There has been some more work done on feline cognition, Christine, but you won't find it in any psychology journal. Feline behavioural cognition, as with canine behaviour, is now a discipline in its own right. Compass animal behaviour courses offer several courses up to HND level in feline behaviour and cognition.
Cats do actively remember, and are just as responsive to observational learning, conditioning, counter-contidioning and implicit learning as dogs. However, they learn slightly differently, have subtly different motivations (and therefore preferred rewards), and interpret environmental cues based largely on vision, while dogs place more emphasis on smell, though both are adept at reading nonverbal, postural behaviour in humans. Cats don't respond to human social cues very well compared to dogs (eg. pointing at something to bring the animals attention to it), and they seem to take more trials to learn new behaviours (especially if said behaviour is outwith a naturalisic behavioural repetoire or context), but they forget less readily than dogs. This means that once they learn something it sticks, with little need for subsequent re-learning. However, the flipside is that counter-conditioning (say to get rid of a previously conditioned fear or phobia) is more difficult in cats, and takes longer (though its by no means impossible - it just takes a bit more time and patience).
In essence, fluffybunny, at some pont your babes may have had an aversive experience associated (correctly or incidentally) with doorbells, strangers etc. and they are just responding in what they believe is an appropriate manner. Alternatively, they may have been poorly socialised before you got them, making them shy and wary of strangers or too many people. Both my cats hide when people come round, but usually they come out to have a nosey (at a safe distance) once they are sure the person is not a threat. This is because my two were not socialised much at all for the first few months of their lives (which is the critical socialisation period). As a result, I know they will never be particularly confident or friendly around strangers or visitors, but neither are they terrified. So long as they are happy to cope with other people in their own way, that's fine by me.
