Vision is there every step of the way when you learn and process information. If vision does not develop efficiently, even a bright child can have difficulties with reading, writing, spelling and math. 

Getman, a renowned behavioral optometrist, stated,"Vision is the dominant mode in the development of intelligence". The more efficient a person is, the higher their score on many intelligence tests. This is why IQ scores can change after vision remediation. We gain the ability to understand and interpret what we see correctly and efficiently as we integrate and utilize the information we gather, learn from it and modify this information by experience. If one or more of these skills are deficient, then the signal through the visual pathway may not be as clear as it could be. This can cause difficulty with learning or performing various tasks, such as reading.
Since 75-90% of classroom learning comes through the visual system, poor visual skills can affect a child's performance. Why?

It takes more energy to use a faulty visual system than it does an efficient one. One out of four children has a vision problem which interferes with their ability to learn efficiently and achieve in school.

It has been estimated that 1 out of 4 children in the U.S. have learning problems. This is roughly 2-7 million children struggling to achieve in school. 25% of ALL children have a vision problem significant enough to affect their performance in school. According to research on just learning disabled populations, the number of kids with significant learning related vision problems soared closer to 85% in their studies. 75% of those identified as learning disabled have their biggest deficit in reading. Out of those children who are reading disabled, 80% of them have difficulties with one or more basic visual skills. Fortunately, these visual deficits can be treated successfully.