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)