Reading is a complex function. It requires a proper retinal image that is received by the brain. There it is analyzed for letter and word recognition and then the semantics is detected. Reading speed slows down when letters are blurred, or do not have enough contrast or luminance or when binocular fusion is hampered. It also slows down when the understanding is difficult, which is why reading one's mother tongue generally is faster than reading a foreign language. Reading skill is therefore influenced by visuo-motor and cognitive (i.e. linguistic, personality, learning, ageing) factors. Reading ability is currently evaluated using various reading tests, which usually have long paragraphs with small print and shorter ones with larger print, to allow determination of reading acuity. To evaluate sustained reading performance, and reading speed, the International Reading Speed Texts (IReST) which contain longer paragraphs with continuous text of fixed size, have been developed. Although clinical reading tests are thoroughly standardized, a significant inter-individual variation in reading speed exists, as a result of the high influence of cognitive factors. When reading, sensorimotor coordination must happen to make a sequence of fixations and saccades so that new information falls onto the fovea. Eye movement efficiency develops almost in parallel with reading ability, in terms of speed and accuracy, as the child progresses from a beginner to a proficient adult reader. Less skilled readers (e.g. children with reading difficulties) typically make more fixations of longer duration and shorter saccades than skilled readers. For an adult reader mean fixation duration is around 220 to 250 ms. However, fixations of variable durations (from 100 ms to over 500 ms) are recorded during a reading session. Although the distribution of fixation durations varies among individuals, due to differences in reading skill, age and other cognitive factors, it is never normal; it always exhibits a pronounced right tail, i.e. an increased frequency of long fixations. Such distributions can be fitted with an ex-Gaussian function, a convolution of Gaussian (normal) and exponential functions, which could provide more efficient analysis. The team in university of Crete recently showed that pre-retinal factors (e.g. text contrast and luminance) influence the mean of the normal distribution, while cognitive factors affect the number of fixations and the mean of the exponential distribution of fixation duration, due to an increased frequency of long fixations. Moreover, inter-individual variation was significantly improved when analysis was performed separately for visuo-motor vs. cognitive components, with most of variability being attributed to the mean of the exponential distribution.

To evaluate functional vision in "simultaneous-image' correction and more specifically visual performance at near, a functional outcome, such as reading speed and ocular fixation times during sustained reading, should be measured. Such recordings could then answer the question how the binocular improvement in reading depends on the difference in acuity between the eyes and the analysis suggested above could help us to distinguish sensory vs. motor components especially in patients with reduced contrast sensitivity.