Ординатура / Офтальмология / Английские материалы / The Neuropsychology of Vision_Fahle, Greenlee_2003

Calcarine sulcus

Fig 7.2 (a) Projection from both halves of both retinae to the LGN and primary visual cortex. (b, c) Topography of the projection of the visual world on to the primary visual cortex. The centre of gaze, corresponding to the fovea, is represented at the occipital pole; the horizontal meridian, i.e. the horizontal line running through the centre, is represented in the depth of the calcarine fissure that runs almost horizontally on the medial side of the occipital cortex (cf. Fig. 7.3). The vertical meridian, the vertical line running through the centre of gaze, separates the projection to the right hemisphere from that to the left hemisphere. It is represented parallel above and below the horizontal meridian. This is to say that each primary visual cortex contains a map of the visual world resembling a (spherical) Mercator’s projection, with the fovea corresponding to one of the poles, and the vertical and horizontal meridians corresponding to, say, the 0 (horizontal) and the 90 and 270 deg meridians (vertical); ((c), lower part). Visualization of the projection to the primary visual cortex.

The stimulus displayed in the upper part of the graph produces the cortical activation displayed in the lower part (compare also Fig. 7.4). (After Fahle 2003b; (c) after Tootell et al. (1982).) (See Plate 2, colour plate section.)

cortex, close to the midsagittal plane of the brain (cf. Fig. 7.3). In the early nineteenth century, neuropsychological investigations by a Japanese ophthalmologist, Tatsuji Inouye (translation: Glickstein and Fahle 2000; see also Wilbrand 1887, 1890; Wilbrand and Sänger 1904, 1917; Henschen 1896; Kleist 1923; Mingazzini 1908; Munk 1878; Foerster 1890), on patients suffering from circumscribed gun-shot wounds of the occipital cortex revealed this representation. Inouye’s investigation, moreover, showed that: (1) the central visual field (1–3 ) is overrepresented in the primary visual cortex: its representation occupies almost one-quarter of this cortex (Fig. 7.4 is overly conservative in this respect); (2) the upper part of the visual cortex, on the upper lip of the calcarine fissure, represents the lower half of the visual field, while the lower part represents the upper part of the visual field; and (3) the representation of the vertical meridian, the vertical line through the fixation point separating the left from the right visual

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Central sulcus

(a)

(b)

Calcarine

fissure

Fig. 7.3 (a) Lateral view of a human brain (that of the author) representing the surface between cortical grey and underlying white matter. Blue, frontal lobe; green, parietal lobe; yellow, temporal lobe; red, occipital lobe, (b) Medial view of the same brain. (See Plate 3, colour plate section for colour coding.)

field, runs parallel to the horizontal meridian and separates the primary visual cortex, Brodmann area BA17 (Fig. 7.5) from the secondary projection, BA18 (Fig. 7.4; Glickstein and Fahle 2000). In many patients, a foveal sparing with a diameter of 0.5–2 is found in blindness of one-half of the visual field, i.e. hemianopia. The underlying cause may be a bilateral cortical representation of the fovea or else a double blood supply for the foveal representation in the primary visual cortex.

Injuries of all structures up to the optic radiation lead to a deficient transmission of information towards the visual cortex, while injuries of the visual cortex lead to a

Fig. 7.4 Visual field representation on the human primary visual cortex as deduced from investigations on gunshot wounds by nineteenth century ophthalmologist Inouye (from Glickstein and Fahle 2000). The upper part of the visual scene (0 –90 ) is represented in the lower part of the calcarine fissure while the lower part of the visual surround (90 –180 ) is represented in the upper part of the primary visual cortex. Hence, the lower part of the vertical meridian separates the primary visual cortex from the dorsal (upper) part of area 18, while the upper part of the vertical meridian is represented on the border between the primary visual cortex and the ventral (lower) portion of area 18, the first extrastriate visual area. As a consequence, only the primary visual cortex contains a continuous representation of the contralateral visual field while the extrastriate areas above and below the calcarine fissure each only represent one quadrant of the visual field. (0 indicates fovea; 90 outer border of visual field.)

deficient cortical representation of parts of the visual field. This distinction is important for rehabilitation (see Chapter 11, this volume). In the case of, say, a retinal defect, the information regarding the corresponding part of the visual field does not even enter the visual system, and there is no possibility of retrieving it by adaptive processes in the brain. (Recovery of function in partially damaged fibres will, of course, improve performance in all types of patients.) On the other hand, defects of representation due to an injury of circumscribed portions of the (primary) visual cortex can, in principle, be overcome by other cortical neurons taking over the analysis of the information from the defective cortical representation areas (cf. Eysel 2002; Gilbert and Wiesel 1992;

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Fibrae arcuatae cerebri

Cingulum

Fasciculus longitudinalis superior

Fasciculus uncinatus

Fasciculus longitudinalis inferior

Fig. 7.6 Main fibre tracts in the human brain.

Fahle and Poggio 2002). Hence, there can be hope for improvement through active restructuring of cortical connectivity after cerebral lesions, while this is not the case for lesions of the retina and optic nerve. Only little hope exists for lesions of LGN and optic radiation since most (90%) of retinal fibres project through these structures, and it is unclear how much visual information is conveyed through other pathways, including the superior colliculus (see Chapters 2, 5, and 9 this volume, and Fig. 7.6). For deficits in reception (retina) or transmission of visual information (optic nerve, tract, radiation, LGN), the only hope presently seems to lie in compensatory behaviours that enable the patients to better cope with their deficits. In the future, neural prostheses and/or transplantations may provide further help.

As mentioned above, optic nerve fibres partly cross at the chiasm, and the crossing fibres stem from the nasal retina representing the temporal half of the visual field. As a consequence, the primary visual cortex of each hemisphere represents the contralateral half of the visual field, and a complete loss of one hemisphere leads to complete blindness in the corresponding half of the visual field, the so-called homonymous hemianopia. Hemianopias often—but not always—are associated with problems of space exploration, with increased latencies for eye movements towards the blind hemifield, and an almost random rather than systematic visual search of the space contralateral to the lesion. Smaller lesions of visual cortex often lead to homonymous quadrantanopia, the loss of vision in one quadrant of the visual field. The reason is that the representations of the upper and lower quadrants are relatively well segregated: there are at least three representations of the upper quadrant bellow the calcarine fissure in cortical areas 17, 18, and 19 (see Fig. 7.7(a)), while the three or more representations of the

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Visual field testing: perimetry

To assess visual function, testing visual acuity is not sufficient, as mentioned above. While acuity is a very sensitive indicator of the quality of the eye’s optical system and of foveal function at the centre of the retina, it only tests a small portion of the visual field and hence of the visual system. Patients are often not aware of defects in their visual fields, similar to our unawareness of the physiological blind spot in the temporal field of view. Hence, testing of the entire visual field is important in patients suffering from symptoms in the field of visual neuropsychology. At least part of the problems the patient experiences may be due to a scotoma, such as in hemianopic dyslexia.

The visual fields were originally tested by bringing in bright objects of different size from the periphery, and by marking at what distance from the centre, i.e. at what eccentricity, they were first detected by the patient (Aubert and Foerster 1857; von Graefe 1856). Smaller objects are first detected closer to the centre than larger ones. Today, many perimeters use stationary rather than moving dots that are presented at a contrast just above threshold of normal observers for each of the visual field positions tested (Aulhorn and Harms 1972). If a patient cannot detect a dot at a given position, this failure indicates a relative damage in the ability to detect differences in luminance at this visual field position and, hence to detect luminance contrast there. (Why and how cortical lesions can lead to diffuse relative scotomata is an interesting and poorly investigated question—one must assume destruction of a (large) percentage of the neuronal population in the corresponding afferent fibre tracts or cortical representation rather than a complete eradication.) Next, the contrast of the dot will be increased to the maximum possible for the perimeter. If the dot is still not detected, the corresponding retinal area is assumed to be completely blind, suffering from an absolute scotoma.

Both methods, moving in dots of defined size and contrast (kinetic perimetry) and testing the contrast required for detection at a given position (static perimetry), make it possible to quantitatively assess the decline of contrast sensitivity from the centre to the periphery of the visual field, and to detect regions where visual stimulation fails. Perimetry is a standard clinical test and, if patients pass this test, their visual fields are considered to be normal—other submodalities of vision such as colour or motion perception are not usually tested in the periphery of the visual field. The reasons for this limitation are manifold. Testing for motion detection is much more difficult technically, patients suffering from isolated defects of, say, motion perception in part of the visual field are rare (and, since this is not a standard test, the few who do will not be found), and perimetry is time-consuming and not popular with patients anyway. There have been some attempts to measure flicker fusion frequency as well as form, orientation, and colour discrimination in the visual field, but these techniques are not used in clinical practice (Aubert 1857; Leber 1869; Wertheim 1894; Ferree and Rand 1920; Aulhorn and Harms 1972; Teuber et al. 1960). Today, it is possible to test the visual field in addition by means of more objective methods, e.g. in patients unable to cooperate. The methods employed are monitoring the pupil response to presentation

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Fig. 7.8 Representation of a simulated scotoma as it appears in functional magnetic imaging of the author’s right hemisphere during stimulation through the right eye. Visual cortex is stimulated by means of a contrast-reversing checkerboard extending, in the right half of the visual field, from the fovea to about 25 deg. eccentricity, with a blank area, without flicker. This artificial scotoma was located at 15 deg. eccentricity and had a diameter around 5 deg., corresponding to the blind spot. For details regarding the method of functional magnetic resonance imaging, see Chapter 4, this volume. (See Plate 4, colour plate section.)

of either luminanceor pattern-defined stimuli (Barbur 1995) or testing the cortical response to visual stimuli by sum potential recordings that may even differentiate between the contributions of different parts of the visual field (Slotnick et al. 2001; see also Chapter 3, this volume) or by functional magnetic resonance imaging (fMRI; see Fig. 7.8 for an example of a scotoma, see also Chapter 4, this volume).

The patients’ task during perimetry is to gaze steadily at a central fixation point and not to move their eyes while a bright dot is presented sequentially often at a hundred or more different positions in the visual field. Whenever the dot appears, the patient is required to press a button, while not being allowed to look at the newly appeared dot, as would be the ‘natural’ reaction. If there are one or several scotomata, the dots within these regions will not be perceived and are presented not just once, but several times, to discriminate between absolute and relative scotomata, further increasing examination time to 15 minutes and more per eye. Recent advances trying to make perimetry more ‘physiological’ include the use of eye-tracking. In this method, the eye of the patient is tracked by a camera and its position measured by a computer. The patient looks at points appearing on a monitor, i.e. he or she performs a saccadic eye movement towards the point (Repnow et al. 1995). If the computer detects a correct direction and amplitude of this saccade, the new dot is considered to be located in an intact portion of the visual field—an assumption not necessarily true as is evident from the phenomenon of blindsight (see Chapter 9, this volume). However, the number of patients showing blindsight spontaneously and with sufficient spatial resolution is low enough not to pose a practical problem. Moreover, even patients suffering from visual field

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defects with blindsight will not move their eyes in the correct direction unless forced to make a saccade in whatever direction but will wait for the next dot they consciously perceive. This ‘gaze’-perimetry is: (1) somewhat faster than the conventional perimetry based on button-pressing; (2) less exhausting for the patient; and (3) uses a monitor to present the stimuli so that the method can, in principle, test the visual field not just for contrast detection. The stimulus serving as the target can be defined by motion among stationary dots, by a (slightly) differing colour, by stereoscopic depth, or a multitude of other features.

The modular structure of visual cortices and its relation to neuropsychology

A failure of visual stimulus analysis can follow not just from blindness for the part of the visual field where the stimulus is presented, as is the case with a scotoma. The failure can also be due to a failure to discriminate the stimulus, i.e. the figure, from its surround. For example, a stimulus that differs from its surround not by its luminance but by a different stimulus attribute such as colour can only be detected by a coloursensitive mechanism. This type of patient would not be blind at any position of the visual field, and perimetry might yield normal results since it only tests the patient’s ability to discriminate between two luminances and, hence, his or her sensitivity for luminance contrast. Still, colour, or motion, or stereodetection might be defective, leading to a circumscribed defect of the corresponding submodality in part of the visual field. These types of defects will be discussed in detail at the beginning of the section on ‘Visual indiscriminations’. Here I will give a short overview of the neuronal basis and some explanations why these defects are still incompletely understood.

During the last decades, electrophysiological investigations in animals (see Chapters 1, 2, and 5, this volume) and later functional Magnetic Resonance Imaging, fMRI (see Chapter 4, this volume) have greatly refined the cytoarchitectonic map initially developed by Brodmann (1909; see Fig. 7.5). We now discriminate between about

40 different areas in the monkey cortex that are all involved in the analysis of the visual world and might represent different modules of processing (see Fig. 7.9). Many of these areas contain a complete topographically ordered representation of the (contralateral half of the) visual field, i.e. their own ordered map of the outer world. Why so many representations and not just one? Comparing the cortex of man with the cortices of less ‘brainy’ animals such as the cat shows that the primary projection cortices for visual, auditory, and somatosensory information constitute a much larger proportion of the brain in these (and other) animals than they do in man. Obviously, the primary sensory cortices are necessary to process the sensory information while the additional cortical areas, often called ‘association’ areas, add additional capabilities to (wo)man not available to simpler animals. It seems that, during evolution, some additional capabilities were built into the cortex—at least partly by duplicating existing areas and devoting them to new tasks. I would like to hypothesize that different cortical areas as

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HC

ER

Fig. 7.9 Schematic view of the cortical areas involved in visual analysis in the monkey brain, based on the pattern of axonal connections between areas, as well as on differences in cytoarchitectonic and electrophysiological properties of single neurons. (From van Essen et al. (1992).) (See Plate 5, colour plate section.)

shown in Fig. 7.9 fulfil at least slightly different tasks during the analysis of the visual world—certainly not a very audacious hypothesis.

Destruction of each one of these areas should lead to a deficit in at least one aspect of visual perception (see the beginning of the section ‘Visual indiscriminations’ for a detailed analysis). These deficits might be quite subtle indeed since, beyond the primary visual cortex, at least some areas receive input from both hemispheres, and defects in one area of one hemisphere can be partly compensated for by the corresponding contralateral area. Moreover, the ensemble of areas works together and, as the results of fMRI investigations demonstrate very convincingly, most stimuli activate quite a number of different areas. So we cannot expect the same clear symptoms from destruction of any of these ‘downstream’ (i.e. ‘higher’!) areas as we get from defects of the primary visual cortex, especially since the system has a fair amount of plasticity even in adults to (partly) compensate for lost functions, perhaps through other areas taking over part of the task. But we can expect that defects of these ‘higher’ cortical areas will