讀文獻看到一句不太確定，thus，the subjects were to rehearse every item during the interitem interval immediately following it 問題: 句子中were to 是什麼?

were=past tense of "be"=are問題在這裏是
The meaning:-
One item followed upon another;
First item goes first,and an interval, and we will follow the second item.
Hence second item following the first at once, but with an interval immediately following.
It is treated as" To Follow" after a short period of interitem interval, to be given to rehearse, to be given to practise the part as a trial performance act over again immediately before the second part !

BE + TO-infinitive

1.) official arrangements: 正式的計畫、安排，又或表示不久的將來幾乎肯定會發生的事情
The Prime Minister is to visit India next month.

2.) official orders: 正式的指示、通知
At the end of the course, all students are to take a written exam.

3.) a possible aim when saying what should be done to reach it: "條件句"中，表示某種先決條件
If you are to work here for more than three months, you must have a residence permit.

還有其他用法，請自行閱讀 http://www.grammaring.com/be-to-infinitive
版大的問題中，were to 的應用屬於第二項，時態是過去

thus，the subjects were to rehearse every item during the interim interval immediately following it
因此，受測者於緊接每一項的短暫間隔復述該項目

were to rehearse 指示要求復述