perk up

Grishamの短編小説Funny Boyを読んでいます。
Emporia howled with laughter, and she was soon joined by Adrian. They laughed until their eyes were wet, until the porch shook, until they were laughing at themselves for laughing so hard. A few of the neighbors perked up and looked from far away. When things were finally under control, she said, "I haven't had sex in so long I've forgotten about it."
動詞の "perk" には元気になる/元気づけるの意味があったと思いますが、上の箇所の意味はそれではちょっと違う気がします。辞書で確認します。
・Cambridge Dictionary of American Idioms: to make something more interesting or active
Usage notes: sometimes used in the form perk up your ears (to cause you to become interested): A new book about using the Internet might not exactly perk up your ears.
なるほど、"perk up" でこの意味を覚えるとよさそうです。