Why indecisive trials matter: Improving the binocular rivalry imagery priming score for the assessment of aphantasia.

Since mental imagery cannot be observed from the outside, it is all the more important to make it measurable. Yet, many so-called mental imagery tasks confuse object and spatial imagery or can be solved entirely without mental imagery, making them inappropriate for the assessment of mental imagery strength. One promising measurement method is the binocular rivalry task by Pearson et al. (Current Biology 18(13):982-986, 2008), which uses mental imagery priming to quantify mental imagery strength. Here, we propose an improved equation for the binocular rivalry priming score to significantly increase its predictive validity. In a sample of 38 aphantasics and 73 controls, we demonstrate that the binocular rivalry priming score calculated by the new equation explains more variance in the self-reported mental imagery capacity than the original equation. The improved binocular rivalry priming score is particularly beneficial when only a few trials are recorded (e.g., due to time or attention constraints) or when people with low mental imagery (i.e., aphantasics) have to be identified. The improved binocular rivalry priming score is advantageous in many situations, making it the preferred measure for future research.