1. At least one storage medium having instructions stored thereon for causing a system to perform a method comprising: processing a first user's pupil activity, using the at least one processor, to determine the first user's first cognitive load for first and second hierarchical elements of a hierarchical user interface (UI) model included on a first page that displays the first and second elements in a first spatial orientation to one another; processing the first user's pupil activity, using the at least one processor, to determine the first user's second cognitive load for the first and second elements of the first UI model included on a second page that displays the first and second elements in a second spatial orientation to one another that is unequal to the first orientation; storing the first user's first and second cognitive loads in at least one machine accessible medium coupled to the at least one processor; and determining a single aggregate cognitive load for one of the first and second elements of the first UI model based on both of the first user's first and second cognitive loads.