Vision

Question 1

What is perception?

Answer 1

Awareness of the elements of environment through
physical sensation (Merriam-Webster)
 Perceptual psychologists study the way we acquire
information about the world via our various senses

Question 2

Light

Answer 2

For an object to be visible, it must emit or reflect
light
 Light can be conceptualised as a wave of
electromagnetic radiation
 One dimension of a wave is its wavelength
 The human eye is only capable of detecting light
within a narrow range of wavelengths
 Within this range of visible
wavelengths, different wavelengths
give rise to the perception of
different colours
 Light also varies in
intensity, leading
to the percept of
brightness

Question 3

The retina

Answer 3

 A thin, light sensitive membrane located at the back of the
eye
 Contains two types of visual sensory receptors (cells that
convert physical input—i.e., light—into electrochemical
signals): rods and cones
 These receptors are connected to other cells
1)Fovea
2) Optic Disk

Question 4

Fovea

Answer 4

a small area in the center of the retina, composed
entirely of cones
 Where visual information is most
sharply focused

Question 5

Optic Disk

Answer 5

 Optic disk: the area of the

retina without rods or cones

Question 6

Rods and Cones

Answer 6

Light is detected by receptors called rods and cones
located at the back of the retina
 Rods and cones transduce light energy into chemical
energy via photopigments – chemicals that absorb
light called opsins

Question 7

Rods

Answer 7

 Long, thin, and blunt
 Highly sensitive to light, but not colour
 Primarily for peripheral and
night vision

Question 8

Cones

Answer 8

 Short, thick, and pointed
 Detect colour
 Good colour vision and high
visual acuity

Question 9

Cone opsins

Answer 9

 This provides the basis for colour vision using cones

Question 10

Rhodopsin

Answer 10

There is just one kind of photopigment in rods
(rhodopsin) which is why rod signals don’t provide
colour information

Question 11

Cones (light)

Answer 11

Colour
6 million 
High acuity 
Fovea and periphery 
Fast dark adaptation 
Low dark sensitivity

Question 12

Rods (dark)

Answer 12

Black and white
 120 million
 Low acuity
Periphery Slow dark adaptation
Low dark sensitivity

Question 13

Photopic

Answer 13

Bright light vision via cones

Question 14

Mesopic

Answer 14

Intermediate light vision via rods and cones

Question 15

Scoptic

Answer 15

Dim light vision via rods

Question 16

Von Kries 1985

Answer 16

von Kries (1895) observed that individuals without rods
were night blind. The reverse applies for individuals
without cones

Question 17

Trichromacy

Answer 17

Normal colour vision in humans is trichromatic :
Cones can contain either L (long), M (medium)
or S (short) wavelength sensitive photopigments
 Our visual system is maximally sensitive to green light
(λmax = 555nm) under photopic conditions
 Evolutionary adaptation?
 All the colours which you can see are due to this
combination of signaling by rods and cones

Question 18

Colour vision deficinies

Answer 18

Retinal ‘colour blindness’ occurs in 8% of the male
population (x-linked), <1% of females
 2 main varieties:
 Anomaly of the photopigment (altered spectral sensitivity)
 Deletion of the photopigment (dichromacy):
 Protanopia – no L cones (red-green colour blind)
 Deuteranopia – no M cones (red-green colour blind)
 Tritanopia – no S cones (blue-yellow colour blind)
 Leads to a decreased ability to see colours and
distinguish colours from one another

Question 19

Opponent-process theory

Answer 19

 Trichromacy theory provides a good account of colour vision,
Hering (1920) proposed an alternative called opponentprocess
theory
 He noted that we tend to categorise colours according to 4
pure colours: red, green, blue, and yellow
 We never describe a colour as yellowish-blue or greenish-red
 Hering proposed that we have two neural processes for colour
vision (plus a third for black-white vision);
 One that, for example, increases its firing rate in the present of red
and decreases in the presence of green
 One that does the same for yellow and blue
 As the cell could not simultaneously have a high and low firing
rate, this explains why we never see blueish-yellow etc.

Question 20

Changes are important

Answer 20

Eyes are making small involuntary movements all the
time
 Constant tremor
 Slow drift
 Microsaccades (fast jerking motion)
 These movements are important to make changes in
images projected on the retina
 What would happen when there is absolutely no
change in the retinal images?

Question 21

Lateral inhibition

Answer 21

When a photoreceptor receives light stimulation, it

inhibits activities of nearby photoreceptors

Question 22

Information reduction

Answer 22

There is a massive convergence of signals as we
move deeper into the retina:
 On average 126 receptors connect to each ganglion
cell
 Thus, a process of information reduction due to limited
bandwidth
 An example of this is the concept of receptive fields,
which seem to form the basic unit of perception
within the visual system

Question 23

Ganglion cell receptive fields

Answer 23

Each ganglion cell receives input from a number of adjacent
receptor cells (retinotopic map)
 These cells form the receptive field of the ganglion cell
 Microelectrode recordings indicate that ganglion cells respond
differently depending on whether the centre or the surround of
their receptive field is illuminated
Ganglion cells are a type of neuron located near the inner surface of the retina and are the final output neurons of the vertebrate retina. Ganglion cells collect visual information in their dendrites from bipolar cells and amacrine cells and transmit it to the brain through out their axon to the brain.

Question 24

Agnosia

Answer 24

Impairment of object recognition ability
Inability to recognize objects

In agnosia, processes such as color, shape, and motion perception are intact

Recognizing a whole object is more than just recognizing its parts

Question 25

Two cortical pathways for vision

1)Ventral Pathway

Answer 25

Occipital  temporal
Processes information about object appearance and identity
Important for object perception

Question 26

Two cortical pathways for vision 2) Dorsal Pathway

Answer 26

Occipital  parietal
Processes spatial information about objects
Important for guiding action

Question 27

Patient DF

Answer 27

Patient DF is a woman with visual apperceptive agnosia who has been studied extensively due to the implications of her behavior for the two streams theory of visual perception. Though her vision remains intact, she has trouble visually locating and identifying objects. Her agnosia is thought to be caused by a bilateral lesion to her lateral occipital cortex, an area thought by dual-stream proponents to be the ventral “object recognition” stream.[1] Despite being unable to identify or recognize objects, DF can still use visual input to guide her action.

Question 28

Optic Ataxia

Answer 28

Intact object recognition
Inability to use visual information to guide action
Associated with lesions in the dorsal pathway (typically in the parietal cortex)

Question 29

Template matching and feature analysis

Answer 29

We tend to recognize different stimuli as the same object (e.g., letter “A”) irrespective of superficial variations

Gestalt principles don’t really help explain why

Another theory of pattern recognition, template matching, doesn’t explain this either

Question 30

Feature Analysis

Answer 30

A visual pattern is perceived as a combination of elemental features
Selfridge’s pandemonium model
What would happen when this pattern is presented on the retina without eye movements?

Whole features (e.g., an entire vertical bar) tend to disappear simultaneously

Question 31

Recognition by component theory

Answer 31

Similar mechanisms have been proposed for 3-D object recognition

An object is first segmented into a set of basic sub-objects (called geons), and then recognized as a pattern composed of geons

Question 32

Neural mechanisms of patter recognition

Answer 32

Neurons in higher-order visual areas respond to increasingly complex patterns
If we keep going down the visual information processing pathway, can we eventually find a cell that responds to a very, very, very specific stimulus?

“Grandmother cell” hypothesis
Weaknesses of the grandmother cell hypothesis
The final percept of an object is coded by a single neuron
However, each neuron’s firing is not so reliable
If that neuron is lost, our perception of the corresponding object would be lost
Perception of novel objects cannot be explained well
Flexibility of object recognition cannot be explained well

Question 33

The grandmother cell

Answer 33

The grandmother cell is a hypothetical neuron that represents a complex but specific concept or object. It activates when a person “sees, hears, or otherwise sensibly discriminates” a specific entity, such as his or her grandmother.

Question 34

Ensemble coding

Answer 34

Object recognition results from the firing of an ensemble of cells (ensemble coding)
More recent evidence suggests that grandmother-cell-like neurons might exist (Quiroga et al., 2005)
But not necessarily in the visual object recognition system per se

Question 35

Phoneme-restoration effect

Answer 35

“The state governors met with their respective legi…latures convening in the capital city.”

Only 1 in 20 participants reported hearing this tone
And this one participant was unable to locate the tone correctly