Spontaneity is a cool usage of the endings ～られる and ～れる. The grammar resembles the passive, but they are semantically different. There are also unique spontaneity verbs. We'll touch on how spontaneity relates to potential phrases.
～られる and ～れる may show spontaneous action or situation. A spontaneous action or situation is one that occurs without the intention of the subject which is often one’s self. There is no will involved. This ending is typically restricted to verbs of thought, cognition, and or feeling. If mentioned, the experiencer is marked by に. The object is marked by が just like with passivization.
1. 人の気配が感じられた。
The presence of (the) people (there) was felt.
2. 行く先が案じられた。
The course was considered.
3. 波の音に驚かれた。
I found myself surprised by the sound of the waves.
4. 昔のことがふと思い出された。
I found myself remembering about the old days all of a sudden.
5. 意気込みが感じられた。
I felt ardor.
6. 空気がおいしいと感じられました。
The air came to be delicious.
7. 入院した祖母の容体が案じられる。
To get concerned about the condition of one's hospitalized grandmother.
8. 秋の気配が感じられる。
To find oneself sensing (the coming of) autumn.
9. 僕にはどうしてもそう思われる。
No matter what, all I think of is that.
10. いかにも不思議に思われた。
It seemed really mysterious.
聞こえる & 見える
The verbs 聞こえる, 見える, as well as other similar looking verbs such as 燃える and 消える also express 自発. So, we'll call these verbs 自発動詞. 聞こえる and 見える predate the potential forms 聞ける and 見られる, and because they don't actually show potential, though potential phrases do ultimately derive from spontaneity phrases due to the fact that having the potential to do something is a characteristic of you that you cannot control, we did not discuss them in the potential lesson.
So, if 聞こえる and 見える do not mean 聞ける and 見られる respectively, we need to see how they differ. Essentially, they show inherent ability whereas 聞ける and 見られる show that you can (if you want), indicating one's intentions can be realized.
聞こえる describes a naturally hearing sensation. It can be defined as "to be able to hear", "to sound", "to be audible", and even "to be famous". 見える may mean "to be able to see". It can also mean "to come into view/to appear". With ように, 見える may show what something looks like. It may also be a very respectful form of the verb 来る.
Examples
11. 雨（の）音の中でも彼らの声が聞こえた。
I could hear their voices even in the middle of the rain.
12. 彼は興奮しているように見えます。
He looks like he's excited.
13. 魚は音が聞こえると思う？
Do you think that fish can hear?
14. 私は彼の講演が聞こえません。
I can't hear his lecture.
15. 年齢相応に見える。
To look one's age.
16. 映画館で今黒沢の映画が見られます。
You can now see Kurosawa's movies in the theater.
17. iPhoneで天気予報が聞けます。
You can hear the weather forecast on your iPhone.
18. 昨日は香具山が見えたが、今日は見えない。
I could see Mt. Kaguyama yesterday, but I can't see it today.
19. 降り出しそうに見える。
It appears that it's going to rain.
20. 隣のテレビの音が聞こえる。
I can hear the neighbor's television('s sound).
21. 新しい眼鏡をかけるとずっとよく見えます。
I see so much better when I wear my new glasses.
22. 彼は右目があまりよく見えない。
He can't see very well with his right eye.
Literally: As for him, his right eye can't see very well.
Historical Note: Long ago ～ゆ was used just like ～られる and ～れる and remains part of many verbs like 燃える and 消える. So, these example verbs come from 燃ゆ and 消ゆ. You may even see these old forms purposely used in songs and poetry. Their roots still end in "y" and they're intransitive and spontaneous in nature.
Bridging Contexts between Spontaneity and Potential
Another verb to note is 思える. It can show spontaneous thought. It can also show the ability of thought which can be seen as coming from 思ゆ or 思われる. 思える and 思われる are almost identical, but the only true difference between the two is that the former is felt to be more objective and the latter is felt to be more subjective.
Before going to examples, note the mentioning of "potential". There are clear instances in which verbs can be interpreted as both a 自発動詞 and a 可能動詞. However, the use of a speech modal such ～てしまう or ～てくる are very important to get both meanings.
Examples
23. A生徒がいつのまにか書けてしまった。
Student A managed to be able to write it before I knew it.
24. びっくりするくらい泣けてきたわ。 (Feminine)
I was surprised at how many tears (I was able to) shed.
24. 自然に笑える映画を作ってみてください。
Try making a movie one would naturally laugh to.
25. 良い作品とは思えません。
I cannot/could not think of it as a good work.
26. 地名の由来はアイヌ語から来ていると思われます。
The place name's origin is thought to come from Ainu.