Step1: 听音频并跟读单词，熟悉它们的发音和意思

optical illusion 视觉偏差

Uncle Sam Army recruitment poster 山姆大叔的招兵广告

magnification 放大n

ambiguity 含糊，模棱两可n

masterpiece 名作n

Step2: 听音频，用简洁的文字回答下面的题目

1. Why does the speaker mention the Uncle Sam Army recruitment poster ?

2. According to the psychologist, what is wrong with the name of Mona Lisa effect?

3. What is the result of the investigation ?

Step2: 关注微信公众号“顶萃英语”，回复关键词“高阶21”可获取参考答案。