Ординатура / Офтальмология / Английские материалы / The Neuropsychology of Vision_Fahle, Greenlee_2003
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responses to eccentrically presented, complex stimuli (plaid background with nine superimposed white crosses), when the subjects were instructed to attend to the left or right visual hemifield. The subject’s task was to detect the presence of a ‘T’ among the crosses. The fMRI responses increased over retinotopically defined visual cortex when subjects shifted attention to that location and this increase was also evident in the late components of event-related potentials (ERPs) measured in separate sessions in the same subjects. This late modulation of the visually evoked response suggests that spatially directed attention has a top–down influence over processing in primary visual cortex. Further evidence for the role of attention in modulating responses in visual cortex is reviewed in Chapter 3, this volume.
Discrimination of direction, speed, and colour of moving stimuli
Several research groups have investigated the effects of task performance on cortical activation with the PET camera or MR-scanner. Such approaches require the subjects to make psychophysical judgements and button-press responses while they view visual motion sequences. In a PET study, Orban et al. (1998) studied changes in rCBF while subjects performed speed discrimination tasks. They reported no increment in response in the motion-sensitive V5/MT region while subjects performed speed discrimination. In contrast, areas 19 and 20 showed significant response enhancements during speed discrimination compared to simple motion detection trials.
As mentioned above, Chawla et al. (1999a) reported the results of three subjects who viewed stimuli defined by colour or luminance contrast. They found speed-dependent responses in V3a and V5 (see also Singh et al. 2000). In a further study, Chawla et al. (1999b) cued subjects to attend to the speed or colour of moving dot stimuli in an eventrelated design. Their results indicate that cued attentional shifts to the speed of the stimuli enhanced responses in V5/MT and attentional shifts to the colour of the dots enhanced the response in V4. Although these effects are small, they suggest a top–down influence of baseline activation in neural circuits in extrastriate visual areas (compare above).
In a similar fashion, Huk and Heeger (2000) reported that area V5 responded more while subjects performed speed discrimination compared to when they performed either direction or contrast discriminations. The stimulus speeds varied around 8 /s for the speed discrimination and 0 /s for the direction discrimination (lower threshold for motion). They found higher responses in MT when the subjects were discriminating the speed of the stimuli. The interpretation of the results is not straightforward, since different speed ranges were used for the different tasks. The lack of enhanced MT activation in the Orban et al. (1998) study compared to the presence of an effect in the Huk and Heeger (2000) study could be related to differences in the methods used to assess brain activity (PET versus fMRI).
Optic flow stimuli
Wide-field motion evoked by eye, head, or body movements has been referred to as optic flow (Koenderink 1986). BOLD responses to optic flow stimuli have been studied
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by Rutschmann et al. (2000). Their subjects viewed dynamic, random-dot kinematograms (RDK) dichoptically. Different speeds ranging from 4 to 13 /sec were used to simulate random, expansion–contraction and rotational motion fields. They also varied the binocular disparity of the flow fields. In two conditions, dichoptic flow fields with and without disparity were presented. Little response selectivity was found in the striate and immediate extrastriate regions. Areas in the cuneus, putatively corresponding to the dorsal parts of V3 and V3a, responded somewhat better to the random walk stimuli than to the other three conditions. The V5/V5a complex showed little sensitivity to the flow patterns of the motion stimulation. In contrast, area KO in the ventral region of V3 appeared to show the greatest selectivity to optic flow. This area also exhibited slightly larger responses to the condition with disparity gradients in the optic flow fields. The results indicate that striate (V1) and extrastriate areas (V2, V3/V3a) respond robustly to optic flow. However, with the exception of a more pronounced response in V3/V3a to random walk, Rutschmann et al. found little evidence for response selectivity with respect to flow type and disparity in these early visual areas. In a large PET study, Beer et al. (2002) reported a similar selectivity to optic flow stimuli in area KO/V3b.
fMRI responses to random-dot motion sequences, in which noise fields were compared to rotation, expansion, and simulated three-dimensional motion, have been reported (Paradis et al. 2000). V5/MT responded well in the random noise–static comparison, but little additional activation was found when the random noise was compared to the coherent motion conditions. Selective enhancement of the V5/MT area during coherent rotational or expansion/contraction motion sequences has been demonstrated by Morrone et al. (2000). However, these enhanced responses could only be obtained when the stimuli rapidly alternated between one of two directions, suggesting that these responses might be more related to the processing of sudden direction changes and not to optic flow as such.
Effects of eye movements on visual-motion responses
An obvious source of experimental error in imaging studies of visual processing is the extent to which the subjects move their eyes during the scanning period. Although movements of the head are restrained by various methods and the effects of head motion can be partially eliminated by postacquisition motion correction (Cox 1996; Woods et al. 1998a,b), the effects of eye movements have been largely ignored in the past. Some form of prescan training has been employed in the hope that the subjects conform to the instructions during the entire scan period. Our experience suggests that this is often not the case especially for long scan periods. The effects of pursuit during motion perception (Barton et al. 1996) and a comparison between saccades and pursuit (Petit et al. 1997) without eye position monitoring have been published. In the next section we outline studies that have attempted to quantify the effect of eye movements on resultant BOLD responses. Some of these investigations proceed to describe cortical areas underlying the control of saccadic and pursuit eye movements.
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Disturbances in eye movement control and double vision are among the most common symptoms following head injury.
fMRI studies of eye movements
Several groups have analysed the cortical responses evoked during saccadic eye movements. To a lesser extent, a few studies have examined the responses evoked during smooth-pursuit eye movements. Below, we review these findings and point out the methodical shortcomings of several studies, which are related to the restraints imposed by the magnetic field in the MR-scanner.
Saccadic eye movements
Saccadic eye movements serve to bring a visual object of interest to the foveal region of the eye. Saccades are fast eye movements, which are both voluntary and ballistic in nature. The brain programmes the saccadic metrics (direction, amplitude) before the neural command signals are sent to the oculomotor nuclei in the brainstem for saccade execution. Before a saccade occurs, attention must be disengaged from a previously attended target, the saccade target must be selected, and the spatial location of this target must be estimated. Several cortical and subcortical regions are involved in the generation of saccades. Among these regions are the frontal eye fields (FEF; Bruce and Goldberg 1985), the dorsolateral prefrontal cortex (Funahashi et al. 1991; PierrotDeseilligny et al. 1991), the supplementary eye fields (Schlag and Schlag-Rey 1987), the posterior parietal cortex (Gnadt and Andersen 1988; Barash et al. 1991a,b; PierrotDeseilligny et al. 1991), the primary visual cortex, the basal ganglia (Hikosaka and Wurtz 1985a,b), and the superior colliculus (Schiller et al. 1979).
Recent functional imaging studies have explored the cortical areas underlying the control of saccadic eye movements in humans (Petit et al. 1993; Paus et al. 1995; Luna et al. 1998; Darby et al. 1996; Bodis-Wollner et al. 1997; Sweeney et al. 1996; Perry and Zeki 2000). In most of these studies, eye movements could not be adequately measured during the scanning sessions. Attempts to use electrooculography have not been very successful, due to the currents induced into the recording equipment during gradient switching (see Felblinger et al. 1996). The exact pattern of eye movements contributing to the cortical activity during imaging thus remained undetermined in these studies.
Changes in the BOLD response related to the task performed could reflect saccade preparation. A favourite paradigm used to compare task effects is the proversus antisaccade tasks. A visual target appearing to the left, say, will evoke a short latency prosaccade. This saccade can, however, be suppressed and the subject can be instructed to look in the opposite direction, thus performing an anti-saccade (Everling and Fischer 1998). The imaging literature is controversial with respect to differences in responses evoked by proand anti-saccades. In a PET study by Paus et al. (1993) and a more recent fMRI study by Muri et al. (1998), no significant differences in FEF activity between these two tasks could be found. In a recent study, Kimmig et al. (2001) compared responses in
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Fig. 4.2 Average time course of the BOLD response in the frontal eye fields (FEF) in three subjects who performed a proor anti-saccade task (from Cornelissen et al. 2002). Correct and incorrect trials are denoted by the different symbols.
proand anti-saccade tasks and report significant differences in the FEF in premotor cortex, as well as differences in the dorsal cuneus and parietal cortex, for these two types of task. These same authors developed a fibre-optic limbus tracking system that is compatible with the magnetic resonance scanning environment. Using this system, Kimmig et al. (1999) reported differences in the pattern of BOLD responses evoked by saccades versus pursuit.
Event-related fMRI of saccadic eye movements
Most of the studies reported in the last section used the so-called ‘block-design’ to study responses related to saccadic eye movements. Usually, these paradigms require the subjects to hold fixation for a prolonged period, say, 30 seconds, and then start performing saccadic eye movements over a 30 second period. This on–off design has the obvious limitation that responses related to the preparation and execution of saccades cannot be disentangled. The event-related design (Buckner et al. 1996) has been introduced to overcome these limitations. Here single-trial events are separated in time and each target onset is triggered by the gradient system of the scanner. The subject performs a single saccade, holds fixation at the peripheral location for, say, 3 seconds, then returns to the centre after the target disappears and the central fixation spot reappears. By repeating this procedure 10 to 20 times for each condition, the responses in saccade-related areas can be examined. In the study of Cornelissen et al. (2002), the subjects were cued to perform proor anti-saccades depending on a colour change of the central fixation spot just prior to fixation offset and target onset. A change from blue to green signalled a pro-saccade, whereas a change from blue to red indicated that the subject should perform an anti-saccade. The responses to these two conditions are shown in Fig. 4.2 and indicate that proand anti-saccades evoked similar activity in the FEF region in prefrontal cortex. Further evaluation of correctly performed and erroneous proand anti-saccades indicated some differences related to task performance (Cornelissen et al. 2002). Connolly et al. (2000) compared activation in prefrontal and
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parietal areas during proand anti-saccades. These activations were compared to those arising during proand anti-pointing to a visual target with gaze remaining straight ahead. The authors report a substantial overlap between areas responding in both tasks, with additional voxels activated during the pointing and anti-tasks. The FEF appears to respond not only during saccade execution, but also during the suppression of saccadic behaviour (required during their pointing task).
Summary
Block-design and event-related methods have also been used to study the effect of task components related to the relative onand offsets of the fixation and target stimuli.
Performance in the so-called ‘gap’ paradigm has recently been compared to that found for the ‘step’ paradigm. In the gap paradigm the fixation point is extinguished 200 ms prior to target onset. This stimulus onset asynchrony (SOA) allows the fixation system to ‘unfixate’ and begin preparation for the next saccade. In the step condition, the fixation stimulus remains on for a short period after the target stimulus has been presented. These different conditions led to different distributions of saccadic reaction times, where more ‘express saccades’ occur in the gap but not in the overlap conditions. An ongoing set of experiments in our laboratory (Özyurt et al. 2001) explores possible differences in the BOLD response patterns to these two types of oculomotor tasks. The initial results suggest that there exist differences in the pattern of BOLD activation for these slightly different saccade tasks. Memory-guided saccades are voluntary eye movements to the remembered locations of previously presented visual targets. The FEF appear to be more active during memory-guided compared to visually guided tasks (Greenlee et al. 2001). In a variant of the memory-guided saccade task, Sereno et al. (2001) had subjects perform centrifugal saccades to remembered locations along a virtual circle in clockwise and counterclockwise directions. The repetitive nature of this task allowed the authors to apply Fourier methods to extract the phase and amplitude of the cyclic BOLD response. The phase of this response provided evidence for a retinotopic organization in an area in the intraparietal sulcus, which they refer to as the putative lateral intraparietal (LIP) area.
Pursuit eye movements
Recent electrophysiological and neuroanatomical studies in monkeys suggest that there is a large overlap in the neural control of (small) saccades, fixation, and pursuit (see Krauzlis and Stone 1999). Smooth pursuit allows us to maintain our gaze on a target, despite the fact that the target is in motion. The mechanisms underlying the perception of visual motion and those controlling resultant pursuit appear to be closely related, at least during the initial stages of pursuit programming (Lisberger and Movshon 1999). Few studies have looked at the effects of pursuit eye movements on the responses in visual and oculomotor areas. Kimmig et al. (1999) showed that the activity in the FEF was greater during saccades than during pursuit, whereas MT (V5/V5a) responded better during pursuit than during saccades. Petit et al. (1997)
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found differences in the location of activation in the FEF related to the type of eye movement performed. Pursuit was associated with activation in the lateral and saccades with activation in the more medial parts of the FEF. Further support for a subdivision of labour in the FEF for saccades and pursuit has been reported by Rosano et al. (2002). Freitag et al. (1998) found that the response in V5/V5a to moving dot stimuli increased during pursuit compared to fixation. Dukelow et al. (2001) first isolated area MST by identifying ipsilateral responses to eccentric motion displays and then characterized this area’s response to visual or self-guided pursuit. The subjects had to pursue either a moving target or an image of their own finger, which they waved in front of their face in the dark. Eye movements were not measured in the scanner, so the intrusion of saccades cannot be ruled out. Nevertheless, the subjects showed selective responses during pursuit and these responses were most pronounced in an area anterior, but adjacent, to MT/V5. These studies point to an extraretinal input into the V5/V5a region, suggesting that this area might be involved in the analysis of object and self-motion (Greenlee 2000). The results of these oculomotor studies indicate that several cortical regions are involved in the programming and execution of saccadic and pursuit eye movements. There is mounting evidence that these different types of eye movements have separate, but also partially overlapping, representations in neocortex. We next outline studies that have explored cortical responses measured during the complex visuooculomotor behaviour required for reading.
fMRI studies of reading
Reading is an important social skill, which, unlike spoken language, has to be acquired in a controlled learning setting over a prolonged period of time during the first years of schooling. Indeed, civilized societies devote considerable resources to ensure that their children learn to read and write with an acceptable level of competence. Which areas of the human brain underlie the ability to read and which of these areas are involved in learning how to read? What goes wrong in the brain functioning of children who, despite normal or above normal intelligence, cannot read at a level appropriate for their age cohort? Which brain functions are impaired in patients with acquired dyslexia following cortical stroke (see Chapter 7, this volume)? Clearly, there are no simple answers to these questions, since the skill of reading is itself a multimodal activity involving the coordination and interaction between visual, auditory, oculomotor, and language areas in the brain. Several brain-imaging studies have looked at reading and disorders of reading (dyslexia) in otherwise healthy subjects. Below we give a brief (and selective) overview of recent studies that have explored the brain associated with reading, with learning how to read, and with reading disabilities. For recent reviews see Shaywitz et al. (1998), Pugh et al. (2000), and Habib (2000).
Normal reading
Reading is a complex oculomotor task, in which the reader shifts his or her gaze sequentially along stationary text. Psychophysical analysis of the visual components of
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reading has been conducted in a series of papers by Legge and co-workers (e.g. Legge et al. 2001). It is usually assumed that orthographic analysis of the individual letters that make up the words is performed by early visual detectors that respond to orientation, spatial frequency, and contrast of the letter elements. Lexical access is a cognitive process, whereby the reader extracts individual words from the letter strings and compares these strings to long-term memory representations of words (i.e. mental lexical) in the respective language. Phonological analysis and access can be compared by having subjects read lists containing proper words (with low and high occurrence frequencies), pseudohomophones (compound words with meaning), and pseudowords (pronounceable non-words; cf. Simos et al. 2002). Language-specific responses are evoked by reading of meaningful words and these activations are primarily located in the posterior part of the left temporal gyrus. Reading pseudowords led to less activation in this area. Reading of both types of words led to additional activation in the superior temporal gyrus, suggesting a phonological analysis in this cortical region (Pugh et al. 1996). The oculomotor component of reading will lead to additional activations in parietal and prefrontal regions (see above). Keller et al. (2001) explored fMRI responses to reading sentences of high and low syntactic complexity where these sentences contained words with either high or low lexical frequency. The authors found the largest responses for sentences containing low-frequency words with high syntactic complexity. Significant frontal lobe responses were also evident in the left hemisphere.
Mirror-script reading and procedural learning
To ensure his privacy and ward off plagiarism, Leonardo Da Vinci wrote his notes in mirror script with his left hand (Richter 1975). Left-handed writers apparently perform mirror writing with greater ease (Tucha et al. 2000). The ability to read mirror script can be learned and, given a sufficient amount of training, mirror script can be read without difficulty. Learning how to read mirror script is a prototypical procedural learning task. The performance of untrained subjects on this task is correlated with measures of visuospatial ability (Schmidtke et al. l996). Several neuropsychological studies have employed this paradigm to study procedural learning in patients. Normal learning was found in 20 patients with prefrontal lesions (Schmidtke et al. 1998). These results suggest that areas beyond those in the frontal-striatal loops are involved in learning how to read mirror script. Other factors involve visual priming of otherwise unfamiliar visual patterns, an increase in the capacity to perform the required mental rotation or inversion, an improvement of working memory capacity required to maintain, combine, and verify decoded letters, and direct recognition of mirror-reversed letters or letter groups.
Three recent fMRI studies examined the patterns of cortical activation in reading spatially transformed versus plain text. Goebel et al. (1998) reported significant increases of BOLD signal along the intraparietal sulcus bilaterally, in the left superior parietal lobule (SPL), the left occipitotemporal cortex, and at the posterior bank of the left precentral sulcus. In a study of Poldrack et al. (1998), similar, bilateral activation was
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found in the posterior SPL, the occipital cortex, parts of the inferior temporal cortex, along the intraparietal sulcus, and also in the cerebellum and pulvinar. The authors suggest that reading of transformed script involves a parietal ‘visuospatial transformation’ area and an occipitotemporal ‘object recognition’ area.
Kassubek et al. (2001) studied 10 healthy subjects while they read visually presented single plain-script and mirror-script words. fMRI responses in naive subjects on day 1 were compared to those acquired on day 2 after an intensive training period of mirrorscript reading. Their results indicate that striate and extrastriate visual areas, associative parietal cortex (BA7 and BA40, superior and inferior parietal lobulus), and the prefrontal cortex (BA6) were bilaterally active during plainand mirror-script reading. Activation in the primary visual cortex (BA17) was stronger during plain script reading compared to mirror-script reading (which is related to a word-length effect—longer words were used in the plain script task). The reverse pattern, i.e. significantly stronger activation during mirror-reading, was seen in BA7 and BA40 (parietal associative cortex) and in BA6 (FEF). After training, FEF and parietal area BA7 bilaterally and right BA40 exhibited a decrease of activation during mirror reading, suggesting their involvement in learning. The training-dependent deactivation of areas that were relatively more active during initial performance indicates that procedural learning of the mirrorreading task is accompanied by a decrease in the demand on the attentional system, on the control of eye movements, and on visuospatial transformation processes. In a follow-up study, Poldrack and Gabrieli (2001) discriminated between skill learning and priming effects arising during multiple training sessions of mirror-script reading. They found decreased activity in several areas related to repetition priming, as well as increases and decreases in activation related to learning. Their findings suggest that procedural learning involves a widespread neural activation of several memory systems.
These studies are provocative, since they suggest that a widely distributed network of visual and oculomotor/motor areas is involved in reading and the ability to learn to read new types of script (such as mirror-script). As such, they suggest that disturbances in reading performance might involve a complex interaction between visual, oculomotor, grapheme–phoneme conversion, language, and learning-related areas in the human brain. The next section outlines selected studies related to problems of reading.
fMRI responses in dyslexia
The phenomenon of dyslexia is characterized by an inability to read at a reasonable level in otherwise normally intelligent and educated persons. Habib (2000) focuses on the problem of grapheme–phoneme transformation in his summary statement: ‘Neuropsychological studies have provided considerable evidence that the main mechanism leading to these children’s learning difficulties is phonological in nature, namely, a basic defect in segmenting and manipulating the phoneme constituents of speech.’ This so-called ‘temporal processing impairment’ theory of dyslexia has received considerable attention in the dyslexia literature (Pugh et al. 2000; Habib 2000).
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Brain-imaging approaches to dyslexia have concentrated mostly on BOLD responses, but some recent work also suggests microanatomical differences in persons with reading impairments. Diffusion-tensor imaging suggests differences in the microstructure of white matter in the left temporoparietal region in dyslexics (Klingberg et al. 2000). The correlation between reading ability and the anisotropy value within the area of interest suggests that dyslexia is associated with a disturbance in the left-hemispheric anterior–posterior connectivity.
In a recent fMRI study, Temple et al. (2000) explored BOLD responses in eight adults with dyslexia and compared their results to those of 10 age-matched controls on rapid versus slow auditory pitch discrimination. Significant activation in the left prefrontal cortex (BA46/9) was evident in the rapid/slow presentation contrasts in the normal readers. This activation was absent in the dyslexics. The results point to a malfunction of the left temporal–prefrontal cortical loop in sound discrimination in adult dyslexics. Training of rapid sound sequences led to a left prefrontal response in two of three dyslexics.
Shaywitz et al. (1998) compared BOLD responses in occipitotemporal and prefrontal areas (Broca’s area) during tasks involving the simple discrimination of letter-case strings (bbBb, bBbb) and non-word rhymes. They compared the responses of normal readers with those of subjects with impaired reading abilities. These authors found an overactivation in Broca’s area and underactivation in the angular gyrus during nonword rhyming. The authors interpret their findings as evidence for a malfunction between orthographic and phonological processes.
Several groups have explored possible visual-related bases for dyslexia using psychophysical and fMRI techniques. Evidence for and against the so-called ‘magnocellular deficit’ in dyslexia can be found in the literature. Eden et al. (1996) reported significantly lower BOLD responses in MT/V5 in dyslexics (compared to controls) during motion perception. Demb et al. (1998a) reported elevated psychophysically determined speed discrimination thresholds in dyslexics and suggest that this joint impairment (i.e. deficit in speed discrimination and reading impairment) might be related to disturbances in the magnocellular input to the visual cortex. In support of this view, Demb et al. (1998b) found fMRI response differences for magnocellular-type stimuli in the MT area. Their results suggest that persons with a reading impairment showed lower BOLD responses overall and a less steep slope in the estimated contrast response functions (BOLD response versus stimulus contrast). They further report a correlation between BOLD response in area V5/MT and reading rate (with a sample size of 5 dyslexics and 5 controls). These findings need to be confirmed with larger sample sizes, but they do suggest that there could be a visual component in dyslexia related to a disturbance of the magnocellular projection from LGN to primary visual cortex (see review by Eden and Zeffiro 1998). Such a disturbance could lead to subnormal activation of motion-selective regions in extrastriate visual cortex. The results reviewed here suggest that fMRI can be used to analyse the cortical responses evoked during reading. As reading demands the coordinated effort of a widespread multimodal representation of form and content,
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the brain responses will necessarily be widespread and complex in nature. By decomposing these tasks into subcomponents, research in this area might be able to dissociate the neural mechanisms underlying each aspect of reading. Such newly gained knowledge might help provide a better understanding of the pathology underlying reading disorders.
Summary
In summary, we have reviewed brain imaging studies of visual motion processing, of oculomotor control, and of reading. The brain-imaging studies all point to V5/MT as an area in the occipitotemporal junction that is involved in several aspects of visual motion encoding. Earlier visual areas, such as V3a and the kinetic occipital area (KO), also exhibit motion-selective responses. These areas appear to contribute to the analysis of motion-defined boundaries required to segment complex visual scenes into figure and ground. Once this segmentation has taken place, visual attention can focus processing capacity to a selected visual target. Focal attention modulates the amplitude of the BOLD signal evoked by these selected stimuli. Attention not only enhances responses to selected targets, but can also enhance responses to selected dimensions of a single target, such as its colour or motion characteristics.
Once a moving target has been segmented from a background and selected as a target for attention, it can be pursued by the observer. Smooth pursuit is associated with activation in motion-selective visual areas, but also in areas in premotor cortex related to the control of eye movements. Task difficulty is a critical variable in oculomotor paradigms. The eye fields in prefrontal cortex can be activated during simple proand anti-saccade tasks, and the magnitude of this activity appears to be, at least to some extent, dependent on the task the subject is performing.
Reading is a complex task that requires visual, oculomotor, and language processing. Brain imaging has been employed to study these areas and to explore differences related to reading ability. Dyslexia is a disturbance of reading ability in otherwise healthy and intelligent individuals. It is our hope that, with the help of fMRI, cognitive neuroscience will be able to provide new insights into the multiple processes underlying visual cognition and disorders of visual cognition.
Acknowledgements
The author thanks the Deutsche Forschungsgemeinschaft (grants: SFB 517, C9, European Graduate School for Neurosensory Systems) for their support and Jale Özyurt and Roland M. Rutschmann for valuable comments on this manuscript.
References
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