Thomas Sutcliffe: It's all right to be unmoved by art

How often do we feel about art what we're supposed to feel? I can only answer for myself and I would guess that it's something like two times out of a 100, which may sound a rather depressing strike rate but seems to me a quite bearable ratio - or, at least, a realistic one. I can easily imagine that I might boost the hit rate a little in conversation, so as not to sound quite so numb and unresponsive, but if I'm honest with myself, low single figures are closer to the truth.

And I don't mean by this that the other 98 encounters are worthless, because they're not. They can be full of interest or diversion, or even confirm a long-held prejudice, which is always satisfying. What I mean is that they don't result in that compulsive swoon of pleasure that a lot of writing about art suggests is the natural state of affairs when we get it on with an artwork, and that a lot of enthusiasts for culture like to imply they're getting on a regular basis. It's possible, of course, that I'm just culturally anorgasmic, but I don't think so. I think people quite often fake it, noisily pretending to a climax that they haven't actually felt.

Still, it's very reassuring when it happens. You know that the organs of appreciation are still operational, and the very rarity of the event gives it an added thrill. It happened to me last week while walking round the Scottish National Gallery of Modern Art's exhibition of Francis Bacon portraits. This was prime territory, it seemed to me, for the faked orgasm. Bacon is (still) in vogue, and now getting to that stage where it's clear that vogueishness has nothing to do with his reputation. So, you expect to encounter words such as "wonderful" and "dazzling" and "breathtaking" - with their familiar accompanying anxiety that your breath won't be taken and you won't be dazzled. And for one or two rooms, that's how it was. It's a fascinating show, thoughtfully laid out and full of arresting paintings, but it wasn't until I came to Bacon's Portrait of George Dyer in a Mirror that I felt that the art was doing the lifting rather than me.

It's not an entirely typical Bacon portrait, this one - isolating his lover in what looks like a circle of light and revealing his profile to one side, in a strangely angular mirror. But what caught my eye was a single long arcing brushstroke that Bacon has put above the figure. In spatial terms, it's ambiguous. Is it a glint of light on a curving transparent screen in front of the figure? Or does it mark the top edge of the concave curve behind him? Or is it simply there to balance the composition (which it does very effectively)? What's most thrilling about the line, though, is not its teasing indeterminacy (it may not even reproduce in a newspaper picture) but the way that it has been marked out in one swift sweep of the brush - a swing of the arm from one side of the canvas to the other. It was done with such briskness that the paint couldn't keep up. It has broken and thinned at various points, returning sharply at the edges of the mark - perhaps as Bacon rolled the brush in his hand and squeezed the paint to the edge of the bristles.

There is another striking paint mark on this canvas - an ejaculated gob of white paint that splats across the lower part of the figure, and seems to offer a rather literal example of Bacon's ambition that his paintings should leave "a trail of the human presence... as the snail leaves its slime". But, perhaps because of its slightly callow suggestiveness, it isn't a patch on the controlled curve above it, which is a real human trace, not a simulated one. There was something about it that was very exciting, and, for me at least, it suddenly clarified the rest of the exhibition, as if everything had popped into focus.

The distinct feeling in many Bacon portraits that the image of the person lies somewhere behind the paint - that a resemblance has been worked over (sometimes as a boxer will work over an opponent) - makes more sense if you think of the brushstrokes as physical gestures. They aren't just the record of a visual impression, or an attempt to replicate it on canvas. They're a record of Bacon imaginatively touching that face - sometimes tenderly and sometimes aggressively. What fantastic strokes they are, too - sometimes like a whiplash across a cheekbone, sometimes a gentle brush, as of fingertips smoothing out an eyebrow. And if you're in the mood, as I was, they are genuinely thrilling - an overworked word that, every now and then, is the only one that will do justice to a reaction that's not just about the brain.