The violation of Fitts’ Law: an examination of displacement biases and corrective submovements

Abstract

Fitts' Law holds that, to maintain accuracy, movement times of aiming movements must change as a result of varying degrees of movement difficulty. Recent evidence has emerged that aiming to a target located last in an array of placeholders results in a shorter movement time than would be expected by the Fitts' equation-a violation of Fitts' Law. It has been suggested that the violation emerges because the performer adopts an optimized movement strategy in which they partially pre-plan an action to the closest placeholder (undershoot the last placeholder) and rely on a secondary acceleration to propel the limb toward the last location when it is selected as the target (Glazebrook et al. in Hum Mov Sci 39:163-176, 2015). In the current study, we examine this proposal and further elucidate the processes underlying the violation by examining limb displacement and corrective submovements that occur when performers aim to different target locations. For our Main Study, participants executed discrete aiming movements in a five-placeholder array. We also reanalyzed data from a previously reported study in which participants aimed in placeholder and no-placeholder conditions (Blinch et al. in Exp Brain Res 223:505-515, 2012). The results showed the violation of Fitts' Law unfolded following peak velocity (online control). Further, the analysis showed that movements to the last target tended to overshoot and had a higher proportion of secondary submovements featuring a reversal than other categories of submovement (secondary accelerations, discontinuities). These findings indicate that the violation of Fitts' Law may, in fact, result from a strategic bias toward planning farther initial displacements of the limb which accommodates a shorter time in online control.

The type 1 submovement conundrum: an investigation into the function of velocity zero-crossings within two-component aiming movements

“…The present study revisited previous datasets featuring a higher than typical incidence of type 1 submovements (Roberts et al 2016 , 2018 , 2022 ). The classic two-component interpretation of speed-accuracy in manual aiming would have it that these submovements are a manifestation of a correction following an initial error within the limb trajectory (Elliott et al 2001 , 2010 , 2017 ; Woodworth 1899 ).…”

“…For full details, see the Method section within Roberts et al ( 2016 ). Participants stood over an LCD monitor (temporal resolution = 60 Hz, spatial resolution = 1024 × 768 pixels), which was connected to an adjacent computer that controlled the experiment using E-prime (Psychology Software Tools Inc., Sharpsburg, PA).…”

“…Taken together, it is of interest to further examine the datasets from our previously mentioned studies; that is, when type 1 submovements tended to dominate (Roberts et al 2016 , 2018 , 2022 ). Specifically, to further examine the function of type 1 submovements, we isolated the incidences of type 1 submovements, and further categorised them as functional and non-functional based on whether the secondary submovement moved the limb closer to the target than the primary submovement (see Khan et al 2006 ; Robinson et al 2014 ).…”

“…Bennett et al 2012 ; Burkitt et al 2015 , 2017 ; Elliott et al 2004 ; Engelbrecht et al 2003 ; Lyons et al 2006 ), there are also situations where a large proportion of aiming movements include type 1 submovements. For example, we have shown a greater incidence of overshooting and subsequent type 1 submovements when targets are sized according to one’s own inherent variability (Roberts et al, 2021), under high-performance-related stress (Roberts et al 2018 ) and aiming to the last-placed target in an array of placeholders (Roberts et al 2016 ). At the same time, these effects were attributed to the possibility of accumulating more potential energy with a view to taking advantage of the viscoelastic properties at the opposing antagonist muscle.…”

Quantification of Mouse Reach Kinematics as a Foundation for Mechanistic Interrogation of Motor Control

“…The relative strength of these intrinsic kinematic relationships could be influenced by the nature of the task, which can dictate strategy. Concomitantly, we found task-dependent alterations in the speed-accuracy tradeoff, with head-fixed reaches displaying greater endpoint error on the slowest instead of the fastest reaches (Dounskaia et al, 2005;Fernandez & Bootsma, 2004;Roberts et al, 2016). Compared to the freely-behaving condition, head-fixed reaches displayed an early upward velocity peak that dominated the speed profile, resulting in high asymmetry.…”