1
|
Jüttner M, Strasburger H, Treutwein B, Landis T, Rentschler I. Complementary deficits in perceptual classification in pure alexia and acquired prosopagnosia - New insights from two classic cases. Neuropsychologia 2021; 155:107820. [PMID: 33676958 DOI: 10.1016/j.neuropsychologia.2021.107820] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2020] [Revised: 01/24/2021] [Accepted: 03/02/2021] [Indexed: 11/30/2022]
Abstract
Pure alexia and prosopagnosia traditionally have been seen as prime examples of dissociated, category-specific agnosias affecting reading and face recognition, respectively. More recent accounts have moved towards domain-independent explanations that postulate potential cross-links between different types of visual agnosia. According to one proposal, abnormal crowding, i.e. the impairment of recognition when features of adjacent objects are positioned too closely to each other, might provide a unified account for the perceptual deficits experienced by an agnosic patient. An alternative approach is based on the notion of complementary visual subsystems favouring the processing of abstract categories and specific exemplars, respectively. To test predictions of these two approaches with regard to pure alexia and prosopagnosia, we present previously unpublished data on digit recognition and visual crowding from two in the neuropsychological literature extensively studied patients, KD and MT (e.g., Campbell et al., 1986; Landis and Regard, 1988; Rentschler et al., 1994). Patient MT, diagnosed with pure alexia, showed pronounced abnormal foveal crowding, whereas KD, diagnosed with prosopagnosia, did not. These results form a distinct double dissociation with the performance of the two patients in other perceptual classification tasks involving Gabor micropatterns and textures, as well as Glass patterns, which revealed a significantly greater impairment in KD relative to MT. Based on an analysis of the specific task demands we argue that prosopagnosia and pure alexia may involve complementary deficits in instantiation and abstraction, respectively, during perceptual classification, beyond any category specificity. Such an explanation appears in line with previous distinctions between a predominantly left-hemispheric, abstract-category and a predominantly right-hemispheric, specific-exemplar subsystem underlying object recognition.
Collapse
Affiliation(s)
- Martin Jüttner
- School of Psychology, College of Health and Life Sciences, Aston University, Birmingham, UK.
| | | | | | | | - Ingo Rentschler
- Institute of Medical Psychology, University of Munich, Germany
| |
Collapse
|
2
|
Abstract
Alexia refers to a reading disorder caused by some form of acquired brain pathology, most commonly a stroke or tumor, in a previously literate subject. In neuropsychology, a distinction is made between central alexia (commonly seen in aphasia) and peripheral alexia (a perceptual or attentional deficit). The prototypical peripheral alexia is alexia without agraphia (pure alexia), where patients can write but are impaired in reading words and letters. Pure alexia is associated with damage to the left ventral occipitotemporal cortex (vOT) or its connections. Hemianopic alexia is associated with less extensive occipital damage and is caused by a visual field defect, which creates problems reading longer words and passages of text. Reading impairment can also arise due to attentional deficits, most commonly following right hemisphere or bilateral lesions. Studying patients with alexia, along with functional imaging studies of normal readers, has improved our understanding of the neurobiological processes involved in reading. A key question is whether an area in the left ventral occipitotemporal cortex is specialized for or selectively involved in word processing, or whether reading relies on tuning of more general purpose perceptual areas. Reading deficits may also be observed in dementia and traumatic brain injury, but often with less consistent deficit patterns than in patients with focal lesions.
Collapse
Affiliation(s)
- Randi Starrfelt
- Department of Psychology, University of Copenhagen, Copenhagen, Denmark.
| | - Zoe Woodhead
- Department of Experimental Psychology, University of Oxford, Oxford, United Kingdom
| |
Collapse
|
3
|
Giofrè D, Toffalini E, Provazza S, Calcagnì A, Altoè G, Roberts DJ. Are children with developmental dyslexia all the same? A cluster analysis with more than 300 cases. DYSLEXIA (CHICHESTER, ENGLAND) 2019; 25:284-295. [PMID: 31332875 PMCID: PMC6771784 DOI: 10.1002/dys.1629] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/17/2019] [Revised: 05/09/2019] [Accepted: 06/20/2019] [Indexed: 06/10/2023]
Abstract
Reading is vital to every aspect of modern life, exacerbated by reliance of the internet, email, and social media on the written medium. Developmental dyslexia (DD) characterizes a disorder in which the core deficit involves reading. Traditionally, DD is thought to be associated with a phonological impairment. However, recent evidence has begun to suggest that the reading impairment in some individuals is provoked by a visual processing deficit. In this paper, we present WISC-IV data from more than 300 Italian children with a diagnosis of DD to investigate the manifestation of phonological and visual subtypes. Our results indicate the existence of two clusters of children with DD. In one cluster, the deficit was more pronounced in the phonological component, while both clusters were impaired in visual processing. These data indicate that DD may be an umbrella term that encompasses different profiles. From a theoretical perspective, our results demonstrate that dyslexia cannot be explained in terms of an isolated phonological deficit alone; visual impairment plays a crucial role. Moreover, general rather than specific accounts of DD are discussed.
Collapse
Affiliation(s)
- David Giofrè
- Department of Educational SciencesUniversity of GenoaGenoaItaly
| | | | - Serena Provazza
- Natural Sciences and PsychologyLiverpool John Moores UniversityLiverpoolUK
| | - Antonio Calcagnì
- Department of Developmental and Social PsychologyUniversity of PadovaPadovaItaly
| | - Gianmarco Altoè
- Department of Developmental and Social PsychologyUniversity of PadovaPadovaItaly
| | - Daniel J. Roberts
- Centre for Cognitive Neuroscience, Division of Psychology, College of Health and Life SciencesBrunel University LondonUxbridgeUK
| |
Collapse
|
4
|
Bormann T, Frings L, Dreßing A, Glauche V, Weiller C. Do all visual deficits cause pure alexia? Dissociations between visual processing and reading suggest “no”. Brain Cogn 2018; 125:69-77. [DOI: 10.1016/j.bandc.2018.05.008] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2017] [Revised: 05/17/2018] [Accepted: 05/21/2018] [Indexed: 01/10/2023]
|
5
|
Sand K, Robotham RJ, Martelli M, Starrfelt R. Visual crowding in pure alexia and acquired prosopagnosia. Cogn Neuropsychol 2018; 35:361-370. [DOI: 10.1080/02643294.2018.1483325] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
Affiliation(s)
- Katrine Sand
- Department of Psychology, University of Copenhagen, Copenhagen, Denmark
| | - Ro Julia Robotham
- Department of Psychology, University of Copenhagen, Copenhagen, Denmark
| | | | - Randi Starrfelt
- Department of Psychology, University of Copenhagen, Copenhagen, Denmark
| |
Collapse
|
6
|
Abstract
Posterior cortical atrophy (PCA) is the canonical “visual dementia,” with affected individuals experiencing a progressive disintegration of their visual world owing to dysfunction and atrophy at the back of the brain. The syndrome, which also affects literacy, numeracy, and gesture, is typically caused by Alzheimer’s disease, but is distinguished from more common amnestic presentations by virtue of relatively preserved episodic memory and insight. Although problems with object and space perception are the most widely reported and investigated symptoms, these higher-order perceptual difficulties are often underpinned by an array of changes in more basic visual and oculomotor processes. Here we review recent studies providing insights into these more elementary aspects of vision in PCA, including fixation stability, saccade generation, point localization, excessive crowding, and factors affecting the effective field of vision. We argue that a more detailed appreciation of these fundamental changes in the early visual system not only will improve the characterization and understanding of this rare clinico-radiological syndrome but will also guide the design of visual aids and strategies aimed at maintaining everyday abilities in individuals with PCA.
Collapse
Affiliation(s)
- Sebastian J. Crutch
- Dementia Research Centre, Department of Neurodegeneration, Institute of Neurology, University College London
| | - Keir X. X. Yong
- Dementia Research Centre, Department of Neurodegeneration, Institute of Neurology, University College London
| | - Timothy J. Shakespeare
- Dementia Research Centre, Department of Neurodegeneration, Institute of Neurology, University College London
| |
Collapse
|
7
|
Yong K, Rajdev K, Warrington E, Nicholas J, Warren J, Crutch S. A longitudinal investigation of the relationship between crowding and reading: A neurodegenerative approach. Neuropsychologia 2016; 85:127-36. [PMID: 26926579 PMCID: PMC4863520 DOI: 10.1016/j.neuropsychologia.2016.02.022] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2015] [Revised: 02/23/2016] [Accepted: 02/26/2016] [Indexed: 11/24/2022]
Abstract
We have previously documented two patients (FOL and CLA) with posterior cortical atrophy who achieved accurate and rapid reading despite deficits in ten measures of visual processing, with two notable exceptions: (1) a measure of visual acuity, (2) a measure of visual crowding. Subsequent longitudinal investigation of these patients was carried out, involving annual tests of early visual, visuoperceptual and visuospatial processing and assessment of reading ability. Follow-up assessments identified the evolution of a particular early visual processing deficit, excessive visual crowding; this deficit has been previously implicated in forms of dyslexia. Consistent with the link between crowding and reading dysfunction, follow-up assessments also revealed deterioration in both patients' reading ability. The current findings demonstrate a neurodegenerative approach towards understanding the relationship between visual crowding and the reading system, and suggest possible mechanisms for how excessive crowding may disrupt word recognition.
Collapse
Affiliation(s)
- Keir Yong
- Dementia Research Centre, Department of Neurodegenerative Disease, UCL Institute of Neurology, University College London, UK.
| | - Kishan Rajdev
- Dementia Research Centre, Department of Neurodegenerative Disease, UCL Institute of Neurology, University College London, UK
| | - Elizabeth Warrington
- Dementia Research Centre, Department of Neurodegenerative Disease, UCL Institute of Neurology, University College London, UK
| | - Jennifer Nicholas
- Dementia Research Centre, Department of Neurodegenerative Disease, UCL Institute of Neurology, University College London, UK
| | - Jason Warren
- Dementia Research Centre, Department of Neurodegenerative Disease, UCL Institute of Neurology, University College London, UK
| | - Sebastian Crutch
- Dementia Research Centre, Department of Neurodegenerative Disease, UCL Institute of Neurology, University College London, UK
| |
Collapse
|
8
|
Roberts DJ, Lambon Ralph MA, Kim E, Tainturier MJ, Beeson PM, Rapcsak SZ, Woollams AM. Processing deficits for familiar and novel faces in patients with left posterior fusiform lesions. Cortex 2015; 72:79-96. [PMID: 25837867 PMCID: PMC4643682 DOI: 10.1016/j.cortex.2015.02.003] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2014] [Revised: 01/30/2015] [Accepted: 02/03/2015] [Indexed: 11/25/2022]
Abstract
Pure alexia (PA) arises from damage to the left posterior fusiform gyrus (pFG) and the striking reading disorder that defines this condition has meant that such patients are often cited as evidence for the specialisation of this region to processing of written words. There is, however, an alternative view that suggests this region is devoted to processing of high acuity foveal input, which is particularly salient for complex visual stimuli like letter strings. Previous reports have highlighted disrupted processing of non-linguistic visual stimuli after damage to the left pFG, both for familiar and unfamiliar objects and also for novel faces. This study explored the nature of face processing deficits in patients with left pFG damage. Identification of famous faces was found to be compromised in both expressive and receptive tasks. Discrimination of novel faces was also impaired, particularly for those that varied in terms of second-order spacing information, and this deficit was most apparent for the patients with the more severe reading deficits. Interestingly, discrimination of faces that varied in terms of feature identity was considerably better in these patients and it was performance in this condition that was related to the size of the length effects shown in reading. This finding complements functional imaging studies showing left pFG activation for faces varying only in spacing and frontal activation for faces varying only on features. These results suggest that the sequential part-based processing strategy that promotes the length effect in the reading of these patients also allows them to discriminate between faces on the basis of feature identity, but processing of second-order configural information is most compromised due to their left pFG lesion. This study supports a view in which the left pFG is specialised for processing of high acuity foveal visual information that supports processing of both words and faces.
Collapse
Affiliation(s)
- Daniel J Roberts
- Research Centre in Brain and Behaviour, Liverpool John Moores University, UK
| | - Matthew A Lambon Ralph
- Neuroscience and Aphasia Research Unit, School of Psychological Sciences, University of Manchester, UK
| | - Esther Kim
- Department of Speech Pathology and Audiology, University of Alberta, Canada
| | | | - Pelagie M Beeson
- Department of Speech, Language, and Hearing Sciences, University of Arizona, USA
| | - Steven Z Rapcsak
- Department of Neurology, University of Arizona, USA; Neurology Section, Southern Arizona VA Health Care System, Tucson, AZ, USA
| | - Anna M Woollams
- Neuroscience and Aphasia Research Unit, School of Psychological Sciences, University of Manchester, UK.
| |
Collapse
|
9
|
Mares I, Custodio P, Fonseca J, Bentes C, Guerreiro M, Guimarães N, Pavão Martins I. To read or not to read: a neurophysiological study. Neurocase 2015; 21:793-801. [PMID: 25700186 DOI: 10.1080/13554794.2015.1013137] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
Abstract
Pure alexia (PA) has been associated with visual deficits or a failure to activate the visual word form area (VWFA). We report a patient with pure alexia due to posterior cortical atrophy, in whom event-related potentials revealed a delay in the P100 component and an absent N170 compared with controls. Furthermore, there was a tendency for a larger delay in P100 latencies associated with incorrectly read words. This suggests that some cases of PA might result from deficits in visual perception, signaled by the P100 early potential which could lead to an inability to consistently activate the VWFA, marked by the absent N170.
Collapse
Affiliation(s)
- Inês Mares
- a Language Research Laboratory, Institute of Molecular Medicine (IMM), Faculty of Medicine , University of Lisbon (FMUL) , Lisbon , Portugal
| | | | | | | | | | | | | |
Collapse
|
10
|
Ryan NS, Shakespeare TJ, Lehmann M, Keihaninejad S, Nicholas JM, Leung KK, Fox NC, Crutch SJ. Motor features in posterior cortical atrophy and their imaging correlates. Neurobiol Aging 2014; 35:2845-2857. [PMID: 25086839 PMCID: PMC4236588 DOI: 10.1016/j.neurobiolaging.2014.05.028] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2014] [Revised: 05/06/2014] [Accepted: 05/31/2014] [Indexed: 12/28/2022]
Abstract
Posterior cortical atrophy (PCA) is a neurodegenerative syndrome characterized by impaired higher visual processing skills; however, motor features more commonly associated with corticobasal syndrome may also occur. We investigated the frequency and clinical characteristics of motor features in 44 PCA patients and, with 30 controls, conducted voxel-based morphometry, cortical thickness, and subcortical volumetric analyses of their magnetic resonance imaging. Prominent limb rigidity was used to define a PCA-motor subgroup. A total of 30% (13) had PCA-motor; all demonstrating asymmetrical left upper limb rigidity. Limb apraxia was more frequent and asymmetrical in PCA-motor, as was myoclonus. Tremor and alien limb phenomena only occurred in this subgroup. The subgroups did not differ in neuropsychological test performance or apolipoprotein E4 allele frequency. Greater asymmetry of atrophy occurred in PCA-motor, particularly involving right frontoparietal and peri-rolandic cortices, putamen, and thalamus. The 9 patients (including 4 PCA-motor) with pathology or cerebrospinal fluid all showed evidence of Alzheimer's disease. Our data suggest that PCA patients with motor features have greater atrophy of contralateral sensorimotor areas but are still likely to have underlying Alzheimer's disease.
Collapse
Affiliation(s)
- Natalie S Ryan
- Dementia Research Centre, Department of Neurodegenerative Disease, University College London (UCL) Institute of Neurology, Queen Square, London, UK.
| | - Timothy J Shakespeare
- Dementia Research Centre, Department of Neurodegenerative Disease, University College London (UCL) Institute of Neurology, Queen Square, London, UK
| | - Manja Lehmann
- Dementia Research Centre, Department of Neurodegenerative Disease, University College London (UCL) Institute of Neurology, Queen Square, London, UK
| | - Shiva Keihaninejad
- Dementia Research Centre, Department of Neurodegenerative Disease, University College London (UCL) Institute of Neurology, Queen Square, London, UK
| | - Jennifer M Nicholas
- Dementia Research Centre, Department of Neurodegenerative Disease, University College London (UCL) Institute of Neurology, Queen Square, London, UK; Department of Medical Statistics, Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel St, London, UK
| | - Kelvin K Leung
- Dementia Research Centre, Department of Neurodegenerative Disease, University College London (UCL) Institute of Neurology, Queen Square, London, UK
| | - Nick C Fox
- Dementia Research Centre, Department of Neurodegenerative Disease, University College London (UCL) Institute of Neurology, Queen Square, London, UK
| | - Sebastian J Crutch
- Dementia Research Centre, Department of Neurodegenerative Disease, University College London (UCL) Institute of Neurology, Queen Square, London, UK
| |
Collapse
|
11
|
Affiliation(s)
- Randi Starrfelt
- a Department of Psychology , University of Copenhagen , Copenhagen , Denmark
| | | |
Collapse
|
12
|
Yong KXX, Shakespeare TJ, Cash D, Henley SMD, Warren JD, Crutch SJ. (Con)text-specific effects of visual dysfunction on reading in posterior cortical atrophy. Cortex 2014; 57:92-106. [PMID: 24841985 PMCID: PMC4194349 DOI: 10.1016/j.cortex.2014.03.010] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2013] [Revised: 02/14/2014] [Accepted: 03/28/2014] [Indexed: 11/13/2022]
Abstract
Reading deficits are a common early feature of the degenerative syndrome posterior cortical atrophy (PCA) but are poorly understood even at the single word level. The current study evaluated the reading accuracy and speed of 26 PCA patients, 17 typical Alzheimer's disease (tAD) patients and 14 healthy controls on a corpus of 192 single words in which the following perceptual properties were manipulated systematically: inter-letter spacing, font size, length, font type, case and confusability. PCA reading was significantly less accurate and slower than tAD patients and controls, with performance significantly adversely affected by increased letter spacing, size, length and font (cursive < non-cursive), and characterised by visual errors (69% of all error responses). By contrast, tAD and control accuracy rates were at or near ceiling, letter spacing was the only perceptual factor to influence reading speed in the same direction as controls, and, in contrast to PCA patients, control reading was faster for larger font sizes. The inverse size effect in PCA (less accurate reading of large than small font size print) was associated with lower grey matter volume in the right superior parietal lobule. Reading accuracy was associated with impairments of early visual (especially crowding), visuoperceptual and visuospatial processes. However, these deficits were not causally related to a universal impairment of reading as some patients showed preserved reading for small, unspaced words despite grave visual deficits. Rather, the impact of specific types of visual dysfunction on reading was found to be (con)text specific, being particularly evident for large, spaced, lengthy words. These findings improve the characterisation of dyslexia in PCA, shed light on the causative and associative factors, and provide clear direction for the development of reading aids and strategies to maximise and sustain reading ability in the early stages of disease.
Collapse
Affiliation(s)
- Keir X X Yong
- Dementia Research Centre, Department of Neurodegeneration, UCL Institute of Neurology, University College London, London, UK.
| | - Timothy J Shakespeare
- Dementia Research Centre, Department of Neurodegeneration, UCL Institute of Neurology, University College London, London, UK
| | - Dave Cash
- Dementia Research Centre, Department of Neurodegeneration, UCL Institute of Neurology, University College London, London, UK
| | - Susie M D Henley
- Dementia Research Centre, Department of Neurodegeneration, UCL Institute of Neurology, University College London, London, UK
| | - Jason D Warren
- Dementia Research Centre, Department of Neurodegeneration, UCL Institute of Neurology, University College London, London, UK
| | - Sebastian J Crutch
- Dementia Research Centre, Department of Neurodegeneration, UCL Institute of Neurology, University College London, London, UK
| |
Collapse
|
13
|
Woollams AM, Hoffman P, Roberts DJ, Lambon Ralph MA, Patterson KE. What lies beneath: a comparison of reading aloud in pure alexia and semantic dementia. Cogn Neuropsychol 2014; 31:461-81. [PMID: 24702272 PMCID: PMC4131257 DOI: 10.1080/02643294.2014.882300] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Exaggerated effects of word length upon reading-aloud performance define pure alexia, but have also been observed in semantic dementia. Some researchers have proposed a reading-specific account, whereby performance in these two disorders reflects the same cause: impaired orthographic processing. In contrast, according to the primary systems view of acquired reading disorders, pure alexia results from a basic visual processing deficit, whereas degraded semantic knowledge undermines reading performance in semantic dementia. To explore the source of reading deficits in these two disorders, we compared the reading performance of 10 pure alexic and 10 semantic dementia patients, matched in terms of overall severity of reading deficit. The results revealed comparable frequency effects on reading accuracy, but weaker effects of regularity in pure alexia than in semantic dementia. Analysis of error types revealed a higher rate of letter-based errors and a lower rate of regularization responses in pure alexia than in semantic dementia. Error responses were most often words in pure alexia but most often nonwords in semantic dementia. Although all patients made some letter substitution errors, these were characterized by visual similarity in pure alexia and phonological similarity in semantic dementia. Overall, the data indicate that the reading deficits in pure alexia and semantic dementia arise from impairments of visual processing and knowledge of word meaning, respectively. The locus and mechanisms of these impairments are placed within the context of current connectionist models of reading.
Collapse
Affiliation(s)
- Anna M Woollams
- a School of Psychological Sciences , University of Manchester , Manchester , UK
| | | | | | | | | |
Collapse
|
14
|
|
15
|
Starrfelt R, Nielsen S, Habekost T, Andersen TS. How low can you go: spatial frequency sensitivity in a patient with pure alexia. BRAIN AND LANGUAGE 2013; 126:188-192. [PMID: 23774289 DOI: 10.1016/j.bandl.2013.05.006] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/12/2013] [Accepted: 05/14/2013] [Indexed: 06/02/2023]
Abstract
Pure alexia is a selective deficit in reading, following lesions to the posterior left hemisphere. Writing and other language functions remain intact in these patients. Whether pure alexia is caused by a primary problem in visual perception is highly debated. A recent hypothesis suggests that a low level deficit - reduced sensitivity to particular spatial frequencies - is the underlying cause. We tested this hypothesis in a pure alexic patient (LK), using a sensitive psychophysical paradigm to examine her performance with simple patterns of different spatial frequency. We find that both in a detection and a classification task, LK's contrast sensitivity is comparable to normal controls for all spatial frequencies. Thus, reduced spatial frequency sensitivity does not constitute a general explanation for pure alexia, suggesting that the core deficit in this disorder is at a higher level in the visual processing stream.
Collapse
Affiliation(s)
- Randi Starrfelt
- Center for Visual Cognition, Department of Psychology, University of Copenhagen, Denmark.
| | | | | | | |
Collapse
|