Thomas Sutcliffe: These pictures can't scare me

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Walking round Tate Britain's Gothic Nightmares exhibition, I was reminded of two of the most potent images I've ever encountered. They aren't famous and I can't even give due credit to their creators, since I was only eight at the time that I encountered them and then not much given to seeking out illustrators' names. One of these images was a picture of a witch, her Bruce Forsyth chin adorned with large hairy warts and her skin the colour of ripe avocado. The other was a single frame from a monochrome strip cartoon, in which the hero fled across the moonlit lawn of a large country house, pursued by crocodile-headed figures.

Both these pictures, which were to be found in books owned by my best friend at the time, terrified me. Indeed I found the witch picture so unnerving that I wouldn't take any risks when I came close - leap-frogging over a whole wedge of pages to make sure that I didn't accidentally open the book at the wrong place. The other picture, for some reason, wasn't beyond contemplation in quite the same way, though whenever I did look at it the plight of the fleeing character was so vividly appalling to me that it gave me problems in getting to sleep.

There was nothing special about either image, of course - it was my naivety that rendered them so thrilling. And it didn't take long to grow out of that trembling susceptability to the "painted devil", the spectacular cowardice that Lady Macbeth scorns when she tells Macbeth that the "sleeping and the dead are but as pictures".

And yet, walking round Tate Britain's exhibition of Gothic and supernatural imaginings I couldn't help feeling some regret that none of these pictures really stirred anything but intellectual or aesthetic response. There was a time when they might have done far more. The star of the show, for example, is Henry Fuseli's famous painting The Nightmare, in which a squat incubus lodges on the torso of a sleeping woman. I think that this might have prompted me to skip a page or two when I was eight. It isn't just that the demon is ugly or that the shadow on the wall behind confirms the scary pointiness of his ears. It's that the eyes address you with such a furious sense of interruption - like some Millwall skinhead distracted from beating up a victim and deciding to transfer his maleficence to you. It is, in paint, the equivalent of the horror-movie cliché when someone makes a noise and the monster's head spins round to identify the source.

It's only a cliché for us, of course, and it's possible that the undoubted sensation caused when Fuseli's painting was first shown at the Royal Academy in 1782 owed at least some of its origin to straightforward terror. These days we have to go to the cinema to get this kick - a kind of neural bypass which dodges the cerebral cortex (the bit that knows that it's just moving shapes on a flat screen) and exploits our instinctive dreads. But the appetite for such thrills is unlikely to have been created by film, and must have subsisted in the past on a diet that we might now find insipid, having got used to stronger tastes. Indeed, if you go far enough back, it isn't difficult at all to find examples of a terror of images that goes beyond even my childhood dread of a watercolour witch. Some forms of iconoclasm have their origin not in sophisticated theology but in straightforward superstition about the potency of certain pictures.

Fear alone is unlikely to explain the vulgar popularity of Fuseli's picture, its ability to draw a crowd. It obviously had a lot to do with sex as well. Can you think of any other painting which presses down quite so impudently on a woman's body? The Nightmare is far more decorous in terms of exposure than some of the other sexualised images in the Tate Britain show, but it is by a long, abandoned stretch easily the most physical - since you can't see it without thinking about what it would be like to be crushed by that squatting figure - or, perhaps, to do the crushing.

And, while the image might look relatively tame in a culture that requires any halfway ambitious young actress to pose as a Hamburg hooker on the cover of lad's mags, it surely did things to its 18th century audience. These days, I suspect, we're too jaded, too dulled by sensation, too sophisticated to feel any such innocent excitement. The fear and arousal have evaporated, to be replaced by analysis and curiosity. It's a pity, really. Sometimes you want your innocence back, not because you value it in itself, but because it's so thrilling to have it taken away from you.