Estimates of various factors to account for activations under the sentence conditions.

We define the estimate of a factor as the largest value that the factor can variably take within an entire sentence. For each factor, its unit load should be invariable among all sentence conditions, making an independent subtraction between estimates of the same factor possible. Separately for long and short sentences, estimates under the Conjoined condition as a reference were subtracted from those under the other Nested and Simple conditions (e.g., the [Nested(L) – Conjoined(L)] contrast abbreviated as Nested’(L); e.g., DoM for Nested’(L), 5–2 = 3). Excluding “number of Merge”, the estimates of which were null for all four contrasts, we regarded “DoM+number of Search” (i.e., adding the estimates of two factors) as an additional factor. Among the nonlinguistic factors, “memory span+counting” and “memory span+number of encoding” were regarded as additional factors, because they were temporal order-related and memory-related factors, respectively.