Halliday & Matthiessen (2014: 544):
Like ‘middle mental’ clauses, ‘effective’ ones either project ideas within a clause nexus or include fact clauses as Phenomenon, e.g.
The first means ‘in my opinion there’s no one here’, with there’s no one here as an idea. The second means ‘there’s no one here, and that worries me’, with there’s no one here as a fact. The fact exists prior to the occurrence of the mental process; but the idea does not – it is brought into existence in the course of the mental process. Thus the second is agnate with there’s no one here, which worries me; but we cannot say there’s no one here, which strikes me. The two are very distinct in speech, thanks to the intonation pattern (see below); the different analyses are given in Figure 7-25.
The difference in structure is clear from the intonation pattern. That of (a) corresponds to I rather think there’s no one here, with falling tonic (tone 1) on here and perhaps a separate falling-rising tonic (tone 4) on strikes/think; that of (b) corresponds to it worries me, the emptiness of the place, a compound tone group with tone 1 on worries and tone 3 on here/emptiness, showing clearly that that there’s no one here is functioning as a postposed Subject. Again, it strikes me is a cognitive process clause, and so can project an idea, whereas it worries me is and emotive one and cannot.