Light and dark adaptation •  The range of light levels over which we can see •  Why do we need adaptation? •  The design problem •  Role of pupil changes •  The duplex retina: four comparisons of rod-based

and cone-based vision •  Dark adaptation and pigment bleaching •  Light adaptation:

Weber’s Law The response of visual neurons

Luminance and retinal illumination

The range of luminances (left) and retinal illumination (right) found in the natural world

Sun

Fluorescent light/bulbs

White paper, full sunlight Candle flame

Comfortable reading

Print read with difficulty White surface, moonlight Threshold for cone vision

White surface, moonless night

Visual threshold

Rod and cone operating ranges

cone vision

day

LUMINANCE RANGE

rod vision

Light reflected from a surface under low and high illumination

The ratio of light intensities reflected from the white surround and the black letter is 9:1 under both low and high illuminations.

Consider and comment:

A white page inside a room reflects less light than a black stone on a sunny beach, yet the page looks white and the stone looks black

The sensations of blackness and lightness depend on the contrast of the stimulus, not on the absolute amount of light reflected off any one part of it.

Luminance difference between ‘L’ and background = 80 units 8000 units

Background = 90 units 9000 units

Response ∝ Difference/Background 80/90 (89%) 8000/9000

(89%)

A piece of white paper that is dimly lit (A) looks white because its luminance lies at the top of its local scale, even though this luminance may be less than that of a piece of black paper that is brightly lit (B).

log illuminance

log

lum

inan

ce

The eye’s sliding scale of brightness

Mechanisms that enable us to see over a wide range of light intensities: •  Pupil changes

•  Duplex retina: rods & cones

•  Dark adaptation & pigment bleaching

•  Light adaptation of the visual system and individual neurons (Webers Law)

Rods & cones: 4 key differences between scotopic and photopic vision •  Contrast sensitivity

•  Distribution of rods and cones

•  Spectral sensitivity of rods and cones

•  Sensitivity to light of rods and cones.

1. Contrast sensitivity functions at three different light levels

Spatial Frequency (cycles/mm on retina)

Spatial frequency (cycles/degree) Se

nsit

ivit

y (1

/thr

esho

ld c

ontr

ast)

2. Distribution of rods and cones visual eccentricity (deg)

spatial density (cells/square mm)

macula lutea

cones

rods

retinal eccentricity (mm)

Convergence

receptors130 million

bipolars20 million

ganglion cells1 million

optic nerve fibres

3. Spectral sensitivity curves for rod and cone vision

Rela

tive

sen

siti

vity

Wavelength (nm)

Purkinje effect

•  A shift in the colour appearance at dusk. •  Reds look darker, blues look brighter

4. Sensitivity to light of rods & cones: Dark Adaptation

Low Lo

g. l

ight

sen

siti

vity

High Time in dark (min)

7 minutes

Duplex function

•  1. Rods are more sensitive than cones (x50)

•  2. There are more rods than cones (x10)

•  3. Ganglion cells have larger RFs for rods than cones (i.e. more post-receptoral summation)

Dark adaptation curves Low Lo

g. l

ight

sen

siti

vity

High Time in dark (min)

Cones Rods

The “design” problem •  To detect differences in luminances across

the visual scene •  Scale the response to these differences

according to the ambient light level

In solving the problem, we must:

1. Have good sensitivity to luminance differences 2. Be able to operate across a wide range of

ambient light levels 3. Cope with a limited neural response range

Contrast sensitivity and operating range

Large operating range but poor contrast sensitivity

Relative light intensity Intensity

Brig

htne

ss

Brig

htne

ss

Good contrast sensitivity but small operating range

Increment threshold curve

WEBER’S LAW

Log background intensity (I)

Log

incr

emen

t th

resh

old

(del

ta I

)

Weber’s Law

∆ I/I = constant

•  Our sensation is determined by the percentage difference in the luminance of a surface relative to its background

•  This holds over a wide range of background (ambient) luminances

I1 I2 I3 Re

spon

se (i

ps)

1 1.5 10 15 100 150 Flash intensity log

Mean light levels

I I

I = 0.5 I = 5 I = 50

A single neuron can shift its operating range according to the mean light level. The light increment (delta I) required to obtain a criterion response is scaled up or down, according to the mean light level. This is known as GAIN CONTROL.

Ganglion cell adaptation

Receptoral adaptation