The Seeing Brain Flashcards
Light is reflected off objects & the ________, _______ & _____ capture light and focus it onto the _________Light is reflected off objects
• Cornea, iris, lens capture light and focus it
onto the retina
• Retina: layer of cells on back of eye that
contains photoreceptors
• Cornea, iris, lens *capture light and focus it onto the * retina
• Retina:
layer of cells on back of eye that contains photoreceptors
Photoreceptors convert ________ energy into ________
Photoreceptors convert *electromagnetic* energy into *neural signals*
How does photoreception work?
▫ Light hits special pigments inside photoreceptors ▫ Results in hyperpolarization of cell ▫ Photoreceptors decrease neurotransmitter release when stimulated with light! (inhibited)
Rods:
▫ Specialized for low levels of light, (important in night vision) ▫ Lower detail, color insensitive
Cones:
▫ Specialized for higher levels of light ▫ Color and detail ▫ Three different kinds of cones specialized for red, blue and green light ▫ Highest density of cones in the fovea of the retina
What is the fovea?
Point of central focus. Highest concentration of cones
How do Photoreceptors convert electromagnetic energy into neural signals?
Signal from photoreceptors transmitted to a chain of neurons in the retina Axons from ganglion cells form the optic nerve and exit the retina en route to the brain Creates a “blind spot” where there are no rods or cones
Blind spot
“blind spot” where there are no rods or cones (no photoreceptors) Pathway to optic nerve to brains visual cortex
Sensation
Transduction of physical energy _> sensation is processed into perception
There are multiple parallel retina-to-brain pathways. Which is the most sophisticated?
Eye to primary visual cortex: geniculo-striate pathway
What happens in the Geniculo-striate Pathway
Axons from the retina come together to form optic nerve. Some axons cross to other side of brain at optic chiasm After optic chiasm, the axons are called the optic tract Synapse in lateral geniculate nucleus (LGN) of thalamus LGN projects to V1 via the optic radiation.
Fovea
The
Seeing as a constructive process
The brain actively constructs a visual representation of the world (perception) and gives meaning to it.
Sensation:(1) To(2) Perception:(3) -
1- a physical stimulus (e.g. light) impinges on receptor, 2- receptor transforms (“transduces”) the stimulus into a neural signal that gets sent to the brain. 3-the brain elaborates and interprets sensations to create a model of the real world
(The seeing brain) Sensation:
a physical stimulus (e.g. light) impinges on receptor, - receptor transforms (“transduces”) the stimulus into a neural signal that gets sent to the brain.
(The seeing brain)
Perception: - the brain elaborates and interprets sensations to create a model of the real world
The physical stimulus: light )
Light = electromagnetic energy (small portion of the electromagnetic spectrum
How does photoreception work? ▫
Light hits special pigments inside photoreceptors ▫ Results in hyperpolarization of cell ▫ Photoreceptors decrease neurotransmitter release when stimulated with light! (inhibited) *very important
What’s the difference between rods and cones?
Rods: ▫ Specialized for low levels of light, (important in night vision) ▫ Lower detail, color insensitive Cones: ▫ Specialized for higher levels of light ▫ Color and detail ▫ Three different kinds of cones specialized for red, blue and green light ▫ Highest density of cones in the fovea of the retina
There are multiple parallel retina-to-brain pathways The most sophisticated pathway is the
Eye to primary visual cortex: geniculostriate pathway Geniculo-striate pathway
Geniculostriate pathways -label image
Visual field pathway -things to notice
Geniculostriate pathway -names of different sections
Axons from the retina
come together to form
optic nerve.
Some axons cross to other
side of brain at optic
chiasm
After optic chiasm, the
axons are called the optic
tract
Synapse in lateral
geniculate nucleus (LGN)
of thalamus
LGN projects to V1 via the
optic radiation.
Contralateral representation in V1
1) Each eye represents both
sides of the visual field:
the left half of each eye
receives light from the right
half of the visual field,
whereas the right half of each
eye receives light from the left
half of the visual field.
2) At the optic chiasm, the
equivalent sides of each eye
merge to form the optic tract.
This means that the left visual
cortex receives information
from the right half of the visual
field, whereas the opposite is
true for the right visual cortex
(they receive information front the contralateral side)
Receptive field
: the region of space that elicits a response from a given neuron
If light is within its receptive field
If light is outside its receptive field
Visual processing along the geniculo-striate pathway
FIRST STOP
Visual processing along the geniculo-striate pathway
- Thalamus (LGN)
LGN
Lateral Geniculate Nucleus (LGN
STOP 1
Contains six layers, three for each eye
- Cells have a center- surround receptive field
- They respond to differences in light across their receptive field (e.g. presence of light in center, absence in surround, or vice versa)
Visual processing along the geniculo-striate pathway
FROM THE THALAMUS TO THE ___
FROM THE Thalamus (LGN) TO THE Primary Visual Cortex (V1)
Next stop: Primary Visual Cortex (also known as V1/Striate Cortex)
Extracts basic information from the visual scene (e.g. edges, orientations, wavelength of light)
- This information is used by later stages of processing to extract information about shape, color, movement, etc.
- Single-cell recordings by Hubel and Wiesel lead to a hierarchical view of vision in which simple visual features (e.g. points of light) are combined into more complex ones (e.g. adjacent points of light may combine into a line)
Single-cell recordings by Hubel and Wiesel lead to a hierarchical view of vision in which simple visual features (e.g. points of light) are combined into more complex ones (e.g. adjacent points of light may combine into a line)
SIMPLE CELLS
Simple cells: combine information from several LGN center-surround cells
• respond to bars of particular orientations, and to points of light along those bars
Types of cells in V1 according to their response patterns
Simple cells: combine information from several LGN center-surround cells
• respond to bars of particular orientations, and to points of light along those bars
▫ Complex cells: combine information from several simple cells.
• Respond to orientation too, but not to points of light along the bar
▫ Hypercomplex cells: combine information from several complex cells.
• Still orientation selective, but prefer lines of certain lengths.
V1 is organized “_______”
Orderly mapping between spatial location of light on the retina and location of V1 receptive fields
• Notice that fovea gets extra cortical representation (“cortical magnification”)
What happens when there is damange to V1?
- Damage to parts of V1 results in blindness for the corresponding region of space.
- The patient cannot consciously report objects presented in this region of space)
HEMIANOPIA
SCOTOMA
QUADRANTANOPIA
CORTICAL BLINDNESS
Damage to parts of V1 results in blindness for the corresponding region of space.
- The patient cannot consciously report objects presented in this region of space)
- However, the patient is still able to make some visual discriminations in the “blind” area (e.g. orientation, movement direction) – this is called blindsight.
BLINDSIGHT
The patient is still able to make some visual discriminations in the “blind” area (e.g. orientation, movement direction) – this is called blindsight.
- However, the patient is unaware of seeing anything.
- If asked, the patient reports that he/she is guessing, but then perform above chance
For an example of blindsight, watch this clip: https://www.youtube.com/watch?v=zOHTpobMFu4
How is blindsight possible
Alternative routes are still available in the event of V1 damage.
This doesn’t mean that they can completely replace V1 function (only coarse discriminations are possible).
Moreover, the information they provide does not reach consciousness (patient is unaware of seeing anything).
V1 - INPUT AND CHARACTERISTICS OF RECEPTIVE FEILD
Extrastriate Cortex
OutsideV1,stillinoccipitallobe • Receptivefieldsgetspatially
bigger, less organized
• Specializedforprocessing particular stimulus attributes
Disorders of color vision
Damage to V4 results in cerebral achromatopsia
▫ World is perceived as drab grey
▫ Retina and V1 responding normally to different wavelengths
▫ Good example of how perception happens in the brain, not at the receptors (sensation may be intact)
Not to be confused with color blindness.
▫ Abnormalities with specific cone types
▫ Problem distinguishing between specific wavelengths of light (typically red vs. green).
Damage to V4 results in
cerebral achromatopsia
▫ World is perceived as drab grey
▫ Retina and V1 responding normally to different wavelengths
▫ Good example of how perception happens in the brain, not at the receptors (sensation may be intact)
Area V5/MT:
Motion perception
Neurons in area V5 respond to particular directions of movement. V5 is also known as MT (“middle temporal”)
Patients with bilateral damage to this region see the world in a series of still frames (akinetopsia)
▫ They can still detect movement in other senses (e.g. hearing, touch), so it’s specific to vision
▫ Akinetopsic patients cannot see an object looming (they see the object once it’s already next to them). They also have trouble performing the simplest actions if they require good movement perception (e.g. pouring tea)
Recap: general principles of vision (indeed, this applies to all sensory systems)
&
SUMMARY
Processing is hierarchical:
- early on in processing, simple perceptual features are
identified (e.g.: edges, orientation, wavelength)
- later on in processing, these features are combined into ever more complex features
Processing of different features occurs in parallel:
- e.g. motion vs. color of objects
- e.g. what is it (identity) vs. where is it (location)
Sensory transduction converts sensory information to neural signals
• The major primary visual pathway goes from the retina to V1 via the LGN
- Neurons have receptive fields
- V1 is arranged retinotopically; edges &
orientation
• V4 = color; V5/MT = motion
Beyond Visual Cortex
- Visual cortex (striate and extrastriate) extracts basic visual information – colors, movement, shapes, edges
- In order for this information to be used it needs to make contact with other types of information:
▫ Where the object is in space (and this can’t be computed from the retinal image alone)
▫ What the object is (knowledge we’ve stored about what things are)
Two Cortical Pathways for Visual Perception
Dorsal (“where/how”) Pathway (Parietal)
Location of objects
Visual control of skilled action directed at those objects (reaching, grasping)
Ventral (“what”) Pathway (Temporal)
Identity of objects
Attach meaning/ significance to objects
As we move down the ventral visual stream…
• Cells fire to more complex and specific stimuli • Receptive fields get bigger
▫ Useful for object recognition
▫ But lose details about object location
• Often sensitive to color
▫ Helps to separate objects from their background
A window into higher visual processing: Visual agnosias (= impaired visual object recognition)
Not Perceptual Blindness: patients do not have a visual field defect
basic discriminations of acuity, brightness, hue are largely intact
Not a Generic Recognition (UNLIKE memory loss)
Deficit: limited to information presented visually
touch, smell, etc. can be used to support successful object recognition
•
apperception
Apperception (still a perceptual stage)
Features are assembled (“integrated”) into objects.
• Why do we perceive the world as a series of objects rather than a jumble of lines?
▫ “Parsing” – grouping lines into objects
Association
Association (semantic stage)
Semantic attributes (meaning) are attached. Memories associated with the object (e.g. what is it used for?) come into play, facilitating recognition).
Stages of object recognition (too simplistic, but it’s a start)
After features (edge orientation, color, motion, etc.)
Apperception (still a perceptual stage)
Features are assembled (“integrated”) into objects.
Association (semantic stage)
Semantic attributes (meaning) are attached. Memories associated with the object (e.g. what is it used for?) come into play, facilitating recognition).
Disorders of apperception: Apperceptive agnosia
Most common after carbon monoxide poisoning, rather than a restricted lesion
Impairments:
Always impaired: Recognition/naming of drawings, photos, and 3-D objects
Generally impaired: Ability to copy, match or parse objects
Apperceptive agnosias are complex and heterogeneous
E.g. Patient HJA (textbook) can copy objects or draw them from memory; may have problems integrating parts into wholes
Common feature is evidence for a disorder of perceptual stage of object recognition
Associative agnosia
Impaired:
Intact:
Impaired:
Recognition/naming of line drawings, photos, and 3-D objects
Intact:
Ability to copy and parse complex objects (to some extent the object is perceived, but not recognized)
Interim summary
• Object recognition requires more than just detecting light and edges (early visual processing):
▫ Apperception:
▫ Association:
▫ Apperception: parse edges into objects
▫ Association: link objects to memories (knowledge about the object)
Category Specificity
• As you get further from V1, visual brain areas become increasingly specialized
▫ Color, motion ▫ What, where
- How far does the idea of visual specialization go?
- Are there domain or category specific areas of the brain?
▫ Does it process one and only one type of information?
• Case study: Are there parts of the brain that are specific to processing faces?
Prosopagnosia (“Face Blindness”)
-inability to recognize people from their faces, not inability to perceive faces - Can result from brain damage, but also developmental form
Individuals with prosopagnosia show disproportionately impaired recognition of faces
Associated with brain lesions, but may also have a genetic component
Deficit restricted to visual domain
May or may not have additional visual impairments
holistic type of processing
Evidence for holistic processing in healthy participants: The Margaret Thatcher illusion
Take home: Recognizing faces may be “special” because it depends on a particular kind of processing (holistic)
Is there a dedicated face region in the brain?
Object recognition: Overall summary
• Recognizing objects takes place in a special processing stream (ventral stream)
▫ Apperception: assembling edges into objects
▫ Association: linking objects to memories (recognition)
• Recognizing faces may be at least somewhat different from how we recognize objects
Two Cortical Pathways for Visual Perception
Dorsal (“where/how”) Pathway (Parietal)
Location of objects
Visual control of skilled action directed at those objects (reaching, grasping)
Ventral (“what”) Pathway (Temporal)
Identity of objects
Attach meaning/ significance to objects
Cones:
PHOTORECEPTOR Specialized for higher levels of light ▫ Color and detail ▫ Three different kinds of cones specialized for red, blue and green light ▫ Highest density of cones in the fovea of the retina
Rods: ▫
PHOTORECEPTOR Specialized for low levels of light, (important in night vision) ▫ Lower detail, color insensitive
Stimulus for vision
Light = electromagnetic energy (small portion of the electromagnetic spectrum) Shorter wavelength = higher energy
Receptive fields
Region of space that illicit a response from a given neuron. (Come in many different sizes and shapes)
o LGN cells ▪ On-center and off-center ▪ Edge detection
LGN - thalamus 6 layers. (three for each eye) Cells have monocular input. Layers alternate inputs from each of the two eyes. The top four are parvocellular layers, two layers from each eye. Parvo (small) LGN cells receive inputs from (small) midget ganglion cells. The bottom two are magnocellular layers, one layer from each eye. Magno (large) LGN cells receive inputs from (large) parasol ganglion cells.
Simple cells • Input • Characteristics of receptive field
Simple cells: combine information from several LGN center- surround cells • respond to bars of particular orientations, and to points of light along those bars
complex cells • Input • Characteristics of receptive field
Complex cells: combine information from several simple cells. • Respond to orientation too, but not to points of light along the bar
hypercomplex cells • Input • Characteristics of receptive field
Hypercomplex cells: combine information from several complex cells. • Still orientation selective, but prefer lines of certain lengths.
Primary Visual Cortex (V1)
The visual cortex is subdivided into a number of separate and distinct regions called visual cortical areas. Later in the semester we will discuss how these different cortical areas are identified and what their functions are. For now, we will concentrate only on the primary visual cortex (also called V1, shown in blue). V1 is located in the Calcarine sulcus in the medial occipital lobe of the brain (near the back of the head, just to the left and right of the middle). V1 is “primary” because the LGN sends most of its axons there, so V1 is the “first” visual processing area in the cortex. V1 processes the information coming from the LGN (as described below) and then passes its output to the other visual cortical areas which are (creatively) named V2, V3, V4, etc.
