Reaction time How I learned to stop worrying and love the ... · Reaction time Roozbeh’Kiani ......

Reaction time

Roozbeh Kiani

Center for Neural Science and Department of Psychology, NYU

How I learned to stop worrying and love the reaction time

What is a decision?

•  A commitment to a proposition or plan of action based on evidence, prior knowledge, payoff, and urgency.

•  Decisions often involve interpretation and contingency.

•  The study of decision-making offers insight into how higher brain areas read out or interpret information acquired through the senses.

Roitman & Shadlen, 2002 Kiani et al, 2011

Motion direction discrimination task

Reward for correct choice

Roitman & Shadlen, 2002 Kiani et al, 2011

Motion direction discrimination task

Psychometric funcBon: Accuracy

Chronometric funcBon: Speed

Rea

ctio

n tim

e [m

s]

Hubel, 1988

Sensory “Evidence”

From: Hubel & Wiesel

From: Celebrini & Newsome

Responses of MT neurons

Britten et al., 1993

Rightward Leftward

Trivial

Easy

Difficult

100 ms

50 s

pike

s/s

– 0 +

Momentary evidence (Δr)

µ= kC

6.4%

99.9%

25.6%

Coherence

Adapted from Caret Van Essen, 2002 URL: http://www.nitrc.org/projects/caret/

Lateral intra-parietal cortex (LIP)

V1 V4

7a 7b

V2d

V2v

Posterior Anterior

MT/MST

Frontal eye field

Roitman & Shadlen, 2002

saccade

motion onset

select Tin

select Topp

Roitman & Shadlen, 2002

High motion strength!

Low motion strength!

Time"Stimulus

on!Stimulus

off!

Spikes/s"

Time"Stimulus

on!Stimulus

off!

Low motion strength!

accumulaBon-‐to-‐bound model

MT: Sensory Evidence

MoBon energy

“step”

LIP: Decision FormaBon

AccumulaBon of evidence “ramp”

Bound

Choose right

Accumulated evidence for rightward target

– +


Accumulated evidence for leSward target

Choose leS

Time

μ= kC


Momentary evidence e.g.,

∆Spike rate: MTRight– MTLeft

Decision variable (v)

Link 1992 Ratcliff & Smith, 2004 Palmer et al, 2005 Laming, 1968 Luce, 1986

Choose left

Time

– +


μ= kC

Choose right

3 degrees of freedom: B, k, t0


Best fi]ng chronometric funcBon

t(C) = BkCtanh(BkC)+ tnd

Predicted psychometric funcBon

P =1

1+ e−2k C B

tnd = 200, 300, 400msB= 0.55, 0.71, 0.83 1/k = 0.03, 0.06, 0.12ms

Gaining intuiBon about the model

Palmer et al, 2005

Gaining intuiBon about the model

t(C) = Bk C

2Pcor C( )−1⎡⎣ ⎤⎦ + tnd Luce 1986, Link 1992

Coupling response Bme and accuracy

(halfway Bme threshold) / (halfway accuracy threshold) ~ 3.5

Why should you care about decision Bme?

Why it isn’t enough to say “I always have fixed sBmulus duraBons in my experiments”? Because the same decision-‐making process applies, curtailing the effect of the sBmulus.

Viewing duration varies from 80ms to 1500ms

Tin Tout

Kiani et al, 2008

Correct response probability improves with stimulus duration

Probability correct

Stimulus duration (ms)

coherence

Kiani et al, 2008

Discrimination threshold (%coh)

Stimulus duration (ms)

data Perfect accumulation

Failure to integrate all the information for longer stimuli

Kiani et al, 2008

Effect of motion pulses on the choice

•  Do early motion pulses have stronger effect on the behavior? •  Does the monkey ignore late motion pulses?

Fixation Targets on

Motion Go

Eye movement Motion pulse: 3.2%, 200 ms

Kiani et al, 2008

Motion pulses bias the monkey’s behavior

-50 -25 0 25 50

Motion strength (%coh)

1

0.75

0.5

0.25

0

Proportion of rightward choices

Kiani et al, 2008

The effect of the motion pulse diminishes by pulse time

Pulse time (ms) 0 400 800

1

0

Shi

ft (%

coh)


Pro

porti

on o

f rig

htw

ard

choi

ce

-20 0 20 -20 0 20

1

0.5

0

1

0.5

0

Pulse time < 175 ms 362-563 ms

175-362 ms > 563 ms

Kiani et al, 2008

Time

Bound

The pulse effect is expected to depend on both the pulse time and the strength of primary motion

0 200 400 600 800

0.3 0.2 0.1 0 -0.1

Pulse time (ms)

Change of rightward choice proportion

The effect of the motion pulse depends on the primary motion strength

0%

12.8%

Kiani et al, 2008

What are the implicaBons?

Visualstimulus

Neuronalresponse

Behavioraljudgement

Britten, Shadlen, Newsome & Movshon, 1993

MT responses depend on motion coherence

0 25 50 75 1000

50

100

150

200

0 10 20 30 40 500

50

100

0 25 50 75 1000

10

20

30

40

50

0 10 20 30 400

25

50

75

100

Firi

ng r

ate

(impu

lses

/tria

l)

Coherence (%)

Preferred directionNull direction

Barlow, Levick and Yoon, 1971



Coherence (%)

Pro

port

ion

corr

ect


Coherence (%)

Pro

port

ion

corr

ect

0

20

40

60

Num

ber

of c

ells

0.1 1 10Threshold ratio (neuron/behavior)

MT cells are as sensitive as monkeys to visual motion

When measured correctly!

Cohen & Newsome, 2009

Saccade

Motion

Targets

Fixation

Time

]

ReactionTime

Hanks, Kiani & Shadlen, 2014

Speed-Accuracy Tradeoff

a b

cBound for Tin

0

A Bound for Tout

0

A

Tin accumulator Tout accumulator

0 20 40 600.5

0.6

0.7

0.8

0.9

1

Motion strength (% coh)

Prop

ortio

n co

rrect

0 20 40 60

0.4

0.5

0.6

0.7

Motion strength (% coh)R

eact

ion

time

(s)

Wednesday, November 6, 2013



a b

cBound for Tin

0

A Bound for Tout

0

A

Tin accumulator Tout accumulator

0 20 40 600.5

0.6

0.7

0.8

0.9

1


Prop

ortio

n co

rrect

0 20 40 60

0.4

0.5

0.6

0.7


Rea

ctio

n tim

e (s

)



0 200 400

0.40.60.8

Time from dots onset (ms)

Nor

mal

ized

firin

g ra

te Accuracy condition

0 200 400

0.40.60.8

Time from dots onset (ms)

Speed condition

−50 0 500123

Motion strength (% coh

Build

up ra

te(N

orm

aliz

ed F

R /

s)

StrongTin

StrongTout

Weak

a b cStrong Tin

Strong Tout

Weak




−400 −200 0

0.5

1

Time from saccade (ms)Nor

mal

ized

firin

g ra

te Accuracy condition

−400 −200 0

0.5

1

Time from saccade (ms)

Speed condition

0 20 40 60

0.8

1

1.2


Nor

mal

ized

FR

befo

re s

acca

de

Faster

Slower

Faster

Slower

a b c




−100 0 100 200 300

0.4

0.6

0.8

1

1.2

Time from motion onset (ms)

Nor

mal

ized

firin

g ra

te

−300 −200 −100 0 100Time from saccade (ms)

0 20 40 600.50.60.70.80.9


Prop

ortio

n co

rrect

0 20 40 60

0.4

0.5

0.6

0.7


Rea

ctio

n tim

e (s

)

Speed vs. Accuracya b

c

0 0.05 0.1 0.150

0.2

0.4

0.6

0.8

Decision time (s)

Urg

ency

sig

nal (

Frac

tion

boun

d)




Saccade

Motion

Targets

Fixation

Time

]

ReactionTime


Integration of priors in the decision-making process

0

0.2

0.4

0.6

0.8

1

Proportion positive choice

−50 −25 0 25 500

500

1000

1500

2000


Reaction time (ms)

0

0.2

0.4

0.6

0.8

1

−50 −25 0 25 500

500

1000

1500

2000


0

0.2

0.4

0.6

0.8

1

−50 −25 0 25 50200

400

600

800


Subject SK Subject LH Monkeys B, I, N, and S

50:50 prior80:20 prior



dt

Accumulatedevidence

A

-A

time

a

b

time

Decision variable

dt

Accumulatedevidence Bias signal

+

(Decision variable)

Momentaryevidence

Momentaryevidence

A

-A



−30 −20 −10 0 10 20 30−100

0

100

200

300

Motion Strength (% coh)−30 −20 −10 0 10 20 30

−100

0

100

200

300

Motion Strength (% coh)

Time from motion onset (ms) Time from motion onset (ms)

250 300 350 400 450 500 550−10

01020304050

Time from motion onset (ms)250 300 350 400 450 500 550

−60−40−20

020

Time from motion onset (ms)

Buildup rate (sp/s )

Time−dependent bias signal (sp/s)

Difference in buildup rate (sp/s )

a b

dc

2

2

300 400 500 600 7000

10

20

30

300 400 500 600 700−20

−15

−10

−5

0e f



Tim

e-de

pend

ent

Bia

s sig

nal (

sp/s

) Integration of priors in the decision-making process


−50 −25 0 25 500

0.2

0.4

0.6

0.8

1

−50 −25 0 25 50200

400

600

800


Proportion positive choice

Reaction time (ms)

a

b




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