Lecture 2:
Early Visual Processing
Read Gazzaniga Chapter 5,
http://homepage.psy.utexas.edu/homepage/class/341K/hayhoe/2016
Date
Topic
Jan 19
Overview of the course: understanding human actions
Jan 21
The Perception Action Cycle: some examples. (Guest
lecture: Dr Matthis)
Jan 26 Early Visual Processing: the eye and visual cortex
Jan 28. Central visual processing: dorsal and ventral streams,
posterior parietal cortex.
(Consequences of stroke in these areas)
Feb 2
Walking and catching: class experiment.
(one versus two eyes)
Visual Perception: what do we want to explain?
How do we get visual information from the world and use it
to control behavior?
What neural processes underlie visually guided behavior?
Different levels: anatomy, neurophysiology, behavior
The eye: turning light into visual signals.
Many basic features of visual perception are determined
by the structure and function of the eye.
The Eye and Retina
A small eye rotation translates into a big change in visual angle
Visual Angle
x
18mm
a
d
tan(a/2) = x/d
a = 2 tan 1 x/d
1 diopter = 1/focal length in meters
0.3mm = 1 deg visual angle
55 diopters = 1/.018
Fundus of the right eye of a human
Origin of ‘red-eye” effect
Types of refractive error
Most of the refraction is done by the cornea.
Presbyopia: stiffening of the lens with age.
Snellen Chart
Photoreceptors
Bipolar cells
Ganglion cells
M= magnocellular, P= Parvocellular
light
Rods and cones
Night vision
Daytime vision
Photoreceptor density across the retina
Cone Photoreceptors are densely packed in the central fovea
Foveal cones are about 0.5 min arc
Snellen Chart – at 20/20
D
Foveal cone diameter is approximately the width of the lines
Hundreds of rods converge onto one bipolar cell
rods
rod bipolar
……
cones
cone bipolar
Little or no convergence in cones
Convergence leads to greater sensitivity (cf catchment area)
Photoreceptors
Bipolar cells
Ganglion cells
M= magnocellular, P= Parvocellular
light
Convergence: many rods converge onto a single rod bipolar cell, and several cones
converge onto a diffuse bipolar cell. This allows the signal to be amplified.
Visual Acuity matches photoreceptor density
Relative visual acuity
Receptor density
Color Vision is a consequence of having 3 different cone types
Blue, green, and red dots
represent the three different
cone types, of a living human
being in a patch of retina near
the fovea.
Each cone type has a different
spectral sensitivity profile.
Color appearance results form
the different pattern of activity
across the 3 cone types.
Consequences of Macular Degeneration
scotoma
The “macular” is the region around the central fovea. Macular degeneration is a
disease of aging but may also occur in the juvenile form.
Fundus of a patient with retinitis pigmentosa
Retinitis is an inherited disease that progresses through early adulthood and leads to
blindness. Gene therapy treatments are being developed. (May co-occur with deafness.)
Other eye diseases:
Glaucoma – high intra-ocular pressure
Diabetic retinopathy
Retinal detachment.
Photoreceptor response to light is sluggish
This is why your computer monitor doesn’t flicker.
Photoreceptor response to light
Cones are much faster than rods – don’t play tennis at dusk!
Retinal ganglion cell receptive fields: how signals are organized
after the receptors.
The receptive field of a cell is the region in space where light leads to a response in
the cell.
Center-surround organization of receptive fields means that light in the center and
light in the surrounding region have opposite effects
Retinal ganglion cell receptive fields
ON and OFF center cells
(What are they for?)
Low sensitivity to blurred images leads to fading
Light Adaptation
The light levels the eye can respond to covers a range of 1010
However, the ganglion cells can only respond between 1 and 200
impulses per second (approx). Therefore the retina must adjust
its sensitivity so that it can signal meaningful changes around a
mean level.
This adjustment is called light adaptation.
The corresponding changes when light level decreases is called
dark adaptation
Pupil can constrict to reduce the amount of light entering the eye.
demo
Major transformations of the light signal in the retina:
1. Anatomical organization of photoreceptors provides high acuity in fovea with
rapid fall-off in the periphery. (photoreceptor density)
2. Color Vision: 3 cone photoreceptors types.
3. Light adaptation – sensitivity regulation - adjustment of operating range to mean
light level. (Light level 1010 range, ganglion cells, 102 range.)
4. Sluggish response – reduced response flickering lights – Temporal
summation/integration – a strong 1 msec flash is equivalent to a weaker 50 msec flash.
5. Convergence of photoreceptors onto ganglion cells also leads to acuity limitations
in the peripheral retina. (1 cone per midget cell in fovea)
Projections of visual signals from retina to visual cortex
The mapping of objects in space onto the visual cortex
Visual consequences of lesions at different locations in the visual p
contralateral hemianop
(cf Homonymous hem
Foveal sparing
Eye movements
Retinotopic Organization and Cortical Magnification
Adjacent points in the world
project to adjacent points in cortex
The brain uses more
physical space for signals
from the fovea than
the periphery
Lateral Geniculate Nucleus
In the thalamus
Two kinds of cells in retina project
to different layers in LGN
M=magno=big
P=parvo=small
K= konio
Signals from each eye are
adjacent in LGN but remain
segregated in different layers
Convergence occurs in V1.
Primary Cortical Sub-divisions
Visual cortex is a layered structure (6 layers). LGN inputs arrive in Laye
to higher visual area., Layers 5,6 output to sub-cortical areas (eg superio
Massive feedback projection from layer 6 to LGN – 800 lb gorilla. Also
inputs from extra-striate cortex)
Cells in V1 respond to moving or flashing oriented bars. Little response t
steady lights. (Note this established the paradigm for neurophysiol invest
vision)
LGN cells have circular receptive fields, like retina.
Not clear what the role of the LGN is.
(Murray Sherman – gates input to cortex)
Oriented cells emerge in V1, probably composed
Of appropriately aligned LGN cells as shown. (Usrey –
dual cell recordings)
Orderly anatomical organization in V1
Cells in V1 are organized into
columns. Orientation preference
gradually changes as one progresses
across cortex. Cells at different depths
Have same orientation preference.
Binocular convergence:
Cells respond more or less to R and L
eye inputs. Ocular dominance varies
smoothly across cortical surface
orthogonal to orientation variation
Note: ocular dominance not equal
to disparity sensitivity (stereo).
Regular large scale organization
of orientation preference across
cortical surface. Does this
simplify signal processing?
What is V1 doing?
Early idea: edge detectors – basis for more complex patterns
Later (1970-80’s) – spatial frequency channels
any spatial pattern can be composed of a sum of sinu
Late 90’s to now:
Main idea about V1 is that it represents an efficient recoding of the
the visual image.
Images are not random. Random images would require point-by-po
like a camera.
Images have clusters of similar pixels and cells designed to pick th
information about spatial variation at different scales (clusters of d