could we use 'about' instead of 'as to' in this sentence:

Russell claimed that we can have a great deal of knowledge as to the structure of stimuli.

when 'as to' and when 'about'?

cheers, Ali
Yes, we can use "about" alternatively. Here is the entry from the Cambridge dictionary:

as to FORMAL
He was uncertain as to which road to take.
There's no decision as to when the work might start.

Both variants are possible. But, to my mind, about structure of stimuli will sound better. In fact, it is better to use the preposition about with the noun knowledge.