SIMULINK and GUIs - A Guide to MATLAB for Beginners and Experienced Users - page 135

Finally,ifyoucreateaGUIwithanAxesboxlikewedid,youmaynoticethat
GUIDEputsintheGUI'sM-fileatemplatelikeacallbacktemplatebutlabeled
“ButtondownFcn” instead. When the user clicks in an Axes object, this type
of function is called rather than a callback function, but within the template
you can write the function just as you would write a callback function. You
can also associate such a function with an object that already has a callback
function by clicking the right mouse button on the object in the Layout Editor
and selecting Edit ButtondownFcn . This function will be run when the
user clicks the right mouse button (as opposed to the left mouse button for the
callback function). You can associate functions withseveral other types of user
events as well; to learn more, see the online documentation, or experiment by
clickingtherightmousebuttononvariousobjectsandonthegridbehindthem
in the Layout Editor.