Retinal Processing

1
Retinal Processing

• Structure of the Retina
• Photoreceptors
• Resolution
• Sensitivity
• Dark Adaptation
• Retinal Ganglion Cells
• Receptive Fields
• Perceptual Effects
• Local Contrast as Information

2
The Retina

•  how do retinal factors affect our perception?
• structure of the retina
• photoreceptors
• rods
• cones
• isomerization

3
Isomerization

• When a photon strikes the visual pigment, it
  ceases to exist as a photon and its energy is
  absorbed by the photoreceptor cell. This leads to
  a change in the electrical polarization of the
  receptor this is transferred to amacrine,
  horizontal bipolar cells eventually to
  ganglion cells.

4
Photoreceptors

• distribution of photoreceptors
• fovea 
• photoreceptors sample the retinal image
• aliasing

5
Resolution

• detail - smaller receptive fields means smaller
  features can be resolved
• visual angle - degrees, minutes, seconds
• visual acuity
• the ability to resolve small detail

6
Visual Acuity

• the ability to resolve small detail
• 20/20 means ability to see letters having line
  width of 1 min arc at 20 ft.
• (entire letter 5 min)

7
Sensitivity

• detection of light
• spatial summation Ricco's Law
• trade-off between area of stimulus and stimulus
  strength for small spots of light that are
  equally detectable.
• temporal summation Bloch's Law
• trade-off between intensity and duration of
  equally detectable lights.

8
Dark Adaptation

• how and why does sensitivity change over time?
• photopic and scotopic state
• duplex retina
• rod/cone sensitivity
• Purkinje shift

9
Retinal Ganglion Cells

•  detail of retina
• receptors, bipolars, amacrine, horizontal
  ganglion cells
• retinal ganglion cells
• collect responses from several receptors
• maintained discharge or spontaneous activity

10
Retinal ganglion cells

• Single cell recordings are used to determine the
  optimal stimuli for the ganglion cells
• A cells receptive field is the patch of retina
  that will respond to stimulation

11
Receptive Fields

• convergence
• lateral inhibition
•  
• centre/surround antagonism
• on center, off center

12
Receptive Fields

• Receptive fields are composed of ON regions and
  OFF regions
• ON regions respond to increases in light, OFF
  regions respond to decreases in light

13
ON and OFF regions

• ON and OFF regions are antagonistic stimulation
  of each produces opposite effects
• The interaction between these regions is called
  lateral inhibition

14
Retinal Processing

• Structure of the Retina
• Photoreceptors
• Resolution
• Sensitivity
• Dark Adaptation
• Retinal Ganglion Cells
• Receptive Fields
• Perceptual Effects
• Local Contrast as Information

15
Lateral Inhibition

• Photoreceptors
• presence of light produces drop in glutamate
• Inverting bipolar cells (On Centre)
• excited by drop in glutamate
• i.e. excited by light
• Non-inverting bipolar cells (Off Surround)
• inhibited by drop in glutamate
• (i.e. inhibited by light)
• therefore excited by dark

16
Retinal Processing

• Structure of the Retina
• Photoreceptors
• Resolution
• Sensitivity
• Dark Adaptation
• Retinal Ganglion Cells
• Receptive Fields
• Perceptual Effects
• Local Contrast as Information

17
Receptive field size

• Receptive field size varies with retinal
  location center fields are small, distant field
  (far from fovea) are large
• Receptive fields determine which stimuli will
  optimally activate each cell

18
Sensitivity versus Resolution

• How do we resolve the tradeoff between resolution
  and sensitivity?
• The Duplex Solution ? rods and cones provide two
  different kinds of film

19
The Duplex Solution

• Rod photoreceptors provide scotopic vision
• High degree of convergence
• Sensitive to low levels of light
• Numerous in the periphery of the retina

20
The Duplex Solution

• Cone photoreceptors enable photopic vision
• Low degree of convergence
• High resolution, require high levels of light
• Numerous in the center of the retina

21
Modelling the Responses

• Using Gaussian distribution
• Difference of Gaussians

22
Retinal Processing

• Structure of the Retina
• Photoreceptors
• Resolution
• Sensitivity
• Dark Adaptation
• Retinal Ganglion Cells
• Receptive Fields
• Perceptual Effects
• Local Contrast as Information

23
Perceptual Consequences

• Neural basis for perceptual effects

24
Perceptual Consequences

• lightness contrast
• Hermann grid
•  
• lightness constancy
•  

25
Lightness contrast

• Objects of equal intensity may appear different
  in lightness
• This is called Lightness Contrast

26
Lightness constancy

• Objects of different intensities sometimes
  appear equal in lightness
• This is called Lightness Constancy

27
Retinal Processing

• Structure of the Retina
• Photoreceptors
• Resolution
• Sensitivity
• Dark Adaptation
• Retinal Ganglion Cells
• Receptive Fields
• Perceptual Effects
• Local Contrast as Information

28
Local Contrast as Information

• cells as "carriers of information"
•  
• cells that encode light level vs. cells that
  encode contrast