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

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or, more generally, on the basis of differences in texture, or spatial frequency (Kartsounis and Warrington 1991; Riddoch and Humphreys 1986; cf. also Davidoff and Warrington 1993; Humphrey et al. 1996; cf. also Warrington and Rabin 1970; Humphrey et al. 1995; Hamsher et al. 1992). (Spatial frequency is, to a first approximation, a measure of the coarseness versus fineness of a pattern, e.g. a grating: a fine sinewave grating has a high spatial frequency, a coarse grating a low spatial frequency.)

We will not deal in detail with further features that can serve to detect borders and hence serve as a prerequisite for figure–ground segregation. Suffice it to say that gradients in the amount of variation over space or time in most of the features outlined above (e.g. differences in luminance contrast) will lead to the perception of borders between the areas differing in regard to this feature. But these features are not tested in neuropsychological patients and, while we plan to extend the range of tests applied in the future, no results are available as yet.

The indiscriminations described here may be related to the so-called cerebral amblyopias that are defined by a loss of clearly defined visual perception without complete loss of vision (see above and Ferrier 1881; Mauthner 1881; Bender and Bodis-Wollner 1978; Riddoch 1917). Patients experience a stimulus presented within the amblyopic region as ‘foggy’ and blunt, and they are unable to detect its form, colour, or motion direction—while they can still discriminate between stationary and moving objects. These are symptoms not unlike those for some patients suffering from blindsight (see Chapter 9, this volume). In some patients, the size of the residual visual field changes and fluctuates over time with occasional ‘obscurations’ (Bender and Teuber 1946, Wilbrand and Sänger 1892; Poppelreuter 1917; Gelb and Goldstein 1922). These fluctuations have a correlate in the visual evoked cortical responses (Zihl and Schmid 1989), and both darkand bright-adaptation may be decreased (Aulhorn and Harms 1972).

Mislocation of objects in space

Mislocalization of objects in the visual field is not uncommon, leading to a perception of objects as closer to the centre of gaze in some but not all patients suffering from scotomata in the peripheral visual field (e.g. Poppelreuter 1917; Riddoch 1935; Ratcliff and Davies-Jones 1972; Beyer 1895). Poppelreuter (1917) found a mislocalization of object positions towards the fovea in patients suffering from peripheral scotomata caused by lesions in the occipital or parietal lobe (cf. also Holmes 1918b ; Hannay et al. 1976; Lenz 1944; Massironi et al. 1990). In hemianopic patients, the subjective ‘straight ahead’ is often shifted and the vertical meridian is rotated (Lenz 1909; Gelb 1926; Zihl and von Cramon 1986). Patients with left-sided lesions, especially of parietal or temporal occipital cortex, experience as ‘vertical’ a line tilted clockwise (up to 20 ). These patients, surprisingly, experience a horizontal line as tilted counterclockwise, and the subjective straight-ahead seems to be shifted to the right (i.e. towards the scotoma; Axenfeld 1894; Liepmann and Kalmus 1900; Bender and Jung 1948; Zihl and von Cramon 1986). These changes were sometimes present (but less pronounced) even

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without accompanying visual field defects (cf. also bisection tasks in neglect patients in the subsection ‘Tests for detecting spatial neglect in patients’).

Neuronal mechanisms for contour and position detection

A highly speculative explanation is that antagonistically organized receptive fields might code visual field position, similarly to the coding of motion direction. Decreasing the influence of one type of input, e.g. of one motion direction, by looking for a while at a waterfall, will result in the perception of the opposite motion direction when subsequently looking at a stationary object. The same neuronal mechanism of adaptation or selective gain change might cause a displacement or sideward shift of the subjective ‘straight-ahead’, coupled with a displacement of the subjective positions of the horizontal and vertical meridians. Patients suffering from these symptoms often suffer from unior bilateral lesions of the occipitoparietal cortex, especially the supramarginal and angular gyri (Lenz 1909, 1944; Gelb 1926; Teuber and Mishkin 1954; de Renzi et al. 1971; Benton et al. 1975; Zihl and von Cramon 1986; cf. also the phenomenon of ‘past pointing’ after lesions of the brainstem and a possible role of eye position control). Another line of explanation is presented in the context of simultanagnosia (see the subsection ‘Visual disorientation in simultagnosia’).

The luminance and colour perceptions of homogeneous areas between the contours are created probably partly by extrapolation, or filling in, from these contours, and partly from a separate area system (see the subsection ‘Different levels and separate channels of visual information-processing’; Welpe et al. 1980; Grossberg 1991). The same may be true for stereoscopic depth, where depth differences seem to be computed only relative to the next contour (see e.g. Fahle and Westheimer 1995).

In textureand motion-defined contours, even the homogeneous areas contain (luminance-defined) contour elements. There, orientation, or distribution of elements, or else motion-speed or -direction are the important parameters to discriminate between figure and ground. This differentiation can be achieved only by mechanisms that group together different visual field positions according to the exact type of feature within a given feature domain, i.e. that discriminate not only between stationary and moving, but between different directions of motion and create an object consisting of (all) elements showing this feature. We can still assume that boundaries are the important feature extracted from the image, but with texture-defined stimuli, boundaries are detected in a more indirect way, or on a second level that compares, e.g. the velocities from motion detectors on the first level, and groups those with similar outputs, thus creating boundaries between areas (second-order processing, see e.g. Zanker et al. 1998; Chapter 4, this volume). An analogue in the domain of luminance contrast would be an area defined by a difference in contrast, rather than in luminance: this would be a contrast in contrast, while the two areas would not differ in mean luminance, so there would not be a luminance contrast at the border between the two areas. This reminds us that, within each of the levels, there may be several steps or sublevels of processing.

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Searching for indiscriminations in the visual field: component perimetry

Due to a process called filling-in, most neuropsychological patients suffering from scotomata in their visual field are not aware of these ‘dark holes’ in their fields. This phenomenon is loosely related to the fact that we are not aware of the blind spot in our temporal visual field caused by the exit of the optic nerve from the eye—in both cases, there is no representation in the (primary) visual cortex of a restricted part of the visual field. Even fewer patients are aware of regions in their visual fields where they do not experience colour, or motion. Given the resulting relative rarity of reports in the literature on patients suffering from relatively isolated defects in detecting, e.g. stimulus motion, with (largely) preserved discrimination of luminance contrasts, we developed a new screening method for visual field defects that should be able to detect such deficits in patients. It is based on a method proposed by Aulhorn and Köst (1988) that was initially developed to diagnose defects in contrast detection.

For this screening method, observers sit close to a monitor screen that displays different types of dynamic stimuli so that a large part of the visual field is stimulated simultaneously. The original method employed visual noise consisting of randomly moving black and white dots presented in the central 30 of the visual field. Virtually all patients suffering from scotomata caused by lesions on the level of retina, optic nerve, optic tract, and optic radiation were able to detect their visual field defects when looking at this stimulus, while only about half of the patients with scotomata due to cortical lesions detected their defects when gazing at the stimulus of the original method (Aulhorn and Köst 1988). We extended the method by adding test stimuli defined purely by differences in hue (colour) without any luminance contrast, i.e. stimuli that were isoluminant and would appear as homogeneous to any colour-blind system (Bachmann and Fahle 2000). A second extension was the use of figures, more precisely checkerboards (or draught boards) defined purely by stereoscopic depth, direction of motion, or time of element appearance—dots were flickered in these latter displays, always appearing a couple of frames later in one type of checks than in the other type. Schematic displays of some sample stimuli of this new method, component perimetry, are shown in Fig. 7.13.

In two studies, we tested around 100 patients suffering from infarctions of either the posterior or medial cerebral arteries with a monitor subtending eccentricities up to around 40 , larger than in the previous studies (Bachmann and Fahle 1998, 2000; Spang et al. 2001). In contrast to the earlier studies that used a smaller monitor, around 90% of these patients were able to subjectively experience their scotomata when looking steadily at these stimuli, often only in the far periphery of the monitor (Bachmann and Fahle 2000). Patients usually experienced their scotomata as regions where the stimulus was not as dynamic as in the rest of the visual field, or was completely stationary, or else as moving more slowly, being less colourful, or missing the checkerboard structure. By and large, the positions of the scotomata the patients drew by felt-tip on

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(c)

Fig. 7.13 Some examples of the stimuli used for component perimetry testing. These are individual stationary frames from a sequence of dynamic stimuli presented to a large part of the visual field of the patients (diameter between about 60 and more than 80 degrees of visual angle). (a)-(c) The stimuli consist either of dynamic noise defined by luminance or hue, by differences in motion direction, depth, or by rotating stimuli.

the monitor were similar to the defects revealed by automated perimetry that was performed on all patients immediately after the tests of component perimetry. Hence the method, requiring about 10 minutes per patient for five different submodalities, is a relatively sensitive screening tool for visual field defects. However, the subjective defects tended to be smaller than the more objective size revealed by conventional perimetry, probably due to filling-in mechanisms.

In the context of visual neuropsychology, the main interest is in patients with discrepancies between the results of conventional contrast perimetry on one hand and those of component perimetry on the other hand. If contrast perimetry yields normal visual fields, while the more subjective component perimetry shows defects, the patient may either be ‘false-positive’, i.e. imagining the defect, or else contrast detection might be relatively normal while some other visual function, such as colour, motion, or depth perception is defective. Figure 7.14 presents the results of one of almost a dozen patients with distinct differences between the results of contrast versus component perimetry that we have found so far. While the visual field for contrast perimetry is almost normal (Fig. 7.14(a)),

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the patient perceives strong differences between his right and left visual hemifields when looking at those patterns of component perimetry that test colour or motion perception (Fig. 7.14(b)). The brain lesion as revealed by MRI is shown schematically in Fig. 7.14(c), showing defects in the left hemisphere that spare the optic radiation and primary visual cortex while involving parts of the temporal and parietal cortex.

Obviously, these rather subjective screening tests have to be complemented by more quantitative measurements such as colour-, motion-, and stereo-perimetry. We are presently developing these methods, partly making use of gaze perimetry (see the subsection ‘Visual field testing: perimetry’). The results of the screening tests strongly suggest that a certain proportion of patients after infarctions of posterior cerebral cortex suffer from circumscribed scotomata of the indiscrimination-type with intact contrast detection, hence largely normal visual fields as tested by conventional perimetry.

Failures to achieve object constancy

In natural environments, objects tend to appear under different circumstances, at different distances, in different orientations, with different illuminations, and at different positions in the visual field. Usually, this variance of appearance does not pose a noticeable problem for visual object recognition in humans. Again, the problem may appear trivial at first glimpse but it is far from trivial, in spite of the ease with which humans usually solve the task. The retinal area covered by a cup at a viewing distance of 5 metres is about one-hundredth of the area covered by that same cup at a distance of 0.5 metres. Nevertheless, the cup appears to have the same size at both distances, due to a neuronal mechanism of size constancy that takes into account the distance of objects when their size is assessed. Disturbances of this mechanism, e.g. in patients suffering from a loss of depth perception (see above), lead to changes in the apparent size of objects presented at different distances: objects perceived as nearer than they actually are appear too small (micropsia), while those perceived as further away than they are appear too big (macropsia) (cf. Holmes 1918a).

Similarly, we recognize objects that are rotated or shifted in the visual field, producing quite different retinal images. This constancy is probably achieved through extensive learning and storage of many typical (or canonical) views of the object, and not primarily by producing a three-dimensional model that is subsequently translated and rotated until it fits the actual position and view of the object in the outer world (Bülthoff et al. 1995; Dill and Fahle 1999) while the final construction of a ‘true’ threedimensional representation in Marr’s (1982) sense cannot be excluded. Patients have been described who experience profound difficulties in matching views of threedimensional objects after rotation or across shifts of perspective (e.g. de Renzi et al. 1969; Warrington and Taylor 1973, 1978; Humphreys and Riddoch 1984, 1985; Levine 1978), usually after lesions in the right posterior hemisphere, especially the inferior parietal lobe (Warrington and Taylor 1973; Warrington 1982). This deficit was called ‘perceptual categorization deficit’ by Warrington and colleagues. These patients have

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relatively few problems in everyday life, probably due to the redundancy present in most visual scenes of everyday life.

The capability to perceive objects in the same colour (colour constancy), irrespective of the illuminant, may suffer an isolated defect. The wavelength sent out by objects depends on their surface properties and on the light source illuminating them. While the surface property is relatively constant and helps in identifying the object, the illuminant is not constant, but varies greatly between light bulbs, neon tubes, and sun- light—and even strongly between the sunlight at midday versus sunrise or sunset. Photographic film correctly mirrors these variations of wavelength composition originating from identical surfaces under different illuminants, leading to clearly different object colours on film. To the human observer, quite to the contrary, the object’s colour stays constant—due to the mechanism of colour constancy. Two studies proved that the ability of colour constancy might be lost in patients while colour vision as such is spared (Rüttiger et al. 1999; see Chapter 8, this volume). Changes of luminance, on the other hand, seem to be automatically compensated for by the fact that cortical neurons are primarily concerned with contrasts rather than with absolute intensities, since object contrasts are independent of the intensity of the illuminant.

Failures to achieve a stable representation of extrapersonal space

Even the discrimination between the effects of ego-motion (self-motion) versus object motion in vision may become defective. There are two possible reasons for the perception of object motion on the retina, namely, motion of the object in the outer world, or else motion of the eye. To discriminate between these two possibilities, and to prevent the brain from misinterpreting retinal image motion as object motion during eye movements, an efference copy is relayed from the oculomotor centres to the visual cortex, containing information on the velocity and direction of the intended motion (Fig. 7.15; von Holst and Mittelstaedt 1950). When an injury of the brain involves the centre mediating this efference copy, the patient is impaired in discriminating between motion of objects versus eye movements—and, consequently, strong subjective object movements will be experienced during each (smooth) eye movement. Indeed, a patient suffering from such symptoms has been described in the literature (Haarmeier et al. 1997), and many more will have not been diagnosed for lack of adequate testing.

The problem of discriminating between motion of the retinal image due to object motion versus eyeor body-motion leads to the more general question of how humans achieve a stable representation of extrapersonal space. Due to the extreme specialization of our central retina, we steadily perform a saccadic scanning of the outer world by eye movements and obviously synthesize from this sequential scanning a relatively stable representation of the visual world. This representation cannot be achieved in the primary or secondary visual cortices, areas 17 through 19, since these areas are retinotopically

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instability of the visual world experienced by the patient as described above (Haarmeier et al. 1997) and probably plays a major role in Balint’s syndrome and simultanagnosia (see eponymous subsection).

There is still another type of representation and that will become important for the understanding of neglect (see the eponymous subsection). That is the allocentric representation related to Marr’s three-dimensional sketch. (But while Marr believed in a real three-dimensional reconstruction of object representations, recent evidence indicates that often a number of different two-dimensional views are stored instead of the computation of a complete three-dimensional reconstruction; see Bülthoff et al. 1995 and cf. Hinton 1981.) The allocentric coordinate system codes objects primarily through the spatial relations between their components. This type of representation allows constancy of object recognition during object rotation. If a watch is rotated by 90 , its retinal image changes considerably, and this is certainly even more true for, say, a human being. Failure in creating an allocentric representation of objects will lead to strong problems for identification of objects presented in unusual orientations, or perhaps even when tilting the head or when lying down.

In summary, constancy mechanisms exist for a number of aspects of visual object perception, such as illuminant, size, object position, and extrapersonal space, and their failure, while probably most often not correctly diagnosed, leads to distinct disturbances in the perception of the outer world (e.g. Riddoch 1935). This ends the description of defects on a relatively early level of visual information-processing, where contours are detected in the visual input and their positions in space are computed. Next, we will deal with the formation of objects from contours, and with the failures of this process.

Apperceptive and associative agnosias

Contour binding, object formation, and their failures: illusory conjunctions and apperceptive agnosias

Following border, or contour extraction, the next level of visual informationprocessing is the binding or grouping (these words will be used as synonyms) together of those borders belonging to the same region or surface and hence to the same object. Binding of contours to a coherent representation of a surface and finally to an object leads to the formation of a gestalt that is distinct from its surround such as in figure – ground segregation. This seems to be an easy feat since humans do it all the time quite effortlessly. But, when researchers in artificial intelligence started to teach vision to their computers, they soon realized the complexities inherent in natural scenes. Objects are not usually presented in isolation but close to other objects. They may be partly occluded by other objects. They are not homogeneous but textured and hence not all contours represent borders, especially if there are shadows cast on to the object. In short, it is not an easy task to combine contours extracted from an image into object representations. Even the intact human visual system sometimes fails in this task. For example, when a (reasonable) number of different forms such as different digits are