After a six-hour test, the optometrist found that Nicholas also had poor visual function skills which made it difficult for his eyes to adjust focus when switching from faraway to near objects, and vice versa.

"He has perfect vision, with no myopia, farsightedness or 'lazy eye' problem," Chow says. "But children can suffer from poor visual function skills that can adversely affect their learning progress."

Visual function skills cover abilities such as being able to train the eye along a line of text, and adjusting focus while viewing objects at different distances. So a child may have perfectly clear vision, but still have difficulty keeping their sight on track.

"Good visual function skills are not innate," explains behavioural optometrist Sa Kai-bong. "When reading, it's necessary for children to run their vision along a line of text, and visually locate each word. Those with poor tracking skills will skip words, or rows [of words]."

Sa, who is assistant manager at the PolyVision Eyecare Centre, says a child's vision may be perfectly clear but still suffer from poor visual function skills.

"This makes comprehension difficult. In time, that child will lose interest in reading."

Parents are often unaware of the importance of visual function, which is why many children with such problems are misdiagnosed as being dyslexic, having poor learning motivation or even as ADHD, experts say.

Yet visual dysfunction is quite common. According to Dr Joel Warshowsky, associate clinical professor at the SUNY College of Optometry and author of , about 20 per cent of school-age children in the US have poor visual function skills and this may lead to behavioural problems, which typically surface when they enter third grade.

"It's the time when they are introduced to small type. Content requires greater focus [by the eyes]," Warshowsky says.

He advocates a range of exercises and techniques to correct problems with processing visual information. Symptoms that can result from overworking the eye muscles in an attempt to better focus the vision - blurred vision, headaches and loss of concentration, for example - are also observed in children suffering from attention deficit disorder, hyperactivity and dyslexia.

When children act up, or fail to respond in class because they have trouble tracking the text, it may be mistaken for indifference, says PolyVision's Sa.

"These children may be scolded by teachers for being slow in class. They may be seated at the back of the classroom.

"After looking at the blackboard, their view becomes briefly blurred when they switch to the textbooks in front of them. This makes it difficult for them to respond as quickly as other students do," Sa says.

"Binocular co-ordination is another important visual function skill. Both eyes need to work as a unit to visually locate a target.

"The brain receives the images from both eyes and combines them to form a single 3-D image.

"Those with poor binocular coordination might develop double vision or a squint. The excessive use of tablets among children also adversely affects the development of children's visual function skills," Sa adds.

To raise parental awareness of visual function skills, PolyVision recently launched a vision assessment programme for primary students. In addition to routine eye checks, each hour-long session will also test the child's visual function skills. Although most children eventually grow out of problems with visual function, Sa says failing to correct them during childhood, when they are most curious and eager to learn, puts youngsters at a disadvantage.

"A child with poor visual function will not have the same problem after he turns 20, even if he doesn't receive any therapy or help. This is because he will have developed the appropriate visual skills by then. But a critical period for learning would have already passed," Sa says.

"One way to develop children's visual function [skills] is to expose them to lots of different experiences. For example, a child playing football on a pitch will get the chance to make use of various visual functions. He has to follow the ball with his eyes, while scanning the whole pitch for his team mates."

Chow is glad her son was diagnosed early. Last year, she enrolled Nicholas in an eight-week remedial programme at PolyVision to boost his visual function and is delighted by the progress that her son has made.

"Sa teaches exercises that must be repeated at home," explains Chow. "One exercise involves him wearing a special pair of spectacles. I place a card filled with graphics close to him and then move it further away to help him adjust his focus. He has shown much improvement, and no longer misses words when reading."

Early correction of visual impairment makes children behave better, as they no longer find reading and focusing as stressful. "Once the cloud is lifted, they are happy and smiling," Warchowsky says.

Additional reporting by Abigail Collier