Randi Starrfelt

Center for Visual Cognition,
Department of Psychology,
University of Copenhagen,
Denmark

 

 

 

 

 

 

New article just accepted for publication in Brain & Language

HOW LOW CAN YOU GO:  SPATIAL FREQUENCY SENSITIVITY IN A PATIENT WITH PURE ALEXIA.

Randi Starrfelt1*, Simon Nielsen1,2, Thomas Habekost,1 and Tobias S. Andersen2

1 Center for Visual Cognition, Department of Psychology, University of Copenhagen, Denmark;    2Department of Informatics and Mathematical Modeling, Technical University of Denmark, Copenhagen, Denmark

 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, LKs 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.

 

Other selected references

Starrfelt, R. & Behrmann, M. (2011). Number reading in pure alexia-A review. Neuropsychologia, 49, 2283-2298.
Notes: It is commonly assumed that number reading can be intact in patients with pure alexia, and that this dissociation between letter/word recognition and number reading strongly constrains theories of visual word processing. A truly selective deficit in letter/word processing would strongly support the hypothesis that there is a specialized system or area dedicated to the processing of written words. To date, however, there has not been a systematic review of studies investigating number reading in pure alexia and so the status of this assumed dissociation is unclear. We review the literature on pure alexia from 1892 to 2010, and find no well-documented classical dissociation between intact number reading and impaired letter identification in a patient with pure alexia. A few studies report strong dissociations, with number reading less impaired than letter reading, but when we apply rigorous statistical criteria to evaluate these dissociations, the difference in performance across domains is not statistically significant. There is a trend in many cases of pure alexia, however, for number reading to be less affected than letter identification and word reading. We shed new light on this asymmetry by showing that, under conditions of brief exposure, normal participants are also better at identifying digits than letters. We suggest that the difference observed in some pure alexic patients may possibly reflect an amplification of this normal difference in the processing of letters and digits, and we relate this asymmetry to intrinsic differences between the two types of symbols
Center for Visual Cognition, Department of Psychology, Copenhagen University, O. Farimagsgade 2A, DK-1353 Copenhagen K, Denmark

Starrfelt, R., Habekost, T., & Gerlach, C. (2010). Visual processing in pure alexia: a case study. Cortex, 46, 242-255.
Notes: Whether pure alexia is a selective disorder that affects reading only, or if it reflects a more general visual disturbance, is highly debated. We have investigated the selectivity of visual deficits in a pure alexic patient (NN) using a combination of psychophysical measures, mathematical modelling and more standard experimental paradigms. NN's naming and categorization of line drawings were normal with regards to both errors and reaction times (RTs). Psychophysical experiments revealed that NN's recognition of single letters at fixation was clearly impaired, and recognition of single digits was also affected. His visual apprehension span was markedly reduced for letters and digits. His reduced visual processing capacity was also evident when reporting letters from words. In an object decision task with fragmented pictures, NN's performance was abnormal. Thus, even in a pure alexic patient with intact recognition of line drawings, we find evidence of a general visual deficit not selective to letters or words. This finding is important because it raises the possibility that other pure alexics might have similar non-selective impairments when tested thoroughly. We argue that the general visual deficit in NN can be accounted for in terms of inefficient build-up of sensory representations, and that this low level deficit can explain the pattern of spared and impaired abilities in this patient
Department of Psychology, Center for Visual Cognition, University of Copenhagen, Denmark. randi.starrfelt@psy.ku.dk

Starrfelt, R. (2009). Aleksi. In A.Gade, C. Gerlach, R. Starrfelt, & P. M. Pedersen (Eds.), Klinisk neuropsykologi ( pp. 45-53). Copenhagen: Frydenlund.

Starrfelt, R., Habekost, T., & Leff, A. P. (2009). Too little, too late: reduced visual span and speed characterize pure alexia. Cerebral Cortex, 19, 2880-2890.
Notes: Whether normal word reading includes a stage of visual processing selectively dedicated to word or letter recognition is highly debated. Characterizing pure alexia, a seemingly selective disorder of reading, has been central to this debate. Two main theories claim either that 1) Pure alexia is caused by damage to a reading specific brain region in the left fusiform gyrus or 2) Pure alexia results from a general visual impairment that may particularly affect simultaneous processing of multiple items. We tested these competing theories in 4 patients with pure alexia using sensitive psychophysical measures and mathematical modeling. Recognition of single letters and digits in the central visual field was impaired in all patients. Visual apprehension span was also reduced for both letters and digits in all patients. The only cortical region lesioned across all 4 patients was the left fusiform gyrus, indicating that this region subserves a function broader than letter or word identification. We suggest that a seemingly pure disorder of reading can arise due to a general reduction of visual speed and span, and explain why this has a disproportionate impact on word reading while recognition of other visual stimuli are less obviously affected
Department of Psychology, Center for Visual Cognition, Copenhagen University, DK-1361 Copenhagen, Denmark. randi.starrfelt@psy.ku.dk

Starrfelt, R. (2007). Selective alexia and agraphia sparing numbers-a case study. Brain and Language, 102, 52-63.
Notes: Department of Psychology, University of Copenhagen, Linnesgade 22, DK-1361, Copenhagen, Denmark. randi.starrfelt@psy.ku.dk
We report a patient (MT) with a highly specific alexia affecting the identification of letters and words but not numbers. He shows a corresponding deficit in writing: his letter writing is impaired while number writing and written calculation is spared. He has no aphasia, no visuo-perceptual or -constructional difficulties, or other cognitive deficits. A similar pattern of performance has to our knowledge only been reported once before [Anderson, S. W., Damasio, A. R., & Damasio, H. (1990). Troubled letters but not numbers. Domain specific cognitive impairments following focal damage in frontal cortex. Brain, 113, 749-766]. This study shows that letter and number reading are dependent on dissociable processes. More interestingly, it points to a common mechanism subserving the perception and production of letters. We suggest that a deficit in a visuo-motor network containing knowledge of the physical shape of letters might explain the pattern of performance displayed by MT

Starrfelt, R. & Gerlach, C. (2007). The visual what for area: words and pictures in the left fusiform gyrus. NeuroImage, 35,  334-342.
Notes: Center for Visual Cognition, Department of Psychology, University of Copenhagen, Linnesgade 22, DK-1361 Copenhagen K, Denmark. randi.starrfelt@psy.ku.dk <randi.starrfelt@psy.ku.dk>
An area in the left fusiform gyrus labelled the Visual Word Form Area (VWFA) is claimed to be especially, or even selectively, responsive to words. We explored how stimulus type and task demands affect activity in this area by conducting a PET experiment where words and pictures were presented in two conditions that differed in demands on shape processing: colour decision and categorization. The subjects also performed an object decision task with pictures only. The imaging data revealed a main effect of stimulus type: rCBF was higher during word compared with picture processing. When compared individually for colour decision and categorization, the difference between words and pictures was only significant during colour decision, although a trend was present during categorization also. rCBF in the VWFA was highest during the object decision task, where only pictures were presented. Our findings indicate that the putative VWFA is activated more by written words than pictures, but only under certain circumstances. As demands on shape processing increase, the difference in activation between words and pictures decreases and can even be abolished. We suggest that activation in the VWFA could reflect shape configuration-the integration of shape elements into elaborate shape descriptions corresponding to whole objects or words. This process may be required to different degrees for pictures and words depending on task demands

Habekost, T. & Starrfelt, R. (2006). Alexia and quadrant-amblyopia: reading disability after a minor visual field deficit. Neuropsychologia, 44, 2465-2476.
Notes: Center for Visual Cognition, Department of Psychology, University of Copenhagen, Linnesgade 22, 1361 Copenhagen K, Denmark. Thomas.Habekost@psy.ku.dk
Reading difficulties caused by hemianopia are well described. We present a study of alexia in a patient (NT) with a milder visual field deficit. The patient had suffered a cerebral haemorrhage causing damage to the left occipital cortex and underlying white matter. NT's text reading was slow and prone to error, but recognition of single letters was preserved. Single word reading was accurate, but slower than normal. On perimetric testing NT initially showed an upper right quadrantanopia, but by attending covertly to this quadrant he could achieve luminance detection except in a small scotoma above the reading line. A whole report experiment showed that letter perception was severely compromised in the quadrant, consistent with cerebral amblyopia. On follow-up testing one and a half year post stroke, a clear spontaneous recovery had occurred, reflected in improved text reading with close to normal eye movements. Still, subtle reading difficulties and oculo-motor abnormalities remained. Overall, the study shows how amblyopia in one quadrant can lead to a characteristic form of alexia

 

 

Anders Gade