Solipsism is the idea that only the self can be known to exist. Introverts have, in my opinion, a solipsistic view of reality in terms of their 7th function.
Ni base, "I am the only potential object (Ne) that exists"
Si base, "I am the only concrete object (Se) that exists"
Fi base, "I am the only emotional object (Fe) that exists"
Ti base, "I am the only thought object (Te) that exists"
The introvert's sense of self comes from the feelings that 7th function objects generate within them. They continually reinforce their identity by staying with the objects that most affect them, to the point where they cannot break the relationship without deflating their ego. Eventually, (I believe) they come to the subconscious conclusion that they ARE those objects, and losing them triggers a defensive reaction, as though they were losing a part themselves. They become blind to 7th function objects that have no internal effect on them, so completely do they filter external reality to match their inner responses.