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The visual
system
Chapter 10
The physical stimulus
Light is a wave…
…and a particle
Psychological dimensions of
light
Hue
Saturation
Brightness
The eye
•Cornea – the main focusing
element
•Lens – adjustable focusing
•Iris – adjust sensitivity and
depth of focus
•Retina – photosensitivity
and much, much more
Structure of the retina
Visualoftransduction
The beginning
vision
a flash of
light in
to photoreceptors
a photoreceptor(hyperpolarization)
produces
Photons-presenting
produce electrical
events
an electrical response
In darkness, there’s a
continuous current in the
outer segment caused by
the circulation of sodium.
In light, sodium circulation
slows down and receptors
hyperpolarize
Disks in outer segments
called lamellae contain a
photopigment
Rhodopsin -- the magic photopigment
Through the wizardry of biochemistry, sodium channels close
Photoreceptors come in different flavours
Spectral absorption curves
Lateral interactions
in the retina help with
several problems
1. Contour sharpening
2. Enhancing sensitivity
Mach bands
A slightly misleading
illustration
We understand the
neural basis of lateral
inhibition because of
work on the horseshoe
crab that is not feasible
in mammals
The duplex retina
The cost of the duplex retina
Central visual pathways
The lateral geniculate
nucleus of the thalamus
Primate lateral geniculate nucleus
Centre-surround antagonism is
the mammalian analogue of
lateral inhibition.
Hubel and Wiesel’s
simple hierarchical model
of visual cortical processing
Simple cells
Complex cells
Columnar organization of VI
Ocular dominance
The hypercolumn
Optical imaging of ocular dominance columns
Optical imaging of orientation tuning
Correlation between optical imaging and
electrophysiological results for orientation tuning
Margaret Wong-Riley and
the cytochrome oxidase story
•autoradiography and activity
•cytochrome oxidase and activity
•intrinsic variability in cyo
Cytochrome oxidase in monkey VI and VII
-blobs and stripes of every stripe
V2 and cytochrome oxidase stripes
Multiple visual representations in cortex
Visual agnosias
•
•
•
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Motion blindness
Prosopagnosia
Cortical colour blindness
Visual object agnosia
Visual processing streams I
Schneider’s Experiment
Tectal undercut
Cortical ablation
Visual processing streams II
• Gordon Holmes
– single patient studies -- it
was obvious that people
without conscious vision
were not ‘blind’
Visual processing streams III –Weiskrantz and
blindsight
In a preliminary test, Weiskrantz positioned a stick in
D.B.s blind spot, either sideways or straight up and down.
He asked D.B. what he saw. The patient said, "I see
nothing."
Weiskrantz persisted. "Am I holding the stick sideways, or
vertically?"
D.B.: "I don't know -- I don't see a stick."
Weiskrantz: "Guess."
D.B.: "Sideways."
Weiskrantz: "Now which way am I holding it?"
D.B. "I don't see a stick."
Weiskrantz: "Guess."
This continued for 20 trials in which D.B.'s performance
was perfect.
Weiskrantz recounts:
"In the interview that followed, and which
was recorded, D.B. expressed considerable
surprise. 'Did you know how well you had
done?', he was asked. 'No,' he replied, 'I
didn't -- because I couldn't see anything; I
couldn't see a darn thing.' 'Can you say
how you guessed -- what it was that
allowed you to say whether it was vertical
or horizontal?' 'No, I could not because I
did not see anything; I just don't know.' (p
24)."
Pohl’s Experiment
Ungerleider and Mishkin’s
Two visual cortical streams
Milner and Goodale’s “Posting”
experiment