Perceptual Systems
Readings, Class 1:
Wolfe Ch 1,2
Kandell et al Chapters 26
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 edGazzaniga, Ivry, Mangun Cognitive Neuroscience Norton, 3rd ed
Visual Perception: what do we want to explain?
How do we get visual information from the world and use it to controlbehavior?
Traditional sub-areas - visual sensitivity color visionspatial visiontemporal visionbinocular vision/ depth
perceptiontexture perceptionmotion perceptionsurfaces, segmentationobject perceptionattentionperceptual learningspatial orientation
eye movements
The constructive nature of perception: a process of guessing the state of the worldfrom 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
Pigment epithelium reduces scatterImportant vegetative functions
iris
pupil
Note – blind spot - cf damage to peripheral retinaVisualize retinal blood vessels.
18mm
0.3mm = 1 deg visual angle
x a
tan(a/2) = x/da = 2 tan-1 x/d
Visual Angle
d
1 diopter = 1/focal length in meters
55 diopters = 1/.018
Most of the optical power of the eye is accomplished by the cornea
Optical correction errors
Presbyopia = stiffening of lens with age so it is no longer variable
Blur circle
Figure 2.9 Photoreceptor density across the retina
Note: color vision in peripheral retina Note: peripheral cones are fatter. Rods similar tofoveal 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)
Sine wave gratings
Acuity is the highest frequency pattern that is just visible – ie the narrowest stripesA similar measure is made by the Snellen letter chart: E
Vernier acuity is an order of magnitude better than grating acuity.How can this be?
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)
Why blue flowers are brighter and red flowers are darker at dusk.
Peaknight day
Note: peak sensitivity in day aboutthe same wavelength as maximumoutput of sun.
Convergence: many rods converge onto a single rod bipolar cell, and several cones convergeonto a diffuse bipolar cell. This allows the signal to be amplified.
M= magnocellular, P= Parvocellular
Midget system preserves acuity inthe central fovea
Horizontal and amacrine cells form inhibitory surrounds of ganglion cells.
Why ON and OFFcells?
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 aretinal 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
Center-surround organization means that responses to uniform lights are reduced
Light spot excites cellDark spot excites cell
Biggest response to a spot in center
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 inCycles 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 regionsThis 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 problemNeed to respond over a range of 1010 – but ganglion cells can only signal 0-200 spikes/sec
Receptor adaptation
Response on different background intensities
tvi curve
ΔI/I = 1
Ganglion cells change sensitivity as well as photoreceptors.
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
The brain uses more physical space for signals from the fovea thanthe periphery
Adjacent points in the worldProject to adjacent points in cortex
Signals from each eye areadjacent in LGN but remainsegregated in different layers.Convergence occurs in V1.
Two kinds of cells in retina projectto different layers in LGN
M=magno=bigP=parvo=smallK= konio
Magno and parvo cells have different spatial and temporal sensitivities.
Function of the differentM and P pathways isunclear.
Note: attempts to Isolate a pathwaypsychophysically 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 thesame across the visual field.
Figure 2.11 Blue, green, and red represent the S-, M-, and L-cones, 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)