Vision

1
Vision

• Chapter 2 and 3 in Sensation and Perception

2
Recap and lab work wk 2

• E-mail check
• confirm lab and tutorial times
• Lab work for week 2
• Signal Detection Procedure and Theory Appendix
  A in the lab book attach results and answer all
  questions
• Others
• 3-D virtual brain Structure of a Neuron Nerve
  Impulse Coding and Stimulus Strength Synaptic
  Strength
• Keep all results as I will be collecting the
  books for marking at various stages during the
  semester.
• recap

3
Light energy into electrical energy

• Light on the retina is converted into electrical
  energy by the visual receptors how?
• A process called visual transduction involves
  the rods and cones
• Distribution in the fovea and peripheral retina
  (Figure 2.15)
• The Outer segment of the visual receptor contains
  stacks of discs which contain the visual pigment
  molecules

4
(No Transcript)
5
Light to electricity

• There are two components in the visual pigment
  molecules opsin and a light sensitive molecule
  called retinal
• Retinal is attached to opsin and is responsible
  for visual transduction
• Retinal absorbs a photon of light and changes its
  shape isomerisation leading to an electrical
  signal

6
Dark adaptation

• The increase sensitivity that takes place as the
  eyes stay in the dark allows us to function in
  dim illuminations
• Initial rapid stage adaptation of cone
  receptors
• Second slower stage adaptation of the rod
  receptors
• Indicated in the dark-adaptation curve

7
(No Transcript)
8
Measuring dark adaptation

• The observer adapts to intense light
• The observer then adjusts the intensity of a test
  light so that he/she can just see it
• Intense light extinguished process of dark
  adaptation begins
• The course of dark adaptation is usually measured
  by the observer adjusting the intensity of the
  test light so it is just visible

9
Dark adaptation

• Our sensitivity to light depends on the
  concentration of the visual pigments.
• The speed at which our sensitivity is adjusted in
  the dark depends on a chemical reaction the
  regeneration of the visual pigments
• Detached retina condition

10
Spectral Sensitivity

• Spectral sensitivity - An observers sensitivity
  to light at each wavelength across the visible
  spectrum

11
(No Transcript)
12
(No Transcript)
13
Q

• Why is our perception of colours and details
  worse in dim illumination than in bright
  illumination?
• Next time you are walking in the dark take note
  of the colours you can see!
• Look up the Purkinje shift.

14
The visual neurons and perception

• The retina converts the light reflected from the
  object we are looking at into electrical signals
• These signals travel through the network of
  neurons in the retina and then out the back of
  the eye in the fibres of the optic nerve.
• How are the electrical impulses in the neurons
  transformed into our experience of seeing? a
  work in progress but there is a lot we do know
• Structure of the retina important for aspects
  of perception such as visual acuity and
  sensitivity

15
The retina

• Neurons in the brain are highly interconnected
• In the retina rod and cone receptors send signals
  to bipolar cells and these send signals to the
  retinal ganglion cells (1 million of these).
• The axons of the retinal ganglion cells form the
  optic nerve
• There are also horizontal cells and amacrine
  cells that transmit signals horizontally across
  the retina

16
(No Transcript)
17
The retina

• Convergence is a process where many neurons send
  signals to one neuron.
• We will discuss how convergence can influence our
  sensitivity to light and our ability to perceive
  details
• Convergence occurs in the retina 126 million
  receptors and just 1 million ganglion cells so
  each ganglion cell receives signals from many
  receptors
• Signals from the rods converge more (leads to
  better sensitivity) than signals from the cones
• No converge in the fovea between cones and
  ganglion cells important for visual acuity

18
The retina

• The importance of convergence for perception
• Better sensitivity - rods
• Better visual acuity cones
• Greater converge in the rods than cones rod
  vision is more sensitive (i.e. our ability to see
  in the dark) than cone vision

19
(No Transcript)
20
Seeing the details

• Cones are good for visual acuity our ability to
  see details in a scene
• Fovea all cones good for visual acuity
• Converge in the rods decreases their ability to
  resolve details - difficulty reading in dim light
• Convergence influences how sensitive we are to
  light and our ability to perceive details

21
(No Transcript)
22
Lateral inhibition

• Inhibition stimulating one neuron can decrease
  the response (firing activity) in another -
• Lateral inhibition transmitted laterally across
  the retina
• Lateral inhibition may influence how we perceive
  light and dark
• Perceptual phenomena that have been explained by
  lateral inhibition Hermann grid, Mach bands

23
(No Transcript)
24
(No Transcript)
25
Simultaneous contrast

• Simultaneous contrast when our perception of
  the brightness or colour of one area is affected
  by the presence of an adjacent or surrounding
  area
• What is the physiological mechanism responsible
  for this effect?
• Can be explained in part using lateral inhibition

26
(No Transcript)
27
(No Transcript)
28
Simultaneous contrast

• An explanation based on lateral inhibition
• But may not be the whole story
• Other visual contrast illusions that may not be
  explained by lateral inhibition Benary cross
  and Whites illusion
• Our perception of light and dark not completely
  determined by lateral inhibition

29
Receptive field

• The receptive field of a visual neuron is the
  area on the retina that influences the firing
  rate of the neuron.
• Excitatory area of the neurons receptive field
• Inhibitory area of the neurons receptive field
• Centre-surround receptive field

30
(No Transcript)
31
(No Transcript)