Partial sensory deprivation can lead to hallucinations, and extreme deprivation to madness, so the 'thoughts' of the monkey's brain removed from its body which was on the apparatus may not have been meaningful or clear thoughts, but nerve cells firing randomly. All pretty grotesque. But it carries an important lesson for scientists.

Question#1: About 'meaningful' above, is it a modifier of 'thought', as 'clear'? Or is it a subjective complement on its own?

Question#2: About 'nerve cells firing randomly', if reworded, is this below the same?

The 'thoughts' of the monkey's brain removed from its body which was on the apparatus may not have been meaningful or clear thoughts; the thoughts may have been nerve cells firing randomly.

Question#3: If 'all pretty grotesque' were supposed to be a complete sentence, would it be 'It's all pretty grotesque'? Or would it be something different?
"Meaningful" and "clear" are two alternate adjective modifers of the noun complement "thoughts."

I'd say that you'd need a comma after "meaningful" for the sense "they may not have been meaningful."

Your rewording of "but nerve cells firing randomly" is correct, in my opinion.

"It's all pretty grotesque" is a correct and natural usage of the expression.
TakaPartial sensory deprivation can lead to hallucinations, and extreme deprivation to madness, so the 'thoughts' of the monkey's brain removed from its body which was on the apparatus may not have been meaningful or clear thoughts, but nerve cells firing randomly. All pretty grotesque. But it carries an important lesson for scientists.
Question#1: About 'meaningful' above, is it a modifier of 'thought', as 'clear'? Or is it a subjective complement on its own?
It's a modifier of thought. (Well, a computer might find it ambiguous, but as a native speaker, I can just 'feel' that meaningful or clear forms a constituent that hangs together as something related to the following noun thoughts. Probably my brain picks up the dissonance of having both an adjective meaningful and a noun phrase clear thoughts as complements, and rejects it. In English we rarely join elements of different functions with a coordinating conjunction like or.)

TakaQuestion#2: About 'nerve cells firing randomly', if reworded, is this below the same?
The 'thoughts' of the monkey's brain removed from its body which was on the apparatus may not have been meaningful or clear thoughts; the 'thoughts' may have been nerve cells firing randomly.
Yes. (I'd add the quotes in the second instance as well, as shown.)

TakaQuestion#3: If 'all pretty grotesque' were supposed to be a complete sentence, would it be 'It's all pretty grotesque'? Or would it be something different?
Yes, or This is all pretty grotesque. Anything generally like that.

CJ

Oops! Avangi wasn't there when I started this post.
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CalifJim Probably my brain picks up the dissonance of having both an adjective meaningful and a noun phrase clear thoughts as complements, and rejects it.
Probably my brain is more forgiving.
Sometimes it takes two words to replace one. I guess it's more common in conversation. We use a term, and think "Hmmm, I wonder if they know what I mean." We "add a comma," and express it another way.

"The thoughts may not have been meaningful" expresses a complete idea, while "The thoughts may not have been clear" doesn't say much. "Clear thoughts" would be necessary as a "substitute" for "meaningful."

Anyway, without a comma, I took it as you did: a constituent that hangs together.

The "thoughts" . . . . may not have been meaningful or clear thoughts . . . .

- A.
Avangi"The thoughts may not have been meaningful" expresses a complete idea, while "The thoughts may not have been clear" doesn't say much.
We'll have to agree to disagree on this one! Both sentences seem equally complete and coherent to me! Emotion: smile

CJ
AvangiSometimes it takes two words to replace one. I guess it's more common in conversation. We use a term, and think "Hmmm, I wonder if they know what I mean." We "add a comma," and express it another way.
"The thoughts may not have been meaningful" expresses a complete idea, while "The thoughts may not have been clear" doesn't say much. "Clear thoughts" would be necessary as a "substitute" for "meaningful."
Yes, that's why I was kind of confused. I thought 'The thoughts may not have been meaningful' itself made sense.

OK, thank you both!
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