With some aphasias, yes and no responses may be unreliable. The person may nod negatively while verbalizing 'yes' or 'yeah' or 'okay.'
To train differentiation of the affirmative from the negative, show 4 pictured objects (e.g. hammer, saw, screwdriver, scissors) and tell the person "I am going to ask you a set of questions all of which you will answer with a "yes" and a head nod." Demonstrate this. Then point to 1 (i.e. hammer) of the 4 pictured objects, asking, "Is this a hammer?" The patient should say "yes." If so, verbally indicate that this is right. If not, then demonstrate the appropriate response. Continue.
After "yes" is established, switch to "no." Give the same instruction. "I am going to ask you a set of questions all of which you will answer with a "no" and a head nod." Demonstrate this. Then, take the same 4 pictures, point to the hammer as before, but now ask, "Is this a screwdriver?" If the person answers "no" praise and verbally indicate that is correct. If the person says "yes" then demonstrate the proper response. Continue.
Track percentage correct and establish 100% differentiation between yes and no responses with this sort of questioning.
Move to personal information or immediate situation yes no questions once you have an established differentiation.