Reading is a complex process that requires the reader’s brain to develop a multitude of neural pathways. The more developed those pathways, the greater the chances the reader will be able to automatize the act of reading. As the many pathways become fully developed, the reader becomes a fluent reader, and comprehension is possible.
A reader has to transfer the visual information of print into phonological information that is familiar to him from the auditory language that he hears daily in order to make meaning out of the print that he is processing. But a reader who has learned sign language, which is spatially visual, has the advantage of having developed neural pathways that connect the visual with language, or meaning. In other words, the baby who has learned sign language has a jumpstart on his ability to transfer visual information into meaningful linguistic information, relevant and critical to reading. He already understands that language or visual symbols convey meaning.
A child whose only exposure to language is auditory begins his development of the neural pathways at a much later age. The child who only knows auditory language must learn to take visual information (print) and make those visual symbols meaningful.