I assume that you use GUIDE. A callback function looks something like this (see below). handles is passed as an input argument. handles may be modified in the function. However, the modified version is lost when leaving the function - if not saved. MATLAB provides a special function guidata to save it, thus at the end of the function guidata shall be called. Next callback function that is called will get the modified handles.
function good_name_Callback( hObject, eventdata, handles )
% hObject handle to volume (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
handles.MyVar = MyVar;
% Save the modified variable, handles
guidata( hObject, handles ) %%<<<<<<<<<<<<<<<<<<<<<<<<<<<<
end
 
In respones to comment
There is nothing special with callback functions. The "magic" is in the value of the callback property (e.g. function handle), which invokes the callback function
@( hObject, eventdata ) ...
MyGui( 'Untitled_2_Callback', hObject, eventdata, guidata(hObject) )
When invoked Matlab supplies values to hObject (and eventdata).
 
"return the handles structure as an output of your function, and accept it in your calling function"   It's difficult to write documentation. There should have been a new paragraph. This does not apply to callback functions when invoked by callbacks of graphic handle objects. Of cource it is possible to call a callback function as any other function and of course local variable values are lost at return (with exception of property values of instancies of handle classes) if not explicitely passed as output arguments. (This text will hopefully be checked and commented on if unclear or wrong.)
