[Animats]

The cutest device for this came from Neuroware in 2011 - neko cat ears connected to an EEG sensor. The system could pick up basic brain states - relaxed, attentive, startled - and moved the ears accordingly.

I saw a group of cosplayers wearing these back then. Someone mentioned the name of one person in the group, and their ears went up. Watching someone play a video game showed them attentive while playing a level, and relaxed while the next level was loading. So, in a limited way, it worked.

(Problems were 1) cost, about US$200, 2) fragile mechanical design, 3) too heavy 4) short battery life 5) ears too big. Someone ought to try this again.)