Principles of Speed & Accuracy

Principles of Speed & Accuracy

Chapter 7

3 types of speed/accuracy tradeoff

spatial & temporal accuracy

temporal accuracy

spatial accuracy

• move a limb (or other object) as quickly as possible to a target, with a goal of minimal error

• participants asked to do a single movement in a particular

goal movement time

repetitive movement-timing

• amplitude and GMT were manipulated

e.g. tapping/clapping along with a beat

Fitts' law

anticipation-timing

continuation task

synchronization task

• error spread of responses used to define W

• continue to tap/clap to the beat when the metronome is turned off

• tap/clap along with a beat set by a metronome

discrete movement-timing

W

= effective target width

e

2 separate timing factors:

• central timekeeping (clock)
• motor implementation

• participants instructed to move a slider along a track over a particular distance in a particular amount of time

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faster beats = greater consistency

GMT decreased with A held constant

receptor

effector

• requires both and anticipation

distance -

independent variable

=

linear increase in W

or

continuation task:

• when will the light arrive?

• how long will it take my hand to hit the button?

GMT held constant with A increased

movement time -

dependent variable

e

external

internal

and

• longer time between beats increases the inconsistency of the central timekeeper

• in these tasks, increased movement speed increases performance (accuracy)

results:

1.

allows for more accurate receptor anticipation

• wait until it's closer to the target

• shorter MT over the same distance were more consistent

2.

rapid movement improves the consistency of the movement

"width"

• improves temporal stability

• shorter MTs are easier to estimate and produce than longer MTs

W

• inconsistency is proportional to the MT

A

"amplitude"

stopwatches!!!

• if MT held constant, increased movement velocity increased consistency

W = a + b (A / MT)

e

generality of Fitts' law

applies to...

MT = a + b[Log (2A/W)]

2

"a law of rapid actions"

• children and adults
• different effectors (hands, feet, underwater, in space, etc.)
• imagined movements
• tool lengths (screwdrivers, etc.)
• computer icon sizes

Log (2A/W) =

average movement time

2

index of difficulty

expressed in "bits"

e.g. number of taps divided by amount of time

ETC....

• difficulty is related to both the distance that is moved and the narrowness of the target

• difficulty is the same for any combination of A and W that has the same ratio

constants

=

b = slope

a = y-intercept

• when the index of difficulty is zero

• indicates degree of sensitivity of the effector to changes in the index of difficulty

• amplitude is half of the target width

depends on:

• effector characteristics (limb size)
• age (old vs. young)
• skill level (amount of practice)

• overlapping targets, so subject taps straight up and down as quickly as possible

*Fitts' law still holds true as long as these factors are unchanged between trials

linear

trade-off:

• increased accuracy requirement results in decreased movement speed

• increase speed at the cost of accuracy
• increase accuracy at the cost of speed

why the difference?

linear = entirely preprogrammed

Fitts' = feedback-based corrections

logarithmic

= controlled MT

= non-controlled MT

vs.

more corrective submovements

fewer corrective submovements

• closed-loop control

• open-loop control

start of movement

end of movement

optimized-submovement

equilibrium point

impulse variability

• rapid initial movement

• movement end-point is programmed

• duration of muscle contraction
• force of muscle contraction

1st submovement

• associated with a specific muscle length-tension equilibrium point

increased force or duration = increased variability

• corrective impulse as target nears

• velocity slows, correction of variability of first impulse made

2nd submovement