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New light shed on post-stroke reading problems

New understanding of left-sided dyslexia could help develop personalised interventions

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Three distinct mechanisms that underly reading errors in stroke survivors have been identified through research, paving the way for personalised interventions and support to be delivered. 

A new study has revealed the cognitive deficits contributing to reading difficulties in people with left-sided neglect dyslexia, a condition that often follows a right hemisphere stroke.

The findings reveal how reading accuracy can be differently affected based on the spatial position of a target word and distractor words in a person’s visual field, shedding light on potential ways to improve post-stroke reading difficulties caused by neglect dyslexia, and increasing potential for post-stroke rehab.

“This study is significant as it differentiates between the mechanisms of whole-word omissions and within-word letter errors, which can often be conflated in neglect dyslexia,” said Dr. Timothy J. Rich, research scientist in the Center for Stroke Rehabilitation Research at the Kessler Foundation. 

“Understanding these distinctions is crucial for developing more effective rehabilitation techniques that are tailored to the specific deficits exhibited by each patient.”

The study involved two separate experiments with ten stroke survivors experiencing left-sided neglect dyslexia, characterised by the omission of whole words on the left side of a page or the initial (left-sided) letters within individual words anywhere on the page. 

These experiments investigated the role of egocentric (viewer-centred) and allocentric (object-centred) spatial frames of reference in reading errors, to discern whether reading errors stemmed from the word’s position relative to the viewer or its position within a two-word frame.

Reading requires information processing of the fixated and the not-yet-fixated words to generate precise control of gaze. In eye movement research on reading, the word or words adjacent to the fixated word are the parafoveal word, while the fixated word is referred to as the foveal word. 

In the first experiment, participants read words presented in their parafoveal vision, with and without distractor words. The second experiment repeated the task with the target words presented in their foveal vision (directly in their line of sight). 

The findings reveal a clear distinction between the types of reading errors in neglect dyslexia. 

Whole-word errors were influenced by the word’s egocentric position, with a significant number of errors involving intrusions from the distractor word. However, this effect was eliminated when the target word was presented foveally, suggesting an egocentric spatial processing deficit. 

Conversely, unilateral paralexias (errors on the left-sided letters of words) remained consistent regardless of the word’s spatial position or the presence of distractors, indicating an allocentric processing deficit within the word itself.

Moreover, the study identified a third deficit – a failure of selective attention – which resulted in whole-word intrusion errors. 

In the presence of distractors, individuals often reported the distractor word instead of the target word, a compelling demonstration of this attentional deficit.

The results suggest that while whole-word errors are subject to spatial, distractor, and attentional effects, unilateral paralexias are not, indicating that different rehabilitative approaches are necessary to address each type of error.

“Given the importance of reading ability to recovery after stroke, there is a pressing need for further studies to develop and refine therapeutic interventions that consider the unique cognitive profiles of those affected by neglect dyslexia,” added Dr Rich. 

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