Fundamental problems affecting mental image vividness comparisons

Subjective ratings of a mental image’s vividness are frequently used to index the similarity between mental and real image phenomenology. However, what it means for a mental image to change in vividness, or to even be perceived at all, is not commonly understood in any concrete terms. Here, the impact of this uncertainty on the validity of vividness comparisons is assessed. Arguments are presented suggesting that vividness ratings cannot distinguish differences in mental image experience from differences in an individual’s self-defined criteria for what constitutes vivid mental imagery, undermining their validity as a tool for comparing the mental image experiences of different individuals. Further, this confound can similarly affect self-identified judgements about whether one experiences a mental image at all (i.e., aphantasia/phantasia). While confounds may affect between-individual comparisons, within-individual comparisons may still have some valid use cases, although they may be largely superseded by other metrics given their limited interpretability. Arguments are also discussed which suggest that vividness ratings and judgements cannot gain construct validity by being correlated with physiological processes until the quasi-sensory nature of mental images is defined. Overall, commonly used vividness ratings seem to be affected by foundational logical flaws which diminish their utility, sometimes entirely. Validly indexing a change in the quasi-visibility of a mental image may require a concerted effort to first define what it means to see a mental image at all, with self-report in general still playing an essential role in mental imagery research, provided it is cautiously interpreted.