You can toggle the default behavior of the 
Highlight and 
Isolate tools. When you select Add to Highlight or Add to Isolated Objects List from the tools’ menus, you can click the top half of the button to add the selected objects. When you select Highlight or Isolate from the menus, the top half of the button clears the existing emphasis and creates a new highlight or isolation for the selected objects.
Highlight and Isolate tools. When you select Add to Highlight or Add to Isolated Objects List from the tools’ menus, you can click the top half of the button to add the selected objects. When you select Highlight or Isolate from the menus, the top half of the button clears the existing emphasis and creates a new highlight or isolation for the selected objects.