New research published in Neuropsychologia provides evidence that adults with dyslexia process visual information differently than typical readers, even when viewing non-text objects. The findings suggest that the neural mechanisms responsible for distinguishing between specific items, such as individual faces or houses, are less active in the dyslexic brain. This implies that dyslexia may involve broader visual processing differences beyond the well-known difficulties with connecting sounds to language.
Dyslexia is a developmental condition characterized by significant challenges in learning to read and spell. These difficulties persist despite adequate intelligence, sensory abilities, and educational opportunities. The most prominent theory regarding the cause of dyslexia focuses on a phonological deficit. This theory posits that the primary struggle lies in processing the sounds of spoken language.
According to this view, the brain struggles to break words down into their component sounds. This makes mapping those sounds to written letters an arduous task. However, reading is also an intensely visual activity. The reader must rapidly identify complex, fine-grained visual patterns to distinguish one letter from another.
Some scientists suggest that the disorder may stem partly from a high-level visual dysfunction. This hypothesis proposes that the brain regions repurposed for reading are part of a larger system used to identify various visual objects. If this underlying visual system functions atypically, it could impede reading development.
Evidence for this visual hypothesis has been mixed in the past. Some studies show that people with dyslexia struggle with visual tasks unrelated to reading, while others find no such impairment. The authors of the current study aimed to resolve some of these inconsistencies. They sought to determine if neural processing differences exist even when behavioral performance appears normal.
“Developmental dyslexia is typically understood as a phonological disorder in that it occurs because of difficulties linking sounds to words. However, past findings have hinted that there can also be challenges with visual processing, especially for complex real-world stimuli like objects and faces. We wanted to test if these visual processing challenges in developmental dyslexia are linked to distinct neural processes in the brain,” said study author Brent Pitchford, a postdoctoral researcher at KU Leuven.
The researchers focused on how the brain identifies non-linguistic objects. They chose faces and houses as stimuli because these objects require the brain to process complex visual information without involving language. This allowed the team to isolate visual processing from phonological or verbal processing.
The study involved 62 adult participants. The sample consisted of 31 individuals with a history of dyslexia and 31 typical readers. The researchers ensured the groups were matched on key demographics, including age, gender, and general intelligence. All participants underwent vision screening to ensure normal visual acuity.
Participants engaged in a matching task while their brain activity was recorded. The researchers used electroencephalography (EEG), a method that detects electrical activity using a cap of electrodes placed on the scalp. This technique allows for the precise measurement of the timing of brain responses.
The researchers were specifically interested in two electrical signals, known as event-related potentials. The first signal is called the N170. It typically peaks around 170 milliseconds after a person sees an image. This component reflects the early stage of structural encoding, where the brain categorizes an object as a face or a building.
The second signal is called the N250. This potential peaks between 230 and 320 milliseconds. The N250 is associated with a later stage of processing. It reflects the brain’s effort to recognize a specific identity or “individuate” an object from others in the same category.
During the experiment, participants viewed pairs of images on a computer screen. A “sample” image appeared first, followed by a brief pause. A second “comparison” image then appeared. Participants had to decide if the second image depicted the same identity as the first.
“The study focused on within-category object discrimination (e.g., telling one house from another house) largely because reading involves visual words,” Pitchford told PsyPost. “It is often hard to study these visual processes because reading also involves other things like sound processing as well.”
The researchers also manipulated the visual quality of the images. Some trials used images containing all visual information. Other trials utilized images filtered to show only high spatial frequencies. High spatial frequencies convey fine details and edges, which are essential for distinguishing letters.
Remaining trials used images filtered to show only low spatial frequencies. These images convey global shapes and blurry forms but lack fine detail. This manipulation allowed the team to test if dyslexia involves specific deficits in processing fine details.
The behavioral results showed that both groups performed similarly on the task. Adults with dyslexia were generally as accurate and fast as typical readers when determining if two faces or houses were identical. There was a non-significant trend suggesting dyslexic readers were slightly less accurate with high-detail images.
Despite the comparable behavioral performance, the EEG data revealed distinct neural differences. The early brain response, the N170, was virtually identical for both groups. This suggests that the initial structural encoding of faces and objects is intact in dyslexia. The dyslexic brain appears to categorize objects just as quickly and effectively as the typical brain.
However, the later N250 response showed a significant divergence. The amplitude of the N250 was consistently reduced in the dyslexic group compared to the typical readers. This reduction indicates less neural activation during the process of identifying specific individuals.
“This effect was medium-to-large-sized, and robust when controlling for potential confounds such as ADHD, fatigue, and trial-to-trial priming,” Pitchford said. “Importantly, it appeared for both face and house stimuli, highlighting its generality across categories.”
The findings provide support for the high-level visual dysfunction hypothesis. They indicate that the neural machinery used to tell one object from another functions differently in dyslexia. This difference exists even when the individual successfully performs the task.
“Our results suggest that reading challenges in developmental dyslexia are likely due to a combination of factors, including some aspects of visual processing, and that developmental dyslexia is not solely due to challenges with phonological processing,” Pitchford explained. “We found neural differences related to how people with dyslexia discriminate between similar faces or objects, even though their behavior looked the same. This points to specific visual processes in the brain that may play a meaningful role in reading development and reading difficulties.”
The researchers propose that adults with dyslexia may use compensatory strategies to achieve normal behavioral performance. Their brains might rely on different neural pathways to recognize objects. This compensation allows them to function well in everyday visual tasks. However, this alternative processing route might be less efficient for the rapid, high-volume demands of reading.
“We expected to see lower accuracy on the visual discrimination tasks in dyslexia based on previous work,” Pitchford said. “Instead, accuracy was similar across groups, yet the neural responses differed. This suggests that adults with dyslexia may rely on different neural mechanisms to achieve comparable performance. Because these adults already have years of experience reading and recognizing faces and objects, it raises important questions about how these neural differences develop over time.”
One limitation of the study is the educational background of the participants. A significant portion of the dyslexic group held university degrees. These individuals likely developed robust compensatory mechanisms over the years. This high level of compensation might explain the lack of behavioral deficits.
It is possible that a sample with lower educational attainment would show clearer behavioral struggles with visual recognition. Additionally, the study was conducted on adults. It remains to be seen if these neural differences are present in children who are just learning to read.
Pitchford also noted that “these findings do not imply that phonological difficulties are unimportant in dyslexia. There is already extensive evidence supporting their crucial role. Rather, our study shows that visual factors contribute to dyslexia as well, and that dyslexia is unlikely to have a single cause. We see dyslexia as a multifactorial condition in which both phonological and visual factors play meaningful roles.”
Determining the timeline of these deficits is a necessary step for future research. Scientists need to establish whether these visual processing differences precede reading problems or result from a lifetime of different reading experiences. The researchers also suggest comparing these findings with other conditions. For instance, comparing dyslexic readers to individuals with prosopagnosia, or face blindness, could be illuminating.
“The next steps for this research are to test whether the neural differences we observed reflect general visual mechanisms or processes more specific to particular categories such as faces,” Pitchford explained. “To do this, we’ll apply the same paradigm to individuals with prosopagnosia, who have difficulties recognizing faces. We believe the comparison of results from the two groups will shed light on which visual processes contribute to dyslexia and prosopagnosia, both of which are traditionally thought to be due to challenges in specific domains (reading vs. face recognition).”
The study, “Distinct neural processing underlying visual face and object perception in dyslexia,” was authored by Brent Pitchford, Hélène Devillez, and Heida Maria Sigurdardottir.
