by Jason D. Yeatman, Ph.D.
The fact that reading depends on the integrity of the visual system does not imply that visual impairments are a common cause of reading difficulties. Indeed, reading might start in the retina and the visual cortex, but successful decoding of even a single written word requires the participation of multiple brain systems including regions of the brain that are specialized for processing language, spatial attention, programming eye movements, and more.
From a neuroscience perspective, reading requires signals to be rapidly communicated between regions of the cortex that are specialized for processing visual, auditory, and language information. An impairment in any one of these systems, or the bundles of wires (axons) that connect them, could cause reading difficulty (Wandell, Rauschecker, & Yeatman, 2011). A comprehensive understanding of reading should build on an understanding of the visual system, which represents one potential bottleneck: If the visual system fails to rapidly and accurately recognize small printed symbols (i.e., letters) then any effort to teach reading skills with conventional approaches will have limited success.
A major achievement in the scientific study of reading has been to demonstrate the central role of phonemic awareness, or the ability to attend to and manipulate speech sounds (phonemes), in the development of skilled reading (Bradley & Bryant, 1983; Wagner & Torgesen, 1987). Impaired phonemic awareness is the most common precursor to dyslexia, and interventions that target phonemic awareness skills have been shown to be the most effective treatment for the average child with dyslexia (Vellutino, Fletcher, Snowling, & Scanlon, 2004).
The importance of phonemic awareness for learning to read, the central role of phonemic awareness impairments in developmental dyslexia, and the success of interventions targeting phonemic awareness skills, have prompted many scientists to minimize the role of the visual system in reading and the role of visual impairments in dyslexia. However, an array of scientific evidence supports the view that many children and adults with dyslexia can have difficulties with a range of visual functions, ranging from the ability to perceive a moving stimulus to the ability to ignore distracting information and attend to pertinent information in a visual scene.
Whether these visual system impairments are typically the underlying cause of reading difficulties, or the downstream result of other non-visual impairments, has been fiercely debated for decades. This debate is unlikely to be resolved anytime soon: Complex, developmental disorders like dyslexia are rarely explained by a single causal factor. The following sections are intended to, first, give a few examples of visual functions that people with dyslexia tend to struggle with and, second, clarify the implications of these findings for treating dyslexia.
A Few Observations about the Visual System in People with Dyslexia
Motion Processing—Disregarding the debate about causality for the moment, several observations have been made about the visual system in children and adults with dyslexia. One of the most extensively replicated findings is that, on average, children and adults with dyslexia show a decreased sensitivity to visual motion (Ben-Shachar, Dougherty, Deutsch, & Wandell, 2007; Demb, Boynton, Best, & Heeger, 1998; Stein & Walsh, 1997).
This effect can be observed in behavioral measurements that ask observers to compare the speed of motion between two different, sequentially presented computer displays. When the speed of motion in the two displays is similar, children and adults with dyslexia experience more difficulty identifying the faster moving visual stimulus than do children and adults with typical reading skills.
This effect is also apparent in brain measurements of a particular region of the visual system that is involved in the perception of motion. Neural responses in this region of cortex show reduced sensitivity to moving stimuli in some children and adults with dyslexia (Demb, Boynton, & Heeger, 1997; Eden et al., 1996). Examining measurements of individual subjects, rather than the average measurements of groups of subjects, reveals that not all people with dyslexia present this impairment in “motion discrimination,” but many do show a pronounced and robust difficulty with this task.
One might ask what motion discrimination has to do with reading. After all, printed text is a static image. Many explanations have been suggested, but a particularly noteworthy study tackled this question by measuring cortical responses to motion in children with dyslexia who were undergoing an intensive, language-based, reading intervention program (Olulade, Napoliello, & Eden, 2013).
They found that the training program induced significant improvements in measures of phonemic awareness and reading skills and also increased cortical responses to motion. This finding is important for two reasons:
- It demonstrates that differences in motion processing can result from differences in reading experience.
- It underscores the fact that, to successfully recognize a word, signals must be rapidly communicated through a network of visual regions before being sent to language regions. Thus, learning to read can affect regions within this network that might not appear to be uniquely related to the task of reading.
While these findings are a clear demonstration that practice with reading can induce changes in the brain’s visual circuitry, there is also evidence showing that differences in visual motion processing precede learning to read. For example, Boets and colleagues demonstrated that measurements of motion perception in pre-readers (age 5) predict future reading development (Boets, Vandermosten, Cornelissen, Wouters, & Ghesquière, 2011).
Together, these findings suggest that there is not a simple causal story: On the one hand, learning to read causes changes in how the visual system processes motion (Olulade et al., 2013), but on the other hand, motion processing deficits are present in pre-reading children who may eventually be diagnosed with dyslexia (Boets et al., 2011). Motion processing deficits will not explain every child’s struggle with reading, and there is currently no evidence showing that motion perception training improves reading.
As scientists, it is our job to understand how motion processing deficits might emerge as (a) a symptom of limited reading experience, (b) an indicator of an inefficiency in the visual system, or (c) a factor that contributes to a difficulty in coordinating the network of visual and language processing regions that must flexibly collaborate to recognize a printed word.
Visuospatial Attention—Many children and adults with dyslexia struggle with the allocation of attention to different objects in a visual field (Vidyasagar & Pammer, 2010). The link between “visuospatial attention” and reading is more straightforward than the link with motion processing. In learning to decode, children must be able to sequentially allocate attention to neighboring letters in a word as they learn to associate sounds (phonemes) with the printed symbols (letters). To become skilled readers, children must be able to focus on individual words within a cluttered page of text, and after a word has been identified, they must rapidly shift their gaze to fixate on the next word in the line. These switches in visuospatial attention must occur rapidly and accurately to support the fluent reading of a page of text.
Tasks that measure visuospatial attention skills take many different forms, such as searching through a cluttered array of images for a particular object, or identifying an object that was briefly presented in peripheral vision. Importantly, these tasks do not involve reading words or recognizing letters; instead, they tap into general visual functions that are used for selecting pertinent information from a complex visual scene.
The scientific literature is clear that, on average, children and adults with dyslexia do not efficiently shift their attention to different locations in the visual field and ignore distracting objects. The hypothesis has been proposed that these attention mechanisms are important for the ability to serially scan through letters in a word, and play an important role in the mapping of print to sound (Vidyasagar & Pammer, 2010).
While more research is needed to fully characterize the role that visuospatial attention plays in learning to read, there are two important findings that are worth highlighting:
- When performing visuospatial attention tasks, people with dyslexia have shown deficits that are just as pronounced (large effect size) as when performing on some linguistic measures, such as verbal short-term memory (Roach & Hogben, 2007).
- In a longitudinal study involving pre-reading children, it was shown that measures of visuospatial attention skills predict future reading skill (Franceschini, Gori, Ruffino, Pedrolli, & Facoetti, 2012). Thus, even though the process of learning to read affects visual processing, differences are already present before children begin reading instruction.
What Do These Visual Processing Deficits Tell Us about Treating Dyslexia?
I have reviewed evidence indicating that at least some children and adults with dyslexia are impaired when performing specific visual tasks. What do these findings tell us about treating dyslexia? Well, at this point in time, not much.
There are concerted efforts in many labs to identify specific manipulations of visual displays (e.g., the spacing between letters in words, different fonts), and specific training programs, that might improve visual functions in a manner that transfers to improved reading skills. But it is too early to make a decisive recommendation on the efficacy of these programs for any particular individual with dyslexia. More often than not, theoretically motivated interventions targeting specific visual functions have no effect on reading skills (Handler & Fierson, 2011).
Ongoing work in my lab and others is aimed at identifying the characteristics of individuals that make them particularly well suited for different approaches to intervention. When we use new brain imaging measurements to non-invasively monitor patterns of brain development in children learning to read, we find that, even among children of the same age, there is considerable variation in the development of visual and language processing circuits. Moreover, the development of brain connections that carry visual information, and the development of connections that carry language information, both contribute in important ways to the development of reading skills (Wandell & Yeatman, 2013; Yeatman, Dougherty, Ben-Shachar, & Wandell, 2012). This work, combined with research aimed at optimizing different approaches to intervention, is likely to lead to substantial improvements in the treatment options that are available to children with dyslexia.
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Jason D. Yeatman, Ph.D., is an Assistant Professor in the Department of Speech & Hearing Sciences and Institute for Learning & Brain Sciences at the University of Washington. His research sits at the intersection of neuroscience and education, capitalizing on new brain imaging technologies to study the biological mechanisms that underlie learning to read.
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