One size Fitts' all? Reconsidering the use of deviations from real-time action imagery in mental chronometry tasks

Mental chronometry tasks commonly presume that real-time action imagery indicates high action imagery ability (AIA). However, factors that systematically influence imagery durations undermine the validity of AIA scores based on deviations from real-time action imagery. Therefore, we introduce and validate a novel approach for measuring AIA using mental chronometry. This approach relies on the relationship between a movement's difficulty and its duration (Fitts' law), with AIA scores quantifying the degree to which this relationship is preserved in action imagery. In a Chronometric Radial Fitts' Task (CRFT), participants (N = 30) executed and imagined both tapping (with a stylus) and clicking (with a computer mouse) radially arranged targets. Single execution and imagery durations were isolated through simultaneous space bar presses. Targets varied in difficulty according to Fitts' law, which held across all four conditions (for both tools, when executing and imagining the movements). The AIA scores derived from the proposed Fitts' law approach outperformed those derived from earlier mental chronometry approaches, being the only scores indicating convergent validity across tools. Using the stylus, larger absolute and relative deviations from real-time action imagery were consistently associated with higher subjective AIA, reinforcing validity concerns. Participants struggled to incorporate the computer mouse's properties into the internal model of the movement, resulting in imagery with the computer mouse relying on internal models similar to the stylus (simpler tool). In addition to offering a refined approach for measuring manipulation- and timing-related AIA, our study highlights the importance of tool familiarity in mental chronometry.