One of the most frequent defects in speech found among the illiterate or the defective is the occluded "s."
In making the normal "s" the tongue is pressed against the hard palate rather tightly, but a narrow groove is left in the middle so that a jet of air passes through. The method of palatography—a very thin black artificial palate covered with chalk is held in the mouth while the sound is being produced—gives a record like that in Figure 1A; the black surfaces indicate where the tongue presses against the hard palate. It is the jet of air through the narrow channel that produces the distinctive sound of "s."
A child with the occluded "s" apparently uses "t" and "d" instead of "s" and "z," saying "tun," "toap," "toup," instead of "sun," "soap," "soup"; or "dode." "Liddy," instead of "doze," "Lizzy."
A palatogram for the