脳内

1. Origins, structure, and basic meaning

• Structure: 脳内, read のうない, denotes the inside of the physical brain or an unspoken process occurring in one’s mind.
• Basic usage: It covers activity within the physical brain and mentally rehearsing an answer before speaking.

2. Common pitfalls and similar expressions

• Inside the brain and inside one’s thoughts
  • It can refer to the physical brain or to an action performed only mentally without being spoken aloud.