If there was a problem in the film it lay in the ungraspable nature of its subject's dilemma; it was as difficult for us to get a hold on the man's disability as it was for him to pick up an egg without smashing it

Imagine you are blindfolded and led to the driving seat of a JCB excavator. Your task is to dig a trench but you have no idea which lever does what and no reliable way of checking the results of your experiments in motion. Then, just as you prepare for a bit of trial and error you are told that the machine is perched on the edge of a 60-foot drop. This slightly clumsy thought experiment is as close as I can get to the kind of sweaty, paralysing panic that must follow the loss of one's proprioception - in plainer terms, the almost unconscious sense of where your limbs are and where they must go if you want to achieve something. Last night's Horizon (BBC2) told the story of Ian Morton, a man who found himself in just such a predicament, robbed by a viral infection of the feedback which reassures us of where we stand in the world.

The film itself did not offer any such models for this weird state - the closest it came being to describe the sensation as similar to the difficulties sometimes experienced by astronauts after long periods in weightlessness - not exactly an illuminating analogy for most of us, even if it provided intriguing clues for those trying to understand what had gone wrong. And if there was a problem in the film it lay (appropriately, perhaps) in the ungraspable nature of its subject's dilemma; it was as difficult for us to get a hold on his disability as it was for him to pick up an egg without smashing it. The puzzlement was only compounded by the amazing forgery of easy, intuitive movement which he has laboriously pieced together over the years - he gestures naturally and though he walks with a slightly unusual gait, the fact that he can walk at all is still a source of amazement to his doctors. Rather than consigning such actions to some sub-processor in the brain he has to calculate the physics of every move and concentrate on its completion. Ian was a butcher when he fell ill - now he's turned himself into a virtuoso of the ordinary. You couldn't help wonder what he might have achieved if that fierce drive had been turned on skills the rest of us don't take for granted.

Stephen Bill's drama The Locksmith has been unsettling some viewers too - its blend of mishap comedy and darker trauma making it difficult ever to be complacent about your sense of balance. Last night's episode was the most remarkable yet. Trimmed of its very brief excursions into the outside world it might have been one of those intense chamber pieces so popular in the Seventies - a sustained and claustrophobic encounter, sweaty with rage and fear. Roland finally captures the man who attacked his wife and, after a long night of mutual incomprehension and imminent torture, the thief is accidentally killed by his daughter Alice. Not all of the discomfort here was intentional - some of the lines are rather stiffly explicit about Bill's themes or articulate in a way that feels like a hand on the scales ("I cry with anger, me. That's how angry I am" said the junkie thief at one point, restoring some moral equilibrium to the scene in a slightly clumsy way). The drama also skids into mawkishness now and then, perhaps because it is difficult to take the sudden swerves in empathy the plot demands. If you're laughing at a character one moment it is awkward, to say the least, to be asked to weep with him the next.

But four episodes in, it is becoming clear that the apparent non-sequiturs (such as the sub-plot about an old lady's betrayal by a bankrupt friend in episode two) are all aligned to the same central notion - the fraying of moral absolutes, of any sense of fixed responsibility (and retribution) for our deeds. And though the curdled blend of genre conventions makes the series less readily digestible at times, it's hard to think that is really a bad thing. We've already got more than enough drama on screen that is designed to soothe and slip down easily - spoon feeding for invalid imaginations.