Pupillary response
The reflexive change in pupil size — to bright or dim light, to cognitive load, and (less obviously) to imagined light. The last of these is reduced or absent in aphantasia, and is the first objective physiological marker of the condition.
The pupillary light response is the reflex by which the pupil contracts in bright light and dilates in dim. Less well known is that the pupil also responds to imagined brightness: when typical imagers vividly picture a bright scene, the pupil contracts as if real light were present, and the magnitude of that contraction tracks how vivid the imagery feels. This finding gives the field a way to study mental imagery objectively rather than relying on self-report.
Kay, Keogh, Andrillon, and Pearson published the key paper in 2022, in eLife. They reported that aphantasic participants showed no imagery pupillary light response, while their perceptual pupillary response (to real light) and their pupil dilation under cognitive load were both intact. The aphantasic pupil works normally; what is absent is the endogenous imagery-driven component. The authors describe their result as the first physiological validation of aphantasia.
The clinical implication is that a pupillometer-based test could in principle become an objective screening adjunct to the self-report VVIQ, though that has not yet translated into routine practice and is unlikely to displace self-report in the near term.