5&6. Vision and pattern recognition Flashcards
what is perception?
Awareness of the elements of environment
through physical sensation (Merriam-Webster)
Perception is not just receiving information
through sensation; it involves interpretation
and recognition of the information
What are the types of perception?
- Visual
- Auditory
- Tactile (haptic)
- Olfactory
- Gustatory
- Object
- Space
- Time
- Speech
- Motion
what is required for an object to be visible?
For an object to be visible it must either emit or
reflect light
what is light?
One way of understanding light is that it is a wave of electromagnetic radiation
– It is a wave, so one dimension of light is its
wavelength
what is the human eye capable of in regards to light?
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
colors
Within the human’s range of visible wavelengths, what do different wave lengths cause?
Within this range of visible wavelengths, different wavelengths give rise to the perception of different colors
How does light vary?
Light also varies in intensity, leading to the
percept of brightness
what do ciliary muscles do?
The ciliary muscles control
the shape of the lens to
accomodate near or far
targets
what happens when the lense is round?
Far Blurred
Near in Focus
what happens when the lense is flat?
Far in Focus
Near Blurred
what is the retina?
A thin, light sensitive membrane located at the back of the eye
what sensory receptors does the retina contain?
Contains two types of visual sensory receptors (cells
that convert physical input—i.e., light—into electrochemical signals): rods and cones
what are the rods and cones connected to?
other cells
what is the fovea?
a small area in the center of the retina, composed entirely of cones
• Where visual information is most
sharply focused
What is the optic disk?
the area of the retina without rods or cones
what do rods look like??
– long, thin, and blunt
– primarily for peripheral and night vision
what are rods highly sensitive to?
– highly sensitive to light, but not to color
what are the rods used primarily for?
– primarily for peripheral and night vision
what do cones look like?
short, thick, and pointed
what do cones do?
detect color
for color vision and visual acuity
what do rods and cones do?
Rods and cones convert light energy into
electrical energy via a photopigments (chemicals
that absorb light)
what are photopigments responsive to?
Different kinds of photopigments are responsive
to different wavelengths of light
– That is, different kinds of photopigments “see”
different colors
why cant rods provide colour information?
There is just one kind of photopigments in rods
(called rhodopsin)—that’s why signals from rods don’t
provide color information
what are the photopigments in cones?
Photopigments in cones (called iodopsin) come in
three variations
what do variations in the photopigments in cones sensitive to?
Each variation is sensitive to a different range of
wavelengths
– Thus, each variation is tuned to a different range of
colors
how do rods and cones have different levels of acuity?
More rods than cones synapse with each ganglion
cell
what are the two major pathways from the retina to the brain?
– retina –> lateral geniculate nucleus (LGN) –> V1
(primary visual cortex in the occipital lobe)
– retina –> superior colliculus (SC) (and pulvinar
nucleus)
what function does the LGN-V1 pathway serve?
conscious aspects of vision (e.g., object recognition)
what function does the SC pathway serve?
unconscious visual functions (e.g., object localization)
What is blindsight?
some people with damage to the LGN-V1 pathway can still react to visual stimuli (e.g. point tonthem) although they claim that they cannot see them!
what do neurons in the V1 respond to?
Neurons in V1 respond to bars of a particular
orientation
what does V1 stand for?
the primary visual cortex
what areas are involved in visual imformation processing beyond the v1?
There are multiple visual
areas in the cortex. Each area performs different
types of computation
what processing occurs in areas of the cortex that appear later in the information processing stream?
in areas that appear later in the information processing stream, neurons respond to more complex patterns
what is our visual perception more closely related to?
Our visual perception is more closely related to neural activity in the higher visual areas
what is the receptive field?
Area of the visual field to which a visual neuron responds
It is very small in the beginning, but becomes
increasingly larger in later stages of visual
information processing
What is visual information processing?
The visual world is broken down into small elements,
and then it is reconstructed
What do ganglion cells do?
Ganglion cells receive input from multiple receptors that are adjacent to each other
what do the two ganglion cells in the retina do to frorm a retinotopic map?
Two ganglion cells that are next to each other in
the retina will have receptive fields that are also
next to each other
– Forming a retinotopic map
What are the properties of neurons in the LGN?
Neurons in the LGN have similar properties
– Center-surround antagonism
– Retinotopic map (i.e., adjacent LGN neurons have
receptive fields on the retina)
How do cells and neurons respond in the LGN ?
ganglion cells and visual neurons in the
LGN respond simply to the presence of a spot of
light on the retina
What are the characteristics and positioning of neurons in the V1?
– Receptive fields of two adjacent V1 neurons are still
adjacent to each other on the retina
– These neurons receive input from multiple LGN
neurons
• This enables V1 neurons to function as bar detectors
What happens to visual neurons in higher order areas and what does this enable?
In higher order areas, neurons have larger
receptive fields by combining input from
multiple lowerlevel neurons
This enables those higherlevel neurons to respond to more complex visual patterns
What is agnosia?
Impairment of object recognition ability
What processes are intact in agnosia patients and what does this cause?
In agnosia, processes such as color, shape, and motion perception are intact. This recognizing a whole object is more than just recognizing its parts
What are the two cortical pathways for vision?
Ventral pathway
Dorsal pathway
What is the ventral pathway, what information does it process and what is it important for?
– Occipital à temporal
– Processes information about object appearance and
identity
– Important for object perception
What is the dorsal pathway, what information does it process and what is it important for?
– Occipital à parietal
– Processes spatial information about objects
– Important for guiding action
Who was patient DF?
– Severe agnosia
– Her visual acuity and nonvisual object recognition are intact
What is optic ataxia and what is it associated with?
– Intact object recognition
– Inability to use visual information to guide action
– Associated with lesions in the dorsal pathway
(typically in the parietal cortex)
What does face perception require?
Face perception requires holistic processing
What was the study on Larry with regard to face perception?
study phase
– Larry had round eyes, a pointed nose, and a
narrow mouth.
– Larry lived in a house with a three-paneled door,
shuttered living room window, and rounded
bedroom windows.
Test phase
- picture of nose (part condition) and whole face (whole condition)*
- picture of larry’s door (part condition) and whole house (whole condition)*
Results:
Face condition
- 65% correctly identified faces in isolated condition
- 75% correct in whole condition
House condition
- 82% in islated condition
- 80% in whole condition
What is involved in the gestalt principles?
- Similarity
- Closure
- Good continuation
- Proximity
Wat do we tend to do in relation to template-matching and feature analusis?
We tend to recognize different stimuli as the
same object (e.g., letter “A”) irrespective of superficial variation
Gestalt principles don’t really help explain why
Another theory of pattern recognition, template
matching, doesn’t explain this either
In the template-matching model what is good matching?
Good matching
(successful perception)
Slight difference between input and template (unsuccessful perception)
How are visual patterns perceived and what model helps to explain this?
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?
(HB on top of eachother)
Whole features (e.g., an entire vertical bar) tend to disappear simultaneously
What does the recognition by component theory suggest?
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
what is the grandmother cell hypothesis?
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?
what are the 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
what is ensemble coding in the grandmother cell hypothesis?
Object recognition results from the firing of an ensemble of cells (ensemble coding)
what has more recent evidence suggested about ensemble coding?
– More recent evidence suggests that grandmothercell-
like neurons might exist (Quiroga et al., 2005)
– But not necessarily in the visual object recognition
system per se
What is bottom up processing?
A? or H?
When the word CAT is presented by the top of the A is cut off to look like a H
What is top-down processing?
recognising the entire word as CAT even though it is presented with the top of the A is cut off to look like a H
what is the word-superiority effect?
Superior recognition of letters in a word context
than alone
– Indicates top-down influence on pattern recognition
what is the phoneme-restoration effect?
Participants were presented with the sentence:
“The state governors met with their respective
legi…latures convening in the capital city.”
Where the … is 120 ms tone prelaced ‘s’
• Only 1 in 20 participants reported hearing this
tone
– And this one participant was unable to locate the tone correctly
