Lecture 11b, Principles of Speed & Accuracy (Schmidt's Law)

Question 1

Linear Speed-Accuracy Tradeoff (Some have started referring to as “Schmidt’s Law”)

Answer 1

• occurs in very fast movements (< 300 ms), where there’s little time to process feedback
• accuracy changes as the duration and distance of the movement vary (i.e. when speed is manipulated)
• in Fitts’ law we manipulate target amplitude and target width and measure movement time or speed changes while keeping or maintaining accuracy at a high level
• whereas, in the linear speed-accuracy tradeoff we manipulate duration and distance of the movement so that we manipulate speed while measuring how accuracy changes

Question 2

Schmidt’s Law (Textbook)

Answer 2

Schmidt’s Law suggests that aiming errors are about the same for various combinations of movement amplitude and MT that have a constant ratio (that is, a constant average velocity). Thus, increases in movement amplitude and decreases in MT can be traded off with each other to maintain movement accuracy in these rapid tasks

Question 3

What kind of movements are Linear Speed-Accuracy Tradeoffs seen in?

Answer 3

Linear speed-accuracy tradeoffs in fast, discrete actions where speed manipulated, accuracy measured

Question 4

SD in movement endpoints (VE) = effective target width (We)

Answer 4

• the variable error or standard deviation of my endpoints is equivalent to my effective target width
• the measure of accuracy was obtained by calculating the standard deviation of variable error of the movement endpoints

Question 5

Manipulating MT & Amplitude results in linear relationship with performance variability (accuracy)

Answer 5

We = a + b(A/MT)
- linear speed accuracy equation
performance is more consistent (less variable) for movements of smaller amplitude and slower MT
- as movement time increases, effective target width also increases meaning people became more variable or less accurate
- at any given amplitude as movement time increased effective target width decreased meaning people become less variable and more accurate
- found a linear relationship between accuracy and amplitude for a constant movement time goal

Question 6

If we combine A and MT what does that give us?

Answer 6

velocity
- manipulating MT & Amplitude (Velocity) results in linear relationship with performance variability
- graph: effective target width (We) as a function of average velocity (A/MT)

Question 7

Fitts’ Law describes the relationship
between which of the following variables
(hint, there may be >1)?
(A)The distance between 2 targets
(B) Bits of information to be processed
(C) The width of each target
(D)Movement Time
(E) Reaction Time

Answer 7

the distance between 2 targets, the width of each target and movement time
- bits of information to be processed and reaction time are related to hick’s law

Question 8

The variable “MT” refers to the average ____________
for a series of movements,
“Log2(2A/W)” represents the __________________ of
the motor task,
“A” represents the target __________ or total distance
between targets, and
“W” is the numerical value representing
target ______________

Answer 8

• the variable “MT” refers to the average movement time for a series of movements,
• “Log2(2A/W)” represents the index of difficulty of the motor task,
• “A” represents the target amplitude or total distance between targets, and
• “W” is the numerical value representing value width

Question 9

According to Fitts’ Law, which of the following
changes would result in an increased Index of
Difficulty for the Fitts’ tapping task?
(A)Increasing the movement time
(B) Increasing the distance between the targets
(C) Increasing the target width
(D)All of the above
(E) Both A and B

Answer 9

increasing the distance between the targets (manipulate this to influence index of difficulty)
- increasing the movement time is incorrect because dependent variable, it is not what we manipulate
- the answer also is not increasing our target width because by increasing target we are decreasing index of difficulty (decreased target width = increased ID)

Question 10

What is Effective Target Width
a measure of?
(A)The distance between 2 targets in a rapid
single-aiming task
(B) The standard deviation of a participant’s end-
points in a rapid-single aiming task.
(C) The size of the target the participant is
aiming for in a rapid-single aiming task
(D) None of the above

Answer 10

the standard deviation of a participant’s end-points in a rapid-single aiming task

Question 11

Logarithmic (Fitts’ Law)
what variables are manipulated and what variables are measured?

Answer 11

variables manipulated
- target amplitude
- target width
variable measured
- movement time

Question 12

Linear (Schmidt’s Law)
what variables are manipulated and what variables are measured?

Answer 12

variables manipulated
- target amplitude
- movement time
- collectively it is velocity that we are manipulating
variable measured
- effective target width

Question 13

1) Logarithmic speed-accuracy tradeoff primarily related to FB (feedback) processing

Answer 13

• as precision demands increase, importance of vision/feedback increases to achieve accuracy
• if you slow your movements down, more able to make small corrections to the movements
• movements are probably controlled using both OPEN and CLOSED-loop processes because have time to use feedback greater proportion of MT occurring after peak velocity
• greater proportion of MT occurring after peak velocity

Question 14

2) Linear speed-accuracy trade-off related to force variability (fast - no time to use FB)

Answer 14

• very fast, temporally constrained movements. errors related to “noise” in the programmed movement
• movements are probably controlled using only OPEN-loop processes (do not see corrective movements)
• faster movements require more force, which will lead to more variability
• equal time before and after peak velocity