Visual Constraints for Sign Language Communication

Production of the signals of any language must to some extent depend on the modality of that production. The vocal apparatus can produce only a limited range of all the sounds that a human ear can detect, and there is certainly a limit to the possible duration and sequencing of these sounds. Similarly, the production of language signals must be related to the perceptual system receiving them. A difference between two sounds must be a difference that the hearer can immediately, accurately, and automatically detect before that difference can be used to convey a distinction between two elements of a language. The production of language signals, therefore, must be constrained by the apparatus used to receive them. We would expect that sign languages of the deaf would evolve in such a way that their units would become less perceptually ambiguous. Since sign languages are received and initially processed by the visual system, we would expect that the rules for the formation of signs of a sign language would be constrained by the limits of the visual system. One of the important limits of human vision is that it is not equally acute in all parts of the field it takes in. When we focus on a point, we can see a great deal of detail in the area immediately surrounding that point, because the image of the point and area falls on the most