Thank you so much for your explanation @Michael Smith and @Nancy Austin. It helped me a lot to understand and to think another way to write exactly. But, is the first sentence that start with "that" also wrong? Thank you!

Your exact rewording doesn't really work in English. "That highly familiar stimuli produce weaker activity than novel stimuli" is a subordinate clause in the original, and if you just remove the "that" and add a comma your sentence is not grammatical, and, worse, does not make sense. You could dump the "that" and turn this into two main clauses by adding an "and" after the comma, though. Because your second main clause is in the passve voice, you'd need to add a "referential pronoun" linking the second clause back to the first, and "this" would be the obvious choice. So you'd have "Highly familiar stimuli produce weaker activity than novel stimuli, and this was interpreted as a neural efficacy. " Does this work for you, or help at all ? https://www.theschoolrun.com/what-is-a-clause Oh, and of course Nancy's version works too - using a "which" (instead of an "and this"), so that the second clause becomes subordinate.

For the second example, you need to add the word "which" between stimuli and was to make the sentence grammatically correct. For example: Highly familiar stimuli produce weaker activity than novel stimuli, which was interpreted as a neural efficacy.