So, I've gone through a good number of results that I got from BNCweb English language corpus, and can conclude that all these three tenses are used with "until now".
If we consider the following sentence: "Until now it has been thought that interferon can do little to prevent the initial entry of viruses into a cell.", why is the usage of the Present Perfect tense here is justified?
As, if I understand it correctly, at the time of the actual utterance, it is no longer thought that interferon can do little to prevent the initial entry of viruses into a cell. And, if I understand it correctly too, "until now" refers to the past. (Or is the word "now" means a more extended period of time?) Why is it not phrased as "Until now it was thought that interferon can do little to prevent the initial entry of viruses into a cell." or "Until now it had been thought that interferon can do little to prevent the initial entry of viruses into a cell."?
But let us consider a different example sentence, in which I think the usage is justified (I'll try to explain why I think so) : "Until now I have abstained from commenting on the possible effects of the Human Genome Project on society in future." I suppose this sentence is fine if the speaker is about to comment on those effects, and would not make any sense if any comments had been made by the time the sentence was uttered.
Another (and the last) example is: Until now we have assumed that banks decide for themselves upon the appropriate ratio in the light of their desire for profit and need for liquidity.
Perhaps it has something to do with the main verbs "think" and "assume"? Or am I missing something absolutely obvious here?
I am asking this because I keep finding a definition that the Present Perfect can be used for activity that lasted up until but not including the present, but in the examples above...the activity seems to have already ended.
I am very much sorry if I have asked too verbose a question, but I thought it would be helpful for me to provide some context sentences.