Vision Flashcards
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
Light
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
The retina
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
Fovea
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
Rods
Long, thin, and blunt
Highly sensitive to light, but not colour
Primarily for peripheral and
night vision
Cones
Short, thick, and pointed
Detect colour
Good colour vision and high
visual acuity
Cone opsins
This provides the basis for colour vision using cones
Rhodopsin
There is just one kind of photopigment in rods
(rhodopsin) which is why rod signals don’t provide
colour information
Cones (light)
Colour 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
Photopic
Bright light vision via cones
Mesopic
Intermediate light vision via rods and cones
Scoptic
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
Trichromacy
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
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.
Changes are important
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?
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
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
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.
Agnosia
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
