The Stiles Crawford effect (SCE)

A failed instrument design led

to the discovery of the SCE

Stiles and Crawford designed a

pupilometer based on the

principle that pupil area is

directly proportional to

perceived brightness.

The design of the pupilometer had two

channels. Each channel illuminated one half

of a bipartite field. One half of the bipartite

field illumination was pupil diameter

dependent. The illumination of the other half

of the bipartite field was independent of pupil

diameter.

Pupil diameter independent

Pupil diameter dependent

Pupil diameter independent

Pupil diameter dependent

Eye’s pupilPupil diameter dependent beam

Pupil diameter independent

The subject’s task was to adjust the

luminance of the pupil independent

channel until the two fields

appeared equally bright. The

luminance of this channel by the

design theory was argued to be

directly proportional to the diameter

of the pupil.

Optical WedgeOpal Glass

X

Maxwellian View Lens

Light

source

Stiles and Crawford’s Pupilometer

Eye

When calibrated against

photographic measurements of

pupil diameter, they found their

instrument increasing

underestimated the pupil

diameter as pupil diameter

increased.

Detailed investigation revealed

that light entering the peripheral

portions of the pupil is less

efficient in eliciting a visual

response than light entering the

center of the pupil.

Eye’s Pupil

Pupil Entry (mm)

Data is often fit with a parabolic function

of the form:

ax2 + bx + c

LRS = ax2 + bx +c

σ = -a

xmax = -(b/2a)

The experimental finding has been

repeatedly verified confirming the

effect is real. However, verification

does not explain why light is not

equally efficient in eliciting a visual

response as a function of pupil

entry.

So why is light that enters the

peripheral regions of the cornea

less efficient in eliciting a visual

response?

Reflection at the optical surfaces of the eye?

Absorption differences?

Retinal effects?

A combination of a variety of effects?

Photoreceptor

Exit pupil of the eye

How would you test the hypothesis that

photoreceptors point toward the the center of

the pupil and not the center of the globe?

Eye’s Pupil

Rela

tive S

ensitiv

ity

Pupil Entry

Rela

tive S

ensitiv

ity

Pupil Entry

N T N T

Eye’s Pupil Ask Subject to

Fixate here

Rela

tive S

ensitiv

ity

Pupil Entry

Rela

tive S

ensitiv

ity

Pupil Entry

N T N T

Eye’s Pupil Ask Subject to

Fixate here

Rela

tive S

ensitiv

ity

Pupil Entry

Rela

tive S

ensitiv

ity

Pupil Entry

N T N T

-4

-3

-2

-1

0

1

2

3

4

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40

Test Probe location (degrees)

SC

E P

ea

k L

oc

ati

on

(m

m)

Nasal Retina Temporal Retina

Nasal

Tem

pora

l

Th

at’

s i

t fo

r th

e S

CE