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.
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Notes
Only 11 of the 16 participants were featured in the reversal versus non-reversal MT analysis of the Main Study. This was because the five participants who were omitted failed to register a reversal/non-reversal submovement for at least one of the target locations.
There was a lower hit rate and greater tendency to overshoot in our Main Study compared to the reanalysis of Blinch et al. (2012). It is noteworthy that the hit rate in the Main Study was similar to previous studies exploring the violation of Fitts’ Law (e.g., 85–90 %, Adam et al. 2006; Pratt et al. 2007), while the seminal work of Fitts and Peterson (1964) indicated a hit rate of approximately 90 % (intermediate to both studies). The reason for the differences between the studies remains elusive, although we suspect it is due to subtle methodological differences. For example, there was an extended practice phase in the Blinch et al. (2012) study, where in the event a participant was inaccurate, they were encouraged to properly land on the target before they could start the experiment for real. There was no such practice phase for the Main Study. These differences are of limited theoretical significance as our main aim was to explore the differences between the conditions held within a single set of participants. In this respect, both studies demonstrated a similar pattern of results (i.e., limited differences between targets for hit rate and a greater terminal overshoot at the last-placed target).
Only 15 of the 20 participants were featured in the reversal versus non-reversal MT analysis of the reanalysis of Blinch et al. There were five participants who failed to register a reversal/non-reversal submovement for at least one of the target locations.
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Acknowledgments
These works were supported by several grants awarded from the Natural Sciences and Engineering Research Council (NSERC) of Canada. We thank Dr. Steve Hansen for his ideas on how best to present the data.
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Roberts, J.W., Blinch, J., Elliott, D. et al. The violation of Fitts’ Law: an examination of displacement biases and corrective submovements. Exp Brain Res 234, 2151–2163 (2016). https://doi.org/10.1007/s00221-016-4618-4
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DOI: https://doi.org/10.1007/s00221-016-4618-4