Cerebral visual impairment

Reviewed by Lotfi Merabet, OD, PhD, MPH

Cerebral (or cortical) visual impairment (CVI), the most common cause of visual impairment in pediatric patients in developed countries,¹ is the umbrella term for brain-based visual disorders that damage or cause maldevelopment of the retrochiasmal visual processing areas in patients without a major ocular disease.

At first glance, these patients can often appear to have normal or near-normal visual acuity levels. However, in reality, many have perceptual visual deficits, a cardinal feature of CVI, which are not typically tested during a standard ophthalmology office evaluation.

Until recently, clinicians could only guess what exactly the affected patients were seeing with any accuracy. Lotfi Merabet, OD, PhD, MPH, from the Laboratory for Visual Neuroplasticity, Department of Ophthalmology, Massachusetts Eye and Ear, Harvard Medical School in Boston, explained that anecdotal evidence provided a strong clue that suggested that children with CVI can have difficulties identifying objects that are presented as abstractions or cartoons. These are “representations of images that depart from naturalistic forms and shapes [and] lack key associative information provided by color. In contrast, individuals with CVI are more likely to correctly identify these same objects when presented as colored photographs,” Merabet said.

CVI collaboration: Leveraging the best of both worlds

Merabet described a recent study² conducted in collaboration with local teachers of the visually impaired; investigators from the Department of Psychology in the Translational Vision Lab at Northeastern University in Boston, Massachusetts; and investigators from the Unit of Child Neurology and Psychiatry at Azienda Socio Sanitaria Territoriale Spedali Civili of Brescia in Italy. This team’s members have done pioneering work to unravel the intricacies of CVI in children in the United States and Italy. The 2 countries employ different approaches investigating CVI, and the collaboration combines the strong points of both continents, according to Merabet.

The Europeans’ work is based on the medical model of CVI; children born with a brain injury are evaluated and diagnosed early at the hospital, followed through the national health care system, and remain under focus within that system. Conversely, in the United States, visual concerns in a child with CVI are often first noticed by parents and teachers of the visually impaired. At the same time, because universal diagnostic criteria are lacking, many children with CVI can fall through the cracks.

An interesting observation is that in the United Kingdom, the health care system has taken things a step further. There, the focus is on identifying and caring for at-risk mothers whose children may develop this neurodevelopmental disorder, effectively trying to cut the disease off at the knees. The US system tends to be more reactionary, in that once CVI is identified, the resources are then focused on affected patients, which helps explain the socioeconomic bias of health care in the United States. “Because investigations into CVI are in the early phases, the collaboration across the Atlantic is crucial,” Merabet said.

The pathway to CVI discovery

The buildup to the study was based on an observation provided by Matthew Tietjen, MEd, CTVI, a study coauthor and teacher of the visually impaired, who noticed a difference in how children with CVI perceived images with their classroom materials. Many of the illustrations were abstractions and cartoons, and he noticed that the children would often misname the objects.

“This is interesting from a scientific standpoint because of the need to understand why this happens, as well as having important educational repercussions. In many cases, children with CVI have to work harder than typically sighted children to get through school material,” Merabet said.

In one anecdote, Tietjen showed a child a picture of a cartoon elephant in frontal view. The child identified the elephant as a Sony PlayStation remote control, presumably mistaking the ears of the elephant for the handles of the remote control and the dark joysticks for the elephant’s eyes.

“As the pictures get further and further from ‘reality,’ it is harder and harder for children with CVI to understand what they are looking at. It has a lot to do with how information is taken into the brain, analyzed, processed, stored, and assigned meaning. The more abstract an image, the more tenuous the object identification,” Merabet said.

This observation by Tietjen resulted in his developing a more systematic, standardized approach to test his observation, with the goal of identifying the drivers of the identifications expressed by the children that cause the misidentifications. Based on an object-naming assessment developed by Tietjen, he and Merabet put together a series of images matched for size, perspective, complexity, and familiarity, called the 2-Dimensional Image Study.²

The investigators chose 12 images of common animate and inanimate objects, each represented from 5 possible image categories. An example taken from the article is the 5 categories of a cat (ie, a color photo, a realistic color drawing, a black-and-white realistic outline drawing, a color abstract drawing, and a black-and-white abstract outline drawing). These representations would disentangle color from form cues and use the color photo as the benchmark for identification, he explained.

The representations of the 12 objects in the 5 categories being evaluated were shown to children on a computer, who pressed the space bar when they named the objects. During this exercise, the investigators used an eye tracker to record where the children were looking from the moment the image appeared to when they identified (or misidentified) an object. Eye tracking was used to quantify gaze behavior, such as the extent of the visual search area explored and the number of fixations made.

“The takeaway is that the patients with CVI fared worse than controls on all 5 outcomes,” Merabet said.

Data analysis showed that the controls scored 100% on image identification. The children with CVI scored worse than controls on all outcomes. “As they moved from abstract to outline to color photo, they improved slightly and identified the images faster, the receiver operating curve trended upward, and the number of fixations decreased,” he said.

The visual search area did not have a clear pattern as a function of the type of image. However, the patients searched a much larger area compared with the controls. For children with CVI, when they erred in their identification of an image, they took more time to do so.

The children in the United States (n = 50) and Italy (n = 50) who participated in the study were not statistically different in terms of age, sex, and disease manifestations. However, Merabet suspects that the children from the United States tended to be from a higher socioeconomic bracket than those from Italy, where the health care system is more accessible.

Manley et al² reported that compared with controls, the children with CVI “showed significantly lower success rates and longer reaction times when identifying objects. In the CVI group, the success rate improved moving from abstract black-and-white images to color photographs, suggesting that object form [as defined by outlines and contours] and color are important cues for correct identification.”

Furthermore, the results obtained using eye tracking with the controls and patients in the study were markedly different, in that the controls exhibited tighter gaze patterns (ie, one fixation to the body and one to the head of the object). Conversely, in patients with CVI, the area of the gaze pattern is much larger and with a greater number of fixations, indicating that the children seem to explore a much larger area (Figure), particularly if the object was misidentified.

“Finally, the distribution of the eye gaze patterns in the CVI group was less aligned with the high saliency features of the image compared [with] controls,” Merabet said. He also noted the importance of having benchmarks to know which features in the images are important to the patients being able to correctly identify the pictures. The investigators used Graph-Based Visual Saliency (GBVS), a mathematical analysis that looks at an image and identifies the features in the image that can be considered the most salient from a “bottom-up” standpoint, such as color, luminance, and edges.

The investigators used the information (the predictive ability) provided by GBVS and combined it mathematically with the individual patient’s gaze pattern, yielding a mathematical comparison between a standard and a response that informs the investigators about how well the 2 agree. The study concluded that the results have important implications in helping to understand the complex profile of visual perceptual difficulties associated with CVI.

Considerations

Merabet pointed out that the exercises described underscored the importance of listening to the observations of the community, such as teachers and parents who often have tremendous insight into the visual difficulties of their students and children. The experience also showed that leveraging the international collaboration provided a larger sample size of patients, allowing more statistically robust findings. With larger study populations, there is also the potential to explore questions regarding differences in factors such as geographic location, disease manifestations, age, race, and socioeconomic differences between the groups.

The implications of these findings impact choices of educational materials used in the classroom. For children with CVI, abstractions and outlines may represent a greater cognitive burden and lost learning time. “For these children to thrive, we must design a world for them that considers their visual needs,” Merabet said.

A pragmatic approach to address CVI is to simplify the visual world for these children by using images with high relevance, which is close to the real world, and minimizing clutter. In addition, giving them more time to process images and checking on them to see whether they are identifying things properly are important. “There are nuances in terms of education that are extremely important. The most important takeaway is that you cannot take for granted that they are following a lesson and understand the content just because their peers are following,” Merabet said.

Finally, he advised that research in CVI should move forward as a community endeavor and strive to answer questions that can have an impact on the lives of these individuals. Without this effort and collaboration, he believes his team would not have achieved the level of research that they did.

The investigators are currently working on a study in which patients with CVI are presented with a real-world image of a lamp and then instructed to find it in a picture of a room. In a second version of the task, a text cue is used—the word lamp—and the participant must read it and find the object in a picture of a room. This analysis is ongoing, but early results suggest that the success rate in CVI is lower and the reaction time is longer to carry out this task. However, one interesting finding was that using a text cue slowed the patients even more compared with a visual cue.³

He pointed out that for these children with CVI, their verbal IQ score, which provides an index of an individual’s overall verbal intellectual abilities, was related to performance. The higher the verbal IQ, the greater their success rate and lower reaction time in identifying objects. This leads to the intriguing possibility that developmental outcomes such as verbal IQ may be important indices of how well an individual can perceive and interact with their complex visual surroundings.

The ultimate goal is to understand how individuals with CVI interact with their visual world and improve accessibility to educational materials for all students, regardless of visual abilities.

References

1. Solebo AL, Teoh L, Rahi J. Epidemiology of blindness in children. Arch Dis Child. 2017;102(9):853-857. doi:10.1136/archdischild-2016-310532

2. Manley CE, Walter K, Micheletti S, et al. Object identification in cerebral visual impairment characterized by gaze behavior and image saliency analysis. Brain Dev. 2023;45(8):432-444. doi:10.1016/j.braindev.2023.05.001

3. Manley CE, Walter K, Bex PJ, Merabet LB. Visual search patterns in cerebral visual impairment (CVI) are driven by saliency cues when exploring naturalistic scenes. Presented at: Vision Sciences Society Meeting 2023; May 19-24, 2023; St Pete Beach, FL. Abstract 4941.