topic vs mental object

topic and mental object both are nouns.

Word	Noun	Adjective	Verb	Adverb
topic	Yes	No	No	No
mental object	Yes	No	No	No

As nouns, mental object is a hypernym of topic; that is, mental object is a word with a broader meaning than topic:

topic: some situation or event that is thought about
mental object: the sum or range of what has been perceived, discovered, or learned

Other hypernyms of topic include cognitive content, content.

topic (noun)	mental object (noun)
some situation or event that is thought about	the sum or range of what has been perceived, discovered, or learned
the subject matter of a conversation or discussion