The phenomenal content of conscious experience—what it feels like to have an experience—is constituted by subjectively attributed meaning, which is intrinsic and non-representational. This subjective meaning is ubiquitous in conscious experiences and is related to Frege's "sense" and Peirce's "interpretant." The authors extend Frege's sense to raw feels of consciousness and argue that both sense and reference play roles in phenomenal experience. They formalize subjective meaning as a relational attribute realized by a map interpreting syntactic structures of a formal system within a semantic space. The image of this map in the mental domain comprises the phenomenal content of qualia, with implications for experience-based theories of consciousness.
A new framework using information-theoretic complexity measures, such as integrated information, has been proposed to quantitatively classify states of consciousness, addressing both phenomenological contents and clinical disorders. However, applying these measures to realistic brain networks is difficult due to high computational costs. This article serves as a lookup table of principle-based and empirically tested measures of consciousness, with emphasis on clinical applicability for assisting diagnosis and therapy. It addresses challenges facing these measures with regard to realistic brain networks and suggests possible resolutions.