I think of interoception as being about the category of things like “thirsty” and "hungry” and "tired" and “sore" and “needing to pee.” Physical body processes.
I think of alexithymia as being about the category of things like "happy" and “angry" and “despairing." Emotions.
Is alexithymia a failure of interoception in relation to a subset of the things to which interoception applies?