Drawing portraits

i have been teaching a disabled man for years and we have done abstract projects mostly. This term he attended the session where we had a model (usuallly we go into another room and he continues with abstract projects). one sketch did look like a portrait but mostly his sketches look like a circular outline with circular lines inside that. Apparently that is his view of the models head. i can show you all pictures. i just can't figure out this way of drawing? Can anyone help me understand? thank you so much