Hiya Tess -
thank you for a compelling post. My form of OCD relates to what you describe about checking and seeing whether something's right. It's a bit like inserting a coin in a coffee machine, only to get it returned numerous times - and only at the 10th or 15th attempt, it 'drops' and I get my coffee, finally. What we do know is that people with OCD may perform less well in so called 'visuo-spatial tasks', which are presented to them in the field of neuropsychology, usually. These tasks are often about perception and decision-making. Also, the research group of Garibotto et al (2010) found striking alterations in white brain matter of patients (also called myelin, it isolates nerve tracts so that signaling is much faster and consumes much less energy). The white matter in their experiment was vital in connecting frontal brain parts to areas that are important in visual interpretation.
I am fascinated by your description of the 'touching' problem. At least it seems certain that there is a strong analogy. Where a checker/starer must repeat his control measure numerous times before the coin drops right, a checker/toucher has to have tactile contact many times before that feeling of 'it is finally right, gasp!' is there. At the moment I wonder if and how the somatosensory part of our brain is involved, i.e. were various body parts are represented, so also our hands and their tactile impressions. I find this highly intriguing, and will do specific literature research on this.
Now, why can you do gardening without those compulsions? My bet, and I am talking from my particular experience: OCD can be seen as evolutionary advantages grown out of all proportion, so that they've become disadvantages. Checking is good, it serves to protect your own territory, you make sure that nothing can go wrong when you're away, and that no one can enter your house. Hoarding can be interpreted as saving things for times of need and want (like animals do with their foodstuffs). Washing means: hygiene. Ordering and symmetry can be important to quickly find materials back when you need them in emergencies. Religious and sexual obsessions may be looked at as normal societal forms of shame, but gone hopelessly out of control.
Sometime ago I wrote: we are perfectly normal, abnormally so. I hope it's clear what I meant by that...
I had the same phenomenon as you do: the excess checking and staring happened only in my apartment. Not with other activities outside of it. I think that this discrepancy can be explained with the need for a safe territory of your own, and the ones you love, which is in line with evolution itself.
See: if I had had my checking compulsions outside my own four walls too, I would never ever been able to cycle to the center of town, because I would have had to check the first traffic light so intensely and long, that it would have gone red way before I'd been able to say to myself: it's really, really green now, Cuthbert...
What a disorder, hey?
Cheers and all the best, Cuthbert.