Transcript lecture1PercSys

Perceptual Systems
Sources:
Wolfe, J, Kluender, K, Levi, D. et al Sensation & Perception 2012 3rd ed Sinauer – 15% discount
and free shipping if ordering online from Sinauer
Kandel, Schwartz & Jessel Principles of Neural Science McGraw-Hill 5th ed
Gazzaniga, Ivry, Mangun Cognitive Neuroscience Norton, 3rd ed
Readings, Class 1:
Wolfe Ch 1,2
Kandell et al Chapters 26
Visual Perception: what do we want to explain?
How do we get visual information from the world and use it to control
behavior?
Traditional sub-areas -
visual sensitivity
color vision
spatial vision
temporal vision
binocular vision/ depth perception
texture perception
motion perception
surfaces, segmentation
object perception
attention
perceptual learning
spatial orientation
eye movements
The constructive nature of perception: a process of guessing the state of the world
from sometimes incomplete sensory data.
Constructive in the sense that it relies on memory representations of past experience
Major transformations of the light signal in the retina:
1. Temporal filtering – visual response slower than input signal.
2. Spatial filtering – local signals are combined across space to varying degrees.
3. Light adaptation – retina modifies responsiveness depending on average light level.
4. Color coding – trichromacy and color opponency
The Eye and Retina
iris
pupil
Note – blind spot - cf damage to peripheral retina
Visualize retinal blood vessels.
Pigment epithelium reduces scatter
Important vegetative functions
Most of the optical power of the eye is accomplished by the cornea
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
Optical correction errors
Blur circle
Presbyopia = stiffening of lens with age so it is no longer variable
Figure 2.9 Photoreceptor density across the retina
Note: color vision in peripheral retina
Note: peripheral cones are fatter. Rods similar to
foveal cones
Visual Acuity matches photoreceptor density
Relative visual acuity
Receptor density
1 foveal cone= 0.5 min arc
Two of the factors limiting visual acuity are
- optics of the eye
- size and spacing of photoreceptors
- (in central fovea, a cone is about 0.5 min arc)
- Grating versus vernier acuity: Snellen (letter chart versus
threading a needle)
Vernier acuity is an order of magnitude better than grating acuity.
How can this be?
Sine wave gratings
Acuity is the highest frequency pattern that is just visible – ie the narrowest stripes
A similar measure is made by the Snellen letter chart:
E
Figure 2.9 Photoreceptor density across the retina
Question: Rods are small and dense. Why isn’t acuity better in the peripheral retina?
Transduction: light into electrical signals
“dark light”
Note sluggish response
Major transformations of the light signal in the retina:
1. Temporal filtering – visual response slower than input signal.
photoreceptor response is slow – increases sensitivity
2. Spatial filtering – local signals are combined across space to varying degrees.
Acuity for fine patterns determined by optics and photoreceptor layout.
3. Light adaptation – retina modifies responsiveness depending on average light level.
4. Color coding – trichromacy and color opponency
Probability of absorption of a photon depends on wavelength
(but receptor doesn’t know what wavelength it absorbed)
Note: peak sensitivity in day about
the same wavelength as maximum
output of sun.
Peak
night day
Why blue flowers are brighter and red flowers are darker at dusk.
Midget system
preserves acuity in
the central fovea
M= magnocellular, P= Parvocellular
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.
Horizontal and amacrine
cells form inhibitory
surrounds of ganglion cells.
Why ON and OFF
cells?
Hecht, Schlaer, & Pirenne, 1942
A single quantum is sufficient to excite a rod photoreceptor.
A few quanta within a small area is sufficient to give a sensation of light.
Measure number of quanta for a just detectable sensation of light – about 100 quanta.
Of those 100 quanta, about 90 are lost on the way to the retina form scatter in the eye.
So 10 quanta incident on the retina lead to a sensation of light.
Light has a Poisson distribution, so the probability that more than one photon falls on
a single rod is very small. Therefore, a single photon must excite a rod, and 10 photons excite a
retinal ganglion cell. This signal is transmitted to the brain with minimal loss and generates a
sensation of light.
Center-surround organization of bipolar and ganglion cells
Light spot excites cell
Dark spot excites cell
Biggest response to a spot in center
Center-surround organization means that responses to uniform lights are reduced
Figure 3.6 Sine wave gratings illustrating low (a), medium (b), and high
(c) spatial frequencies
These grating stimuli are called “Gabor patches”. Spatial frequency is measured in
Cycles per degree, and contrast is a measure of the difference in intensity between
light and dark bars.
Figure 3.7 The contrast sensitivity function (red line): the window of visibility
Perceptual consequences of center surround antagonism
Brightness is coded by the differences in illumination between adjoining regions
This results from center-surround organization.
Perceptual consequences of center surround antagonism
Brightness is coded by the differences in illumination between adjoining regions
Major transformations of the light signal in the retina:
1. Temporal filtering – reduced response to high temporal frequencies – Temporal
integration – a strong 1 msec flash is equivalent to a weaker 50 msec flash.
2. Spatial filtering:
- Anatomical organization of photoreceptors provides high acuity in
fovea with rapid fall-off in the periphery. (photoreceptor density)
-Convergence of photoreceptors onto ganglion cells also leads to
acuity limitations in the peripheral retina. (1 cone per midget cell in fovea)
- Center-surround antagonism reduces sensitivity to uniform fields.
3. Light adaptation
4. Color coding
Light adaptation: the problem
Need to respond over a range of 1010 – but ganglion cells can only signal 0-200 spikes/sec
Ganglion cells change
sensitivity as well as
photoreceptors.
Response on different
background intensities
tvi curve
ΔI/I = 1
Receptor adaptation
Perceptual consequence of light adaptation: hard to tell ambient light intensity
Loss of sensitivity at low temporal frequencies (slow rate of change of intensity) is a
consequence of light adaptation (sensitivity changes with average light level)
(afterimage fading)
Figure 2.17 Dark adaptation curve
Sensitivity recovers when the retina is in the dark, rapidly for cones, slowly for rods.
(afterimages)
Major transformations of the light signal in the retina:
1. Temporal filtering – reduced response to high temporal frequencies – Temporal
integration – a strong 1 msec flash is equivalent to a weaker 50 msec flash.
2. Spatial filtering:
- Anatomical organization of photoreceptors provides high acuity in
fovea with rapid fall-off in the periphery. (photoreceptor density)
-Convergence of photoreceptors onto ganglion cells also leads to
acuity limitations in the peripheral retina. (1 cone per midget cell in fovea)
- Center-surround antagonism reduces sensitivity to uniform fields.
3. Light adaptation – sensitivity regulation - adjustment of operating range to mean
light level. (Light level 1010 range, ganglion cells, 102 range.)
4. Color opponency. Organization of 3 cone photoreceptors into color opponent
signals (Luminance, Red-Green, Yellow-Blue)
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
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.
Magno and parvo cells have different spatial and temporal sensitivities.
Function of the different
M and P pathways is
unclear.
Note: attempts to
Isolate a pathway
psychophysically were
unsuccessful
Figure 2.17 Dark adaptation curve
Cone Photoreceptors are densely packed in the central fovea
Note: despite lower density of cones in peripheral retina, color vision is basically the
same across the visual field.
Figure 2.11 Blue, green, and red represent the S-, M-, and Lcones, respectively, of a living human being in a patch of retina at
1 degree from the fovea
Two of the factors limiting visual acuity are
– optics of the eye
- size and spacing of photoreceptors
- (in central fovea, a cone is about 0.5 min arc)