The research found that people who watched a movie in 3D had improved cognitive skills compared to those who watched it in 2D.

【翻译】研究发现，观看3D电影者较之观看2D电影者，更加提高了他们的认知能力。

句子含义是：3D电影的效果能够提高人的认识水平。

关于这个句子涉及3个动词时态问题：

1. The research found：“研究发现”，显然指过去“发现”。

2. that people (who watched a movie in 3D) had improved cognitive skills：宾语从句that people had improved cognitive skills中，使用过去完成时had improved，说明早于found，合乎情理。也就是说“提高认知能力”在先，“发现”在后。

3. who watched a movie in 3D 和who watched it in 2D：两个定语从句在时态方面是一个层次，是对过去事实的客观陈述。因此，过去完成时had improved 与定语从句watched 没有时间比较关系、而且也不强调时间比较关系！它只强调跟主句found存在时间的先后关系。

如果把found 改为一般现在时，那么：

The research finds that people who watch a movie in 3D have improved cognitive skills compared to those who watch it in 2D.