Aesthetic-Usability Effect

I've started reading a very good book, Measuring the User Experience, by Tom Tullis and Bill Albert, that discussed methodologies for, well, measuring the user experience.  The book covers  a fascinating range of topics including ways to measure facial expressions, track eye movements, puillary dilation, etc.  Yes, all this can actually be done at this point.

They also cover much more prosaic approaches like testing whether someone successfully complete a predefined task.

What strikes me about this is that their definition of "user experience" is so obviously a superset of both "usability" and common definitions of "visual appeal". So, this has got me thinking... is there really a meaningful distinction between usability and aesthetics at least with regard to software and web design? 

In my first blog post I sited a 2009 study by Christine Phillips & Barbara S. Chaparro that looked at the impact of visual design on perceptions of usability. There are a number of blog posts and articles that explore the interrelationship between aesthetics and usability but the most widely read and one of the earliest was Mark Boulton's 2005 piece: Aesthetic-Usability Effect.

As the first sentence in Mark's article states:

The Aesthetic-Usability Effect is a condition whereby users perceive more aesthetically pleasing designs to be easier to use than less aesthetically pleasing designs. 
My problem is that the definition of aesthetic is this:

pertaining to, involving, or concerned with beauty, emotion or sensation as opposed to pure intellectuality

So, the discussion is already ridiculously muddied by these definitions since to say something is concerned with sensation is a meaningless statement. What isn't concerned with sensation? You sense your brakes have failed or that the car has just crashed into a telephone pole. Ok, so toss out that part and let's focus on the other parts of the definition.

Even the other parts are problematic because they are conclusive statements. Using the above definition, we most definitely cannot equate the terms "visual appeal" and "aesthetic appeal" since we know that aspects of visual appeal (i.e. font or color) can impact usability and the definition of aesthetic relates only to beauty. 

Until you do some testing, you really have no idea if the changes are "purely aesthetic" in nature. The choice of font or color can obviously impact the usability in the extreme.

I think many visual or sensual elements of a product that have real "usability" implications are misunderstood as being "purely" aesthetic in nature or in their appeal. For example, I think it would be difficult to find any aspect of the design of Apple products like the iPod, iPhone or iPad that were purely "aesthetic" in nature, according to this strict definition.

So why is there a debate at all? 

I think it's because determining the usability impact of many design choices or various visual elements can be time consuming and tedious and involve regression analyses and so forth. Why is it so time consuming and tedious? Because cognition is largely a black box and optimizing design often involves a lot of trial and error. In addition, the available options are usually not well defined and most people don't have a very good vocabulary for talking about them.