Abstract
The classic theorem of Paul Fitts' (1954) asserts that the combined effects of movement amplitude and target width (index of difficulty: ID) define movement times (MT) for goal-directed reaches. Moreover, Fitts' theorem states that reaches yielding the same ID produce equivalent MTs regardless of the response's amplitude and width combination. The present study examined the utility of Fitts theorem in the context of reaches to virtual targets wherein width- and amplitude-based ID changes (3, 4, 5 and 6 bits of information) were presented in separate blocks (Ã la Fitts) and randomly interleaved on a trial-by-trial basis. In addition, we examined whether finger- (Experiment 1: N = 12) and stylus-directed (Experiment 2: N = 12) reaches influence MT/ID relations. Experiments 1 and 2 showed that blocked and random trial presentations elicited a linear increase in MT as a function of increasing ID (Fs > 61); however, slopes for MT/ID relations were markedly shallower in the width as compared to the amplitude manipulation. Moreover, slopes for amplitude-based ID manipulations were equivalent across finger- (b = 101 ms; R2 = 0.99) and stylus-directed (b = 134 ms; R2 = 0.99) reaches whereas slopes for width-based manipulations were shallower in the former condition (finger-directed: b = 5 ms: R2 = 0.87; stylus-directed: b = 58 ms: R2 = 0.99). Importantly, examination of reach endpoints indicates that the differences observed here cannot be tied to between-condition differences in error rates. Thus, the present findings add to the extant literature insomuch as they demonstrate that the information-processing capacity of the motor system is not broadly reflected by a simple line function.Acknowledgments: Supported by NSERC.