Chapter 6 Principles of Speed, Accuracy, and Coordination

6

Principles of Speed, Principles of Speed, Accuracy, and Accuracy, and CoordinationCoordination

C H A P T E R

Objectives

• This chapter will help you to understand the following:

– Speed–accuracy trade-off in simple aiming movements

– Logarithmic and linear relationships between speed and accuracy

– Relationship between timing accuracy and movement time

– Principles of bimanual timing and role of self-organizing principles

Figure 6.1

Fitts’ Law

• Fitts’ Law states that MT is constant whenever the ratio of the movement amplitude (A) to target width (W) remains constant.

• In addition, Fitts found that the MT increased as the ratio of A to W increased by either making A larger, making W smaller, or both.

(continued)

Fitts’ Law (Continued)

• MT = a + b [Log2(2A/W)]

• where a and b are constants (the y-intercept and slope, respectively) and A and W are defined as before.

• The term Log2(2A/W) is referred to as the index of difficulty (ID).

• Fitts' Law says that MT is linearly related to the index of difficulty.

Figure 6.2

Linear Speed–Accuracy Trade-Off

• The linear speed–accuracy trade-off suggests that for various combinations of movement amplitude and MT that have a constant ratio (a constant average velocity), the aiming errors are about the same.

• Increases in movement distance and decreases in MT can be traded off with each other to maintain movement accuracy in rapid tasks.

Figure 6.4

Figure 6.5

Sources of Error in Rapid Movements

• In summary, this is why increasing the speed of a rapid movement contributes to its inaccuracy:

– The relative contraction forces of the various participating muscles are a major factor in determining the ultimate trajectory of the limb.

– The inconsistency in these forces increases with increased force.

(continued)

Sources of Error in Rapid Movements (continued)

– When MT decreases, more force is required.

– When amplitude increases, more force is required.

– More force generates more variability, which causes the movement to deviate from the intended trajectory, resulting in errors.

Figure 6.7

Very Forceful Movements

• Here's what results when a movement requires very high levels of muscular contractions (greater than about 70% of the subject's capabilities):

– Increasing speed by reducing MT can decrease spatial and timing error.

– Because a greater muscular force requirement increases accuracy, adding inertial load to the movement can decrease error, up to a point.

(continued)

Very Forceful Movements (continued)

– An inverted-U relationship exists between spatial accuracy and force requirements, with least accuracy at moderate levels of force.

Movement Timing

• Skills with purely temporal goals seem to follow somewhat different principles than those having purely spatial goals.

• Decreasing the MT has the effect of decreasing the timing error for skills with temporal goals, making the movement more accurate in time, not less.

Figure 6.9

Bimanual Aiming Tasks

• Bimanual Fitts’ task

• Both limbs could be assigned identical tasks with either low or high IDs, or the limbs could also be assigned to different (incongruent) tasks

• The explanatory power of Fitts’ Law is reduced when separate and incongruent task demands are required of two limbs

(continued)

Bimanual Aiming Tasks (continued)

• This finding could be a result of an attempt by the executive to deal with an overloaded attentional demand by issuing a single motor program that controls both limbs

• Conclusion is supported by other bimanual research, and these findings support a view on which the MT and kinematics for both limbs are not determined independently but rather by a joint command

Continuous Bimanual Timing

• When controlling the continuous movement of two limbs, each with its own spatial or temporal goal (or both), because the movements are ongoing, the executive can do the following:

– Use a common movement command to control the movements of both limbs

– Switch attention rapidly between the executions of the two tasks

Coordination as a Self-Organization Process

• The notion of the motor program is not without its critics.

• Investigators from the self-organization perspective hold that the regularities of movement patterns are not represented in programs but rather emerge naturally out of the complex interactions among many degrees of freedom.

Speed–Accuracy Trade-Off Reconsidered

• The increased complexity of coordinating two movements also provides more flexibility, such that increases in speed result in changes to the coordination pattern in order to maintain stability.