Sleuthing subjectivity: a review of covert measures of consciousness.

Consciousness is inherently private, but overt behaviors like facial expressions and vocalizations are unreliable indicators because they can be deceptive, absent, or conflict with research goals. This review examines emerging covert physiological measures—including eye, skin, respiratory, and heart signals—that can disclose consciousness without relying on overt behavior. It also addresses the challenge of distinguishing physiological signals linked to conscious versus unconscious neural processing and considers the ethical implications of infringing on the innate privacy of consciousness.

Study at a glance

Abstract

Consciousness is private. Although conscious beings directly access their own conscious experiences, the consciousness of others must be inferred through overt report: observable behaviours - such as overt facial expressions, vocalizations and body gestures - that suggest the level, state and content of consciousness. However, overt report is limited because it can be erroneous (for example, resulting from wilful deception or being subject to recall error), absent (for example, during sleep and paralysis) or conflict with research goals (for example, in no-report paradigms and resting-state studies). These limitations encourage the search for covert measures of consciousness: physiological signals that disclose consciousness without relying on overt behaviour. This Review highlights emerging covert measures of consciousness in humans, including eye, skin, respiratory and heart signals. We also address the challenge of distinguishing physiological signals linked to conscious versus unconscious neural processing. Finally, we consider the ethical implications of infringing on the innate privacy of consciousness.