When did Asimov decide to unify the Foundation and Robots series?

Question

Originally Isaac Asimov's Robots and Foundation series were separate entities, but were later unified. The first book in which I remember the connection being made clear was Foundation and Earth (1986) in which

it transpires that R. Daneel Olivaw from the Robots series has lived for millenia, using his pyschic abilities to shape the course of human history. This is expanded upon in the Foundation prequels where it turns out he directly worked with Hari Seldon and was a driving force in the creation of the Foundation.

The original series were completed in the 1950's with Second Foundation (1953) and The Naked Sun (1957) being the last in each series. At this point there is clearly no connection between either series as far as I can tell

Nothing was then written in either series for 25 years until Foundations Edge (1982) and The Robots of Dawn (1983) restarted both series. It seems possible that Asimov at this point was intending to merge the series as several plot points that are important in Foundation and Earth are established.

Notably the introduction of a psychic robot in The Robots of Dawn and a whole planet linked by telepathy in Foundations Edge, which R. Daneel sees as a possible future evolution for the entire galaxy.

So my question is this: When did Isaac Asimov first decide to combine the two series? At some point in the 25 year years between 1957 and 1982? Or was it not in fact until after he had already restarted both series. Has he ever gone on record as to when this happened? Bonus points if he made any comments on why he decided to do this.

Answer 1

Asimov's text in I. Asimov, which was the basis for the Wikipedia summary quoted by Athena Widget, doesn't exactly state when he made the decision to merge the two universes, but it says that he began to implement the merge with Robots and Empire (1985):

[...] my pleasure with The Robots of Dawn led me to write a fourth robot novel. In the fourth book, Elijah Baley would be dead, but I had already decided that the robot, Daneel Olivaw, was the real hero of the series, and he would continue to function.

Still, the fact that my robots were becoming increasingly advanced with each robot book, made it seem stranger and stranger that there were no robots in my Foundation series.

Carefully, I worked out a reason for it and, in doing so, I could see that it was going to be necessary to tie my robot novels and my Foundation novels together into a single series. I intended to begin that process with the upcoming fourth robot novel, and to give a hint of my intention I was going to call it Robots and Empire.

[...] To my surprise and considerable chagrin, the del Reys argued strongly against my plan to fuse the two series into one. They said the readers would prefer to have the two separate [...]

I stumbled away very dispirited and explained the situation to Kate Medina. [...] She said, “What is it you want to do, Isaac?”

I said miserably, “I want to tie the series together.”

“You’re the writer. Do it.” [...] I went ahead and wrote Robots and Empire and clearly began the process of fusing the two series.

^{I. Asimov, 1994, chapter 145}

However, this is inconsistent with what Asimov wrote in an earlier essay! Here he claimed that he started the process with The Robots of Dawn (1983):

In Foundation’s Edge, I made a point of referring to various non-Foundation novels of mine and indicating how they might fit into the Foundation universe. (A few of the critics disapproved of this, but I’m darned if I know why. They are my books and I can do as I wish with them, I should think.)

A major embarrassment were the Lije Baley novels which can in no way be fitted into the Foundation Universe. The Lije Baley novels contain societies based on strains of humanity with life-spans of up to four centuries and supported by elaborate systems of robots. The Foundation novels have ordinary short-lived strains of humanity only, and no robots at all.

In Foundation’s Edge, I finally mentioned robots, and in The Robots of Dawn I amused myself by letting the plot serve the function of pointing the Lije Baley universe in the direction of the Foundation universe.

^{My Projects, IASFM October 1983}

I think the apparent inconsistency can be explained by seeing this as a gradual process: In Foundation’s Edge (1982), Asimov mentioned robots, but those were not necessarily the same ones as in the robot novels. In The Robots of Dawn (1983), he began to converge the two series, and in Robots and Empire (1985), he made the merge explicit.

Answer 2

Source

In his memoir I. Asimov (1994), Asimov explained that following his commercial and critical success with The Robots of Dawn, he decided to write Robots and Empire with the intentions of making Daneel, "the real hero of the series," the novel's protagonist; and that Robots and Empire would create a bridge to the later volumes of his future history. About this second aim, Asimov said that he was persuaded against it by Lester del Rey and Judy-Lynn del Rey, his long-time friends and the editors of Del Rey Books, who thought that the fans of Asimov's series of novels would rather that Asimov kept the Robot and Empire/Foundation universes separate. On the other hand, his editors at Doubleday Books - his hardcover book publisher - encouraged Asimov to do what deep-down he wanted to do. From then on, Asimov proceeded with his plans for unifying the two series.