What is spiritual aspect of art in Buddhism?

It's an interesting question and I don't know an answer from a Buddhist source.

One insight may be that, I read that an fMRI of someone concentrating on a task (including a task like "art", I imagine) shows less brain activity than usual -- more specifically, activity in fewer regions of the brain.

So, you might think that concentrating on a task implies a lot of activity, but it's somewhat the opposite, maybe its "fewer distractions", i.e. when distractions have subsided.

I read that for an activity which requires concentration (i.e. writing software) the practitioner takes some time, many minutes, to get in the zone:

(idiomatic) In a mental state of focused concentration on the performance of an activity, in which one dissociates oneself from distracting or irrelevant aspects of one's environment

There's a description I liked in Zen and the Art of Motorcycle Maintenance (emphases in the following quote are mine):

Well, those were the commonest setbacks I can think of: out-of-sequence reassembly, intermittent failure and parts problems. But although setbacks are the commonest gumption traps they’re only the external cause of gumption loss. Time now to consider some of the internal gumption traps that operate at the same time.

As the course description of gumptionology indicated, this internal part of the field can be broken down into three main types of internal gumption traps: those that block affective understanding, called "value traps"; those that block cognitive understanding, called "truth traps"; and those that block psych*m*tor behavior, called "muscle traps." The value traps are by far the largest and the most dangerous group.

Of the value traps, the most widespread and pernicious is value rigidity. This is an inability to revalue what one sees because of commitment to previous values. In motorcycle maintenance, you must rediscover what you do as you go. Rigid values make this impossible.

The typical situation is that the motorcycle doesn’t work. The facts are there but you don’t see them. You’re looking right at them, but they don’t yet have enough value. This is what Phædrus was talking about. Quality, value, creates the subjects and objects of the world. The facts do not exist until value has created them. If your values are rigid you can’t really learn new facts.

This often shows up in premature diagnosis, when you’re sure you know what the trouble is, and then when it isn’t, you’re stuck. Then you’ve got to find some new clues, but before you can find them you’ve got to clear your head of old opinions. If you’re plagued with value rigidity you can fail to see the real answer even when it’s staring you right in the face because you can’t see the new answer’s importance.

The birth of a new fact is always a wonderful thing to experience. It’s dualistically called a "discovery" because of the presumption that it has an existence independent of anyone’s awareness of it. When it comes along, it always has, at first, a low value. Then, depending on the value-looseness of the observer and the potential quality of the fact, its value increases, either slowly or rapidly, or the value wanes and the fact disappears.

The overwhelming majority of facts, the sights and sounds that are around us every second and the relationships among them and everything in our memory...these have no Quality, in fact have a negative quality. If they were all present at once our consciousness would be so jammed with meaningless data we couldn’t think or act. So we preselect on the basis of Quality, or, to put it Phædrus’ way, the track of Quality preselects what data we’re going to be conscious of, and it makes this selection in such a way as to best harmonize what we are with what we are becoming.

What you have to do, if you get caught in this gumption trap of value rigidity, is slow down...you’re going to have to slow down anyway whether you want to or not...but slow down deliberately and go over ground that you’ve been over before to see if the things you thought were important were really important and to—well—just stare at the machine. There’s nothing wrong with that. Just live with it for a while. Watch it the way you watch a line when fishing and before long, as sure as you live, you’ll get a little nibble, a little fact asking in a timid, humble way if you’re interested in it. That’s the way the world keeps on happening. Be interested in it.

At first try to understand this new fact not so much in terms of your big problem as for its own sake. That problem may not be as big as you think it is. And that fact may not be as small as you think it is. It may not be the fact you want but at least you should be very sure of that before you send the fact away. Often before you send it away you will discover it has friends who are right next to it and are watching to see what your response is. Among the friends may be the exact fact you are looking for.

After a while you may find that the nibbles you get are more interesting than your original purpose of fixing the machine. When that happens you’ve reached a kind of point of arrival. Then you’re no longer strictly a motorcycle mechanic, you’re also a motorcycle scientist, and you’ve completely conquered the gumption trap of value rigidity.

Another is just that, I wonder why something like a garden or a rock garden is attractive. I don't know why this is so, but I find I like a clean and tidy home -- with the emphasis on "clean and tidy" rather than "home" because that's true of other places too -- like class-rooms, other people's homes, offices, public spaces, etc. (and presumably monasteries). It's hard to describe the experience, maybe aesthetic, maybe some temporary "peace of mind" at least relatively. I think you see that in some of the Zen stories: sweeping the floor, washing the bowl after eating, being tidy with them when you take your sandals off.

As for "doing" art I used to find with some activities -- physical exercise for example -- that even if I didn't always like "doing" it (the 'present' tense), I liked "having done" it (the 'perfect' tense). So I was reluctant to start the "work" sometimes, but did because I knew I liked the result of working. Maybe later I found myself enjoying the activity itself -- possibly when developing some mastery. I'm told there's a "sweet spot" for an activity to be enjoyable or engaging, including for children -- it should be neither too difficult ("impossible") nor too easy ("boring"). I suspect this is "relishing" of a type which classic Buddhism warns against as being addictive, but what can you do? Maybe check that the activity is relatively harmless at least or beneficial in some way.