Transcript Basic Visual Processes

Basic Visual Processes
Anatomy and physiology of the
visual pathway and a few other
pertinent facts to get us started
Anatomy of the eye
Visual angles
Distances in eyes are always given as
angles:
Thumb width at arm’s length= 1.5 degrees
Fist width at arm’s length=8-10 degrees
Hand span at arm’s length=20 degrees
One degree at retina=0.3 mm
Visual angle of one cone=0.0084 degrees = 0.5 minutes of arc
Measuring the effects of optics
Line and point spread functions

If we assume linearity, then we can use these
to predict what any stimulus presented to the
lens will produce on the retina
The point spread function
-Measuring these involves a number of very tricky problems
-essentially, what is involved is measuring the reflected light that
leaves the eye as a pinpoint of light is presented to it.
-remember that what we’re looking at here is what gets through
the optics and not necessarily what is seen.
Contrast and spatial frequency
Gratings
Square wave gratings
Sine wave gratings
Varying contrast and frequency
Contrast=Imax/Imin/2*Iav so
can be changed by
varying top or bottom of
fraction
-variation in spatial frequency is
usually given in cycles/degree
Spatial frequency
Jean Fourier
Any complex waveform can be
represented as the sum of a series
of sine waves of different
frequencies and amplitudes
Measuring optics
Modulation transfer function

A different and handy way to represent spread
functions
-what is measured is the
ability of the eye’s optics
to transmit variation in
contrast at a variety of
different spatial
frequencies
Measuring optics
Chromatic aberration
Because refraction by any
medium depends on
wavelength, the optical
quality of the eye varies
with wavelength.
Resolving power is lowest
at short wavelengths
The retina
Receptor subtypes
The beginning of vision
-presenting a flash of light to a photoreceptor produces
an electrical response
Disks in outer segments
called lamellae contain a
photopigment
TRANSDUCTION
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
Rhodopsin -- the magic photopigment
Through the wizardry of biochemistry, sodium channels close
The sensitivity of visual
transduction
7 photons produce a perceptible
response (Hecht, Schlaer and Pirenne)
1 photon closes about 106 ion channels
The receptor mosaic
Spatial frequency limits of the receptor
mosaic

-using interferometry to get past optics
-when light waves collide, the
effects can be destructive or
constructive. Thomas Young
showed that you can set up
conditions where these patterns
of construction and destruction
produce a fine grating. In fact,
this was an early, powerful
demonstration of the wave
nature of light.
Laser interferometry
-takes advantage of diffraction to
produce very fine gratings behind
eye’s optics
The receptor mosaic
Aliasing
-Nyquist frequency is
the spatial frequency
above which confusion
can occur.
-demonstration of
aliasing
The receptor mosaic
Can measure the sampling frequency of the cones using aliasing. The
prediction is that when the Nyquist frequency is exceeded, the apparent
spatial frequency of gratings should begin to decrease because of aliasing.
The frequency at which this happens depends on the types of cones
involved.
Long and Medium wavelength cones – 60 cycles/deg
Short wavelength cones – 3 cycles/deg
This corresponds to retinal spacing of 10 arc minutes for S-cones and 0.5
arc minutes for M and L cones
The S-cone mosaic
The S-cone mosaic
Spatial frequency of S-cones makes a nice
match with point spread function
Using adaptive optics to visual
living photoreceptors
Using the same technology
as in the Hubble telescope,
you can compensate for
optical imperfections using a
deformable mirror.
Spectacular
images of in
vivo
receptors
Cone mosaic summary
• It is possible to measure the optical properties of the eye
separately from visual function per se using various forms of
refractometry.
•It is possible to measure some simple properties of the
photoreceptor array using laser interferometry
•It is also possible to visualize the cone mosaic directly using
various staining techniques or, in vivo, using adaptive optics
•All of these measurements suggest ultimately that the receptor
array is a very good optimized match for the eye’s optics. In other
words, the organization of receptors is as good as, but no better
than, the eye’s optics
•There are significant differences between L and M cones and S
cones. Again, though, these differences match the way that the
eye’s optics respond to light with different spectral properties
Photoreceptor array
The images show how the photoreceptor array varies
with retinal eccentricity
The duplex retina
Retinal circuitry
Note important differences in
connections of rods and cones
Cones can often have fairly direct
access to retinal ganglion cells via
bipolar cells.
Rods most often synapse with rod
bipolar cells which, in turn,
synapse with amacrine cells
Note the midget and parasol retinal
ganglion cells.
Midget and parasol RG cells
Dendritic field sizes of RG cells
Parasol dendritic fields are much larger than
midget fields.
It takes many more midget cells to cover
visual field than parasol cells
Lateral geniculate nucleus
About 90-95% of retinal
ganglion cell axons land
here.
Layers 1 & 2 are
magnocellular and layers
3-6 are parvocellular
So-called intercalated
layers (between 6 layers)
may be a special
koniocellular pathway,
dedicated to S-cone
transmission
M and P pathway response to
contrast
*Note that these show response to contrast and not illumination. This is a
very important distinction – visual systems respond (non-linearly) over about
5-6 orders of magnitude for illumination but the range of illumination in a
typical scene is rarely more than one order of magnitude. By and large,
visual systems are linear for contrast.
Behavioural effects of lesions to
P or M layers of monkey LGN
Spatial frequency effects of P
Filled circles = magnocellular lesion
Open circles = parvocellular lesion
Temporal frequency effects on both
but M system very important for high
frequencies
Retinal ganglion cell receptive
fields
Steven Kuffler
Linear summation of centre and
surround responses
Graphical representations of the
receptive field
1-d
2-d
Can measure CSF for single cells
-low frequency dropoff tells about size of RF
-high frequency dropoff tells about size of centre of RF
CSF for a typical LGN cell
Adding the time dimension
Notice that temporal properties of centre and surround stimulation differ a bit
One way of describing this is to say that such receptive fields are spacetime inseparable. If they were separable, the spatial profile of the rf would
differ over time only by a scalar value. There ARE rfs like this, but not in
retina or LGN.
Phase reversing gratings
-this is one common way to measure the temporal properties of rf’s
Notice how CSF varies depending
on both spatial and temporal
frequency
The DOG operator
-this is a very common and useful way of modelling the spatial and
temporal structures of receptive fields
Adaptation and contrast
normalization
-Notice that the CSF does depend on intensity, but only really for low
intensities. At higher intensities, contrast sensitivity changes very slowly.
The retinogeniculostriate pathway
The structure of cortex
Cortex is laminar and connections are very precisely organized
Orientation selectivity
Hubel and Wiesel’s
simple hierarchical model
of visual cortical processing
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
Colour coding in blobs
-non-oriented colour opponent cells
A somewhat opposing view
Blobs represent convergence of iso-orientation contours so orientation
preference is less obvious. Also, because these regions would be active
with all contours, one might expect overall higher cytochrome oxidase
levels
Directional
selectivity
Disparity sensitivity
V2 and cytochrome oxidase stripes
Lennie’s view
A portrait of the visual cortical systems based on simple considerations of area
“Specificity” of rf properties
Contrasting views