Finding common mechanisms behind the crowding effect
4 July 2024
We funded research examining if crowding effects in children with amblyopia (lazy eye) share mechanisms with adults’ peripheral vision crowding, revealing visual processing insights.
Visual crowding refers to the disruptive effect of clutter on object recognition.
While it is most prominent in adult peripheral vision, crowding also affects foveal vision (the central part of vision used to look at words or objects) in typically developing children and individuals with strabismic amblyopia.
More on visual crowding
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Visual crowding refers to the disruptive effect of clutter on object recognition. Crowding is a normally occurring visual effect but we do not know why it happens.
Example of the crowding effect is illustrated below. When focussing on the central cross below, it should be easy to identify the isolated‘backwards-C’ in your peripheral vision (left), and difficult to identify the same letter flanked to either side by distractor/flanker letters (right, where the target is the middle element).
The occurrence of crowding in adult peripheral vision and an amblyopic foveal vision raises the questions: do these crowding effects share the same underlying mechanism?
The study
This study, carried out by UCL experimental psychologist, Dr John Greenwood and his team, investigated crowding errors in peripheral and foveal vision across three groups:
- Typical adults
- Children aged 3 to 8 years with typical vision
- Children with strabismic amblyopia
What is amblyopia?
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A lazy eye, also known by its medical term amblyopia, is a vision condition that occurs in childhood. A lazy eye in children can develop between birth and the age of seven years old, when one eye is used less than the other. As a result, the affected eye is not able to build a strong link with the brain and will have reduced vision.
Typically, amblyopia only affects one eye, although in rare instances, both eyes may be affected due to a strong glasses prescription.
What are the causes of amblyopia?
A lazy eye occurs when the connection between the brain and the retina in the eye does not develop properly in a child’s early years.
This can be a result of other conditions, such as a squint, obstacles in the vision field such as a droopy eyelid or cataract, or differences in prescription between the eyes.
As a result the eye may receive a reduced amount of light, experience a lack of focus or see differences between images in the eyes. As the affected eye receives fewer visual signals, the vision deteriorates and the child becomes more dependent on their stronger eye. If left untreated, central vision in the affected eye may never develop properly.
The different types of amblyopia
There are different types of amblyopia, including refractive, deprivation and due to strabismus (squint).
Strabismic amblyopia is the most common. It is caused by an imbalance in the eye muscles that results in them crossing and being unable to work in sync with one another. This means that the eyes do not point in the same direction, which leads to vision problems.
The research team measured the visual acuity (the ability to see fine detail) in every participant to determine the minimum target size at which each observer could judge the orientation of the target element. This is to control for the known reduction in acuity in amblyopia.
They also measured crowded acuity to determine the extent of the area of crowding around the target location in the fovea.
1 in 50 children
are affected by amblyopia (lazy eye), based on current estimates
Each participant then performed an orientation-matching task to measure the perceptual effects of crowding.
Observers were asked to adjust a reference stimulus to match the perceived orientation of a target element. Crowding effects were measured with flankers positioned at different angles around the target.
Systematic crowding effects
The results showed striking similarities in how crowding manifests across the different groups. In adults’ peripheral vision, crowded errors followed a systematic pattern:
- Observers were both accurate and precise when reporting the orientation of isolated targets without clutter.
- Small target-flanker differences in orientation led to assimilation errors, where observers indicated intermediate orientations between the target and flankers.
- Large target-flanker differences in orientation led to substitution errors, a mixture of responses near either the target or flanker orientations.
Interestingly, the crowding errors seen in the foveal vision of children, both typically developing and those with amblyopia, followed the same systematic assimilation and substitution patterns.
A unified explanation
The researchers successfully modelled these effects using a population pooling model of crowding.
This model suggests that the systematic disruptions to vision from crowding are caused by an excessive integration of objects by visual system.
This means that the visual system perceives and combines multiple visual elements into one picture, making it difficult to distinguish between individual objects accurately.
The coherence between the model predictions and empirical data across all three groups suggests a common underlying mechanism governing crowding in the peripheral vision of adults, the foveal vision of children and those with amblyopia.
Dr John Greenwood, experimental psychologist at UCL
Based on this common mechanism, the researchers can make important predictions for a diverse range of conditions where elevated crowding has been observed, including posterior cortical atrophy, dyslexia and infantile nystagmus.
The model can help determine which brain processes contribute to these perception and crowding errors.
