Lecture 19 - Speed and Accuracy

Question 1

Speed vs Accuracy

Answer 1

High speed - Low Accuracy
Low Speed- High Accuracy

ex: Football vs surgery

In general, move quicker at expense of accuracy

Question 2

Take home message

Answer 2

The motor system is often
faced with conflicting goals and has to achieve some type of balance

Question 2

Accuracy of Aiming Movements Decreases with Speed (test)

Answer 2

Participants made rapid, goal-
directed aiming movements from 1
of 3 starting positions to a spatial
target while holding a stylus.

Results: Peter the accuracy the slower the speed

error increases as speed does, increased variabilty

Question 2

Fitts Theorem Explained

Answer 2

High speed, low index of difficulty
Low speed, high index of difficulty

Movement time (MT) increases as movement amplitude (A) increases

MT is constant for a fixed ratio of movement amplitude (A) to target
width (W) (1:1 ratio)

MT increases as the target width (W)
decreases (i.e., accuracy demand increases) or amplitude of movement
increases

Amplitude increase means high index
increase with means lower index

Question 3

Fitts Theorem Equation

Answer 3

Fitts (1954) discovered the relation between movement amplitude, accuracy, and MT could be described by a log-linear equation

Fitts’ Theorem: MT = a[log2(2A/W)] + b
where:

a = slope
A = movement amplitude
W = target width (i.e., accuracy demand)
b = y-intercept
Index of difficulty (ID) = [log2(2A/W)]

Question 3

Speed-Accuracy Tradeoffs Across the Lifespan

Answer 3

Higher intercept for older
adults means baseline
movements (unconstrained)
are slower than young adults.

Higher slope for older adults
means speed-accuracy
tradeoff is more severe than
young adults

Young adults make smoother, faster
movements with fewer corrections than older adults

Question 4

Daily examples

Answer 4

Bigger buttons cause they want u to see/more important

Frequently used areas (keys) are
larger to reduce accuracy demand
and save time

Infrequently used areas (keys) are
smaller to increase accuracy demand
at the expense of time cost. This also
minimizes risk of error

Question 5

Motor Variability is Proportional to the
Amount of Force Produced

Answer 5

Large mu=large mvc
Small mu=smaller mvc

Scaling of force variability is
proportional to the amount of force
produced

Force variability peaks at 75%
maximal voluntary contraction (MVC) and decreases slightly until 100%
MVC

At 75% of MVC most motor units are already on. Fused tetnus means muscles are still contracting = less forced is produced due to fatigue (less variabilty)

Variability = the more contraction

Question 6

Variability of Contraction Depends on the Properties of Motor Unit Recruitment

Answer 6

Innervation Number: The number of
muscle fibers innervated by a single
motor neuron. Slow twitch (fatigue
resistant) have smaller innervation
numbers than fast twitch (fast
fatiguing) muscle fibers

Fast twitch muscle fibers
tend to contribute to larger motor
units than slow twitch muscle
fibers

The twitch potential of a single fast twitch fiber can be 20x larger than a
typical slow twitch fiber. Thus, activating larger fibers can increase the
variability of force production during voluntary actions