moving target, for example, LM reported that when she poured tea “the fluid appeared to be frozen like a glacier,” and when crossing the road “when I’m looking at the car first, it seems far away. But then, when I want to cross the road, suddenly the car is very near.” In other words, the moving object appears to jump rather than move smoothly, as experienced by normal individuals when viewing moving objects under stroboscopic illumination, for example, at a disco. Hemiakinetopsia has also been described, and it is always contralateral to the lesioned side; however, it is not commonly detected because of the masking effect of the concurrent visual field loss.

Disorders of Stereopsis—Astereopsis

The ability to perceive three-dimensional (3D) images (stereopsis) is one of the most fascinating visual phenomena but the least well understood. This visual function involves processing disparities between the two-dimensional images viewed and perceived from a slightly different angle by the left and right eye. The brain uses these retinal disparities to extrapolate the distance of the objects to guide our vergence eye movements and to provide our perception of stereoscopic depth. This process develops innately but may be impaired by ophthalmologic disturbances before the adult level of stereoacuity is achieved. As a result, approximately 5% to 10% of the population is unable to appreciate stereopsis, primarily as a result of uncorrected childhood strabismus or amblyopia, presumably because of disruption of the normal development of binocular cortical neurons.

The physiology of stereopsis is rather complex because numerous cortical areas are involved. Much of the striate and extrastriate cortices have some role in stereopsis. Binocular visual interactions first occur in the V1 neurons, which respond selectively to retinal disparity. In addition, area V2 and V3 contain retinal disparity-sensitive neurons that act similarly to V1 neurons. The current evidence also suggests that both the dorsal and ventral streams are involved in stereopsis. The depth information from the dorsal stream is probably used to localize objects accurately in visual space and guide vergence eye movements, whereas the information from the ventral stream establishes a richer perceptual

representation, including detailed 3D representations. However, although several studies have suggested a dominant role of the right hemisphere in the perception of depth from stereopsis, a unilateral lesion does not abolish stereopsis.50,51

Some patients with astereopsis or stereoblindness complain of difficulty perceiving distance, perspective, depth, or thickness; however, the majority are unaware of any deficits. This could be the result of anosognosia, but the ability to use other cues such as motion and linear perspective to compensate for the lack of stereoacuity could explain this observation. Impaired stereopsis in an acquired form has been reported in Alzheimer’s disease,52 tumors,53 and strokes,54 following surgical excisions for the treatment of intractable epilepsy,55 and in traumatic head injuries.56

Visual Object Agnosia

Visual object agnosia is characterized by a difficulty to recognize or identify familiar objects using visual information, when this difficulty cannot be explained by other cognitive impairments such as a disorder of intelligence, attention, and language or impaired peripheral visual processing (acuity, brightness discrimination, depth perception, visual field, and color vision). Generally, a diagnosis of visual object agnosia should be made when a patient misnames an object if he or she is unable to describe or mime the use of the object presented visually but is then able to do so correctly when he or she can feel or hear the object.

Visual object agnosia was classified by Lissauer57 into two categories: apperceptive, in which higher order visual perception and therefore recognition is impaired, and associative, in which perception is largely intact but recognition is selectively impaired.However, the distinction between these two categories is not always clearcut and some perceptual disorders may exist in each. Despite this, Lissauer’s categorization remains a useful framework.

In apperceptive visual agnosia, the patient is not able to recognize the shape of objects and, therefore, has difficulty recognizing, copying, matching, or discriminating simple visual stimuli and drawing an object. Although the visuoperceptive disturbance has been likened to the image seen on a TV screen connected to a faulty aerial, it has been proposed that apperceptive visual agnosia is the result of damage to those components of object recognition that normally allow the construction, by recourse to stored representations, of an object-specific structural description.58 This was based on a small group of patients who had difficulty in object recognition only when the perspective was unusual or the lighting was uneven. Others have suggested that the major deficit is a more global deficit in the perceptual integration of shape elements into coherent wholes.59 The lesions causing apperceptive visual agnosia tend to be typically diffuse and posterior. The typical white matter lesions suggest that it results from some form of disconnection possibly of local intralaminar connections rather than neuronal loss. Apperceptive visual agnosia has been reported in carbon monoxide or mercury poisoning, anoxia, or bilateral PCA stroke.60

In contrast, associative visual agnosia is a condition in which the individual is able to see normally and appreciate the shape of an object, but as Teuber61 so succinctly described “the visual percept is stripped of its meaning.” Hence, such patients are able to copy, draw, and match objects62 but are unable to