Depicts the top of the head. The modern form of the this component looks like 田 (field), but is unrelated.

Change in sound

思 and 囟 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in Old Chinese.
思 *[s]ə
囟 *[s]ə[r]-s

Change in form

Due to historical stylistic changes, this component is less similar to 囟 than it was in ancient scripts.