意思：那些最初没有被奖励,而是在开始十天后开始奖励的老鼠马上开始显出减少错误的现象,但是很快就变得和一直被奖励的老鼠有相似的出错率.
“that were initially unrewarded but began to be rewarded only after the tenth day ”是修饰第一个Rats的,此句子中的第一个定语从句,“showed an immediate reduction in errors and soon became similar in error rate to the rats”是第一个Rats的动作,而“that had been rewarded consistenly”是修饰第二个rats的,是此句的第二个定语从句.