Fitts' index of difficulty: A useful theorem for speed-accuracy relations in the oculomotor system


The present discussion will outline work examining whether ID-based speed-accuracy relations hold for goal-directed eye movements (i.e., saccades). Indeed, although the oculomotor literature has shown that amplitude-based ID changes elicit a robust increase in movement time (MT) it is largely unclear whether width-based ID changes similarly influence MT. This distinction represents an important test of Fitts' theorem and the assertion that movements yielding the same ID produce equivalent MTs regardless of the response's amplitude and width combination (i.e., unitary MT/ID relations). To that end, participants completed saccades in separate conditions that manipulated the amplitude and width characteristics of a target object. Importantly, the separate amplitude and width conditions were equated for ID (i.e., 3.34, 3.67, 4.06 and 4.61 bits of information) – a manipulation that provided a test of whether saccades are governed via unitary MT/ID relation. MTs for primary saccades across amplitude and width conditions increased as a function of increasing ID (R2=0.99 and 0.76); however, the slope of the MT/ID relation for the amplitude condition (18 ms: CI95%=5) was steeper than the width condition (3: CI95%=2). Further, aggregation of MTs for primary and secondary saccades did not enhance the explanatory power for MT/ID relations (R2=0.97 and 0.54). Thus, the present results demonstrate that, for saccades, the fixed parameter nature of Fitts' ID cannot be applied to a continuous range of veridical movement amplitudes and target widths.

Acknowledgments: Supported by NSERC