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Flashcards in Vision Deck (35)
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What is perception?

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



For an object to be visible, it must emit or reflect
 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


The retina

 A thin, light sensitive membrane located at the back of the
 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
2) Optic Disk



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


Optic Disk

 Optic disk: the area of the
retina without rods or cones


Rods and Cones

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



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



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


Cone opsins

 This provides the basis for colour vision using cones



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


Cones (light)

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


Rods (dark)

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



Bright light vision via cones



Intermediate light vision via rods and cones



Dim light vision via rods


Von Kries 1985

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



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


Colour vision deficinies

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


Opponent-process theory

 Trichromacy theory provides a good account of colour vision,
Hering (1920) proposed an alternative called opponentprocess
 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.


Changes are important

Eyes are making small involuntary movements all the
 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?


Lateral inhibition

When a photoreceptor receives light stimulation, it
inhibits activities of nearby photoreceptors


Information reduction

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


Ganglion cell receptive fields

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.



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


Two cortical pathways for vision
1)Ventral Pathway

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


Two cortical pathways for vision 2) Dorsal Pathway

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


Patient DF

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.


Optic Ataxia

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


Template matching and feature analysis

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


Feature Analysis

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