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Speed, accuracy and anticipation

Bio 39March 5, 2019

From athlete’s point of view

• Keys are:– Size– Strength– Anticipation

In contrast, play of games

• Ability of player’s nervous system to– Make good decisions and plans– Execute

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• From the nervous system’s point of view all sporting movements represent a compromise between:

1.Speed (baseball)2.Accuracy (golf)3.Anticipation

“Fast ball” Sports

• Baseball• Tennis• Soccer• Volleyball• Ice Hockey (& field hockey)

Lags versus delays

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Some definitions

• Reaction time (RT) = interval of time between onset of stimulus and initiation of motor response

• Movement time (MT) = time to complete movement

• Response time = RT + MT

The classic confrontation

Time for ball from pitcher to home plate

Depends on how fast the pitcher can pitch

367-614 ms !!!!

Corresponds to throwing between 60-100 mph

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Reaction time of batter

• TS = Tr(x) + Tm

• Where– TS is the response

time– Tr is the reaction time– Tm is the movement

time

Reaction and movement times (sum equals the response time)

Response times of batters

• Average batters Ts 400 – 450 ms• Very good batters: Ts 300 – 350 ms• TM can’t be change much (200 ms), so

Tr 100-150 ms for very good batters– 43 ms for information concerning ball

position and velocity to be transferred from retina to cerebral processing centers

– Cerebral processing times are 50-100 ms !!!

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But there is much more going on!

• Hick’s law

– Reaction time increases linearly as information load increases

Sequential model

• Decide where ball is going• Make a neural program to tell the

muscles what to do• Swing the bat

• RESULTS SUGGEST THAT THIS MODEL IS NOT POSSIBLE!

Speed versus accuracy• When attempting to

thread a needle it is important to move slowly because the eye of the needle is very small.

• In other words, we must sacrifice speed in order to gain accuracy

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R. S. Woodworth (1899)Experiment

– Draw lines with a pencil back and forth between two lines

– Stop at each line, but keep pace with a metronome

– Use metronome to change speed

– Measure variability of errors

Paul Fitts (1912-1965)

• Mathematician who revolutionized how we think about movements

• Formulated Fitts’ law

A Fitts’ tapping task

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Fitts observed the following

• Movement time (MT) decreases as amplitude decrease

• MT varies with A

• Movement time decreases as width of target increases

• MT varies with 1/W

• Combining these observations he proposed that– MT varies as A/W

where A/W is the index of difficulty

What is the difficulty of this task? What is the movement time?

How can we take into account the last observation?

MT varies with log (A/W)

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Fitts law

MT = a + b log ( 2A/W )

• Each limb of the body obeys Fitts law– A is arm– W is wrist– F is fingers

• Different slopes imply different sensitivities– Smaller slope means

greater sensitivity

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Fitts law is very robust. Holds for:

• Single aiming movements in air or water

• Moving a joystick, handle or cursor• Throwing darts• Eye movements

What is the explanation for the speed-accuracy trade-off?

• Early explanations:– Stressed role for closed-loop control

mechanisms– Two phases to movement

• Open loop phase that resulted in a movement toward the target

• Closed-loop correcting phase that adjusted the trajectory of the movement when an error was detected

Speed accuracy trade-off holds even for very rapid aiming movements:

suggests that role for closed-loop control is minimal

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Force-force variability

Ask subjects to produce forces against an immovable handle to reach target a given distance away

Speed-accuracy is a feature of central motor planning for the movement

• Remember the role of muscle co-activation in speed-accuracy trade-off

• Further evidence:

Fitts law holds for imagined movements!

Timing

The timekeeping responsible for the duration of internal events such as timing of an agonist burst or the internal timing of responses as a whole

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One major exception to Fitts law:

Batter’s dilemma: balance spatial errors against timing errors

Choice 1• Swing fast:

– Means that can wait longer as the pitch travels towards home plate before swinging

– Consequently • Increase the processing time• Decrease timing error• Hit the ball harder

– Suffer increased spatial errors• Strike out more often, more pop-ups, grounders

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Choice 2

• Slow the bat swing– Means initiate swing early in the wind-up– Positions the bat more accurately by

reducing spatial error– BUT

• Less time to process information• Increased timing error• Ball hit with less force• More foul balls

Fastball pitcher on the mound

• Hit ball:– Tends to be popped up more often– Ball tends to go to mid-infield

• Why?– Small timing error, variable spatial

performance

Slow throwing pitcher

• Batters tend to hit the ball solidly to all parts of field

• Why?– Small spatial errors but variable timing

performance

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What about anticipation?

Anticipation

Requires that performers coordinate or in some way synchronize their motor responses to an external event

– Examples: hit a baseball, timing the snap count in football, keep the beat in music

Possible to distinguish three types of anticipation:

1. Receptor anticipation2. Perceptual anticipation3. Effector anticipation

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Receptor anticipation

• Involves the anticipation of the arrival of an external event when some aspect of the event can be tracked in some manner

• Stimulus is prime component in the anticipation process

• Example:– Determining when a baseball will arrive at home

plate. Why? – batter visually tracks the progress of the ball

Perceptual anticipation

• Involves the anticipation of an external event that cannot be directly tracked but occurs with some degree of regularity or under a set of conditions

• Example: base runner anticipating when to steal a base. Why?– Runner uses the regularity of the pitcher’s

movements and situation

Effector anticipation

• Involves the process of determining how long one’s own movements or movement component will take to initiate and execute

• Important to all movement skills

• Closely related to concept of risk

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Soccer goal keepers

• Guess where the ball will go– Have early “check-point”:

• Successful stop: if everything seems consistent then dive to where prediction indicates

• Unsuccessful stop: If ‘check-point” not passed then do not do better than chance to stop shot

Modified sequential model?

• Anticipate where ball if going BEFORE it is thrown

• Make a neural program to tell the muscles what to do

• Swing the bat• Surprisingly this model is also likely

not to be true!!

Conclusion

• Sequential model for the batter’s brain is not possible

• In other words the brain of the batter must be making decisions about swinging the bat while swinging the bat!

• How is this possible?

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Motor Control

Mirror Neurons (located in IF)

Cricket batters

• Expert batters– Predict where ball will go from the windup– Better predictors than novice batters– Perform better facing human bowlers than

bowling machines

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Model for anticipatory motor control

• Neural programming for anticipation, timing and motor control occur, at least in part, concurrently

Pitcher’s problem• Their own windup

gives important information to the batter’s brain that helps the batter hit the ball

What is pitcher to do?• Deception• Distraction

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The classic confrontation: Why did Casey strike out?

• Battle over the management of crucial neural resources between batter and pitcher

Why did Casey strike out?

• Ignored first two pitches– No contribution to anticipatory skill

• Angry– Irrelevant information, loss of focus

• Played to the crowd– Distraction of attention

PLAY BALL !