One of the important concepts to understand in motor control is about Fitts Law. What is it primarily about?
Fitts, in his experiment, asked the participants to tap as fast as they could on two targets that had a width of W (in inch) and separated by a distance of D (inch). Both W and D could be manipulated (independent variables). He calculated the resulting average movement time (MT in ms) by dividing the trial duration by number of taps.
In the above equation, log2(2D/W) is commonly termed as the index of difficulty (ID). Supposing ID = log2(2D/W),. Changing either D or W can vary the ratio 2D/W. Therefore, the ratio could change from 1 (denominator is higher than the numerator; no gap between targets, D=1, W=2, 2D/W ratio=1), 2(numerator is four times higher than the denominator, D=2, W=1, 2D/W ratio=4), 3 (numerator is eight times higher than the denominator; D=1, W=.25,2D/W ratio=8), 4(numerator is sixteen times higher than the denominator, D=2, W=.25, 2D/W ratio=16), 5 (numerator is thirty two times higher than the denominator, D=3, W=.5, 2D/W ratio=32), 6 (numerator is sixty four times higher than the denominator, D=4, W=.25, 2D/W ratio=64) and so on. Specifically, as the ratio 2D/W goes exponentially, the index of difficulty (i.e., log2(2D/W)) goes as a logarithmic function. In a logarithmic relationship, as x changes, there is an initial steep change of y after which the change becomes slower. Therefore, as you keep increasing D and decreasing W (thereby increasing 2D/W), after a certain time, the index of difficulty rises slowly.
Now this index of difficulty is linearly proportional to movement time (MT). This is what is the Fittz Law! i.e., as the index of difficulty increases the time to process it increases too.
MT = a +b[log2(2D/W)]
Ref:
- Fitts, P.M., (1954). The information capacity of the human motor system in controlling the amplitude of movement. Journal of Experimental Psychology, 47, 381-391.
- Richard A. Schmidt, Timothy D Lee, (1999. Motor Control and Learning (Third Edition). Human Kinetics.
