Chapter 5 Flashcards
Visual Receptors (photoreceptors)
- Are receptors in the back of your eye specialized to absorb light and transduce it into an electrochemical pattern in the brain
- That is, they can take the light and turn it into a receptor potential – a de- or hyperpolarization of the receptor membrane
Law of Specific Nerve Energies [placeholder]
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Müller (1838): one neuron’s action potential always conveys the same kind of information
- brain “sees” the activity of optic neurons and “hears” the activity of the auditory neurons
- In other words, the brain somehow interprets action potentials from auditory nerves as sound, from optic nerves as light, and from olfactory nerves as odors
Pupil
- opening in the center of the eye that allows light to pass through
Lens
- focuses the light on the retina, controlled by ciliary muscles to make it thicker or thinner as needed
Retina
- back surface of the eye that is lined by visual receptors
- light from above strikes bottom and light from below strikes top
- light from left strikes right side and vice versa
Blind Spot
- In both eyes
- Where the optic nerve leaves the eye to talk to the brain. In this spot, there are no visual receptors
Route Within the Retina [placeholder]
- Light travels through the eye to the back and hits the retina
- On its way, it passes through ganglion, bipolar cells and horizontal cells in the middle of the eye. It does so without distortion.
- Once it hits the retina, it gets sent forward to bipolar and horizontal cells (instead of the action potential going to the optic nerve immediately, it gets sent forward to the horizontal cells, bipolar cells, and ganglion cells)
- The cells are transparent
- Bipolar cells send the message even more forward to ganglion cells
- The ganglion cells’ axons join together, loop around, and finally travel back to the brain
- The optic nerve is made up of axons of ganglion cells
–> the point where optic nerve leaves the eye does not have receptors and is our blind spot
Fovea and Periphery of Retina [placeholder]
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Macula
3mm X 5mm center of retina with greatest ability to resolve detail
Fovea
- the center of the macula, provides most detail (so your sharpest, clearest vision comes here – when you read, you focus the words on your fovea)
- each visual receptor here has a direct pathway to the brain through one bipolar cell and one ganglion cell
- provides exact location of a point of light (acuity)
- As opposed to your peripheral vision where bipolar and ganglion cells get input from several visual receptors
For animals
Birds of prey have more visual receptors on the top of their retina to see clearer things below them
Rodents have more visual receptors on the bottom of their retina the see clearer things above them
Birds of pretty have 2 fovea
The periphery of retina provides…
- better sensitivity to dim light
- can’t detect exact location or shape of light but convergence enables detection of very faint light
Rods and Cones [placeholder]
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Rods
- visual receptors that are abundant in the periphery of the retina
- respond best to low light (dark) conditions
Cones
- visual receptors that are abundant in and around the fovea
- respond best to bright light conditions
- essential for color vision
- There are only cones in the fovea; in there periphery there are both rods and cones, although the further away from the fovea you get, the more rods and less cones there are
Rods and Cones [Info Dump]
- About 120 million rods and 6 million cones
- But, each cone has direct line to brain while many rods share same line
- Both rods and cones contain photopigments,
Photopigments
- chemicals that release energy when struck by light
- light is absorbed and a chemical reaction occurs that releases energy
The Mammalian Visual System
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The Mammalian Visual System within the eyeball [Info Dump]
- rods and cones (visual receptors) synapse on bipolar cells
- But they also synapse on horizontal cells – long cells that run the other way (horizontally) and that in turn make inhibitory synapses on the same bipolar cells mentioned above
- So bipolar cells get an excitatory message from the rods and cones and also get inhibitory messages from horizontal cells
- bipolar cells then synapse on ganglion cells
- axons of the ganglion cells leave the back of the eye as the optic nerve
Horizontal Cells
long cells that run the other way (horizontally) and that in turn make inhibitory synapses on the same bipolar cells mentioned above
Optic chiasm
- The optic nerves from both eyes travel back into the brain and meet
- At the chiasm, the inside half of the axons of each eye cross over (in humans – in animals, it depends)
- most of the optic nerves go through the Lateral Geniculate Nucleus (visual perception area) of the thalamus to the primary visual cortex (where is that?)
- number of neurons within this path varies widely among people by a factor of 2 or 3 – accounts for the large differences among people in their abilities to detect brief, faint, or rapidly changing visual stimuli
- some of the optic nerves split and end in the superior colliculus
Retina and Lateral Geniculate Pathways [placeholder and key terms]
- Ganglion cells that start in the retina and make up the optic nerves fall into 3 categories: parvocellular, magnocellular, and koniocellular
Parvocellular
- smaller ganglion cell bodies and small receptive fields, located near fovea
- detect small details and color
- all axons go to lateral geniculate nucleus
Magnocellular
- larger ganglion cell bodies and receptive fields, distributed fairly evenly throughout the retina
- respond to moving stimuli and large overall patterns
- not color sensitive
- most axons go to lateral geniculate nucleus
- some go to the superior colliculi
Koniocellular
- small ganglion cell bodies that occur throughout the retina
- many functions (truthfully, we’re not exactly sure what all they do!)
- axons go to lateral geniculate nucleus, thalamus and superior colliculus
Many different types of ganglion cells implies analysis of information from the beginning
Pathways in Cerebral Cortex
- Most visual information from lateral geniculate nucleus goes to primary visual cortex (aka area V1 or striate cortex (striate means striped – it looks striped in appearance)
–> first stage of visual processing
- Output of V1 goes to secondary visual cortex (V2)
–> second stage of visual processing which transmits visual information to additional areas
–> feedback loop to V1
–> V1 and V2 also exchange information with other cortical areas and thalamus
Magnocellular Path
- Ventral Branch
- Dorsal Branch
Ventral Branch
to temporal cortex is sensitive to movement
Dorsal Branch
to parietal cortex integrates vision with action
Parvocellular Path
to temporal cortex is sensitive to details of shape
Mixed parvo/magnocellular path
to temporal cortex is sensitive to brightness and color
Ventral Stream or “what” pathway
- The 3 visual paths to the temporal cortex
- specialized to identify and recognize objects
doral stream or “where” or “how” pathway
- The visual path to the parietal cortex
- helps the motor system find objects and determine how to move to them, grasp them, etc.
Visual Field
- the whole area of the world that you can see at any given time. The part you see to your left is your left visual field, the part to your right is your right visual field
Receptive Field
- the part of the visual field to which any one neuron responds is that neuron’s receptive field
Mechanisms of Processing in the Visual System cont.[info dump] [place holder]
- Borders in our visual field identify where one object stops and another starts.
Lateral Inhibition
- The retina’s way of sharpening contrasts to make borders clear
- Each active receptor and it’s visual path tends to inhibit the visual path of neighboring receptors
- an active receptor excites both a bipolar and horizontal cell; in turn, horizontal cell inhibits bipolar cell, but net potential is excitatory on bipolar
- but, horizontal cell does inhibit neighboring bipolar cells on border of visual field
- effect is to heighten contrast: receptors inside visual field are excited and those on border tend to be inhibited
Slide 22 on Chapter 5 is what you suppose to do on MS whiteboard
Slide 23 (look on powerpoint and do diagram)[WATCH HER VIDEOS]
Brightest 6 and 10
Darkest 5 and 11
The Shape Pathway in the Cerebral Cortex [placeholder]
Cells of the visual cortex: 3 types
Simple cells (small receptive field
- Only respond to bar-shapes, with the correct orientation, in the correct spot
Complex cells (large receptive field)
- Only respond to bar-shapes, with the correct orientation, but it can be in any spot
End-stopped or hypercomplex cells
- Only respond to bar shapes, with the correct orientation, it can also be in any spot, but ALL of it must fit in the receptive field (with the other 2, only most if it needs to fit)
Visual Agnosia
- Inability to visually recognize objects despite otherwise satisfactory vision
- Can describe object but doesn’t know what they are, e.g., key, stethoscope, smoking pipe
Prosopagnosia
- inability to recognize faces
- can still read and recognize person by their voice
- Can still tell by the face whether the person is male or female, young or old, just can’t recognize it!
- Caused by damage to inferior temporal cortex area, fusiform gyrus (face blindness), especially active in recognition of faces – believe it may be specifically wired just to recognize faces! (or perhaps anything we spend a lot of time identifying different types of. E.g., new research shows people good at identifying various cars, birds, etc. have this area light up when doing so).
The Trichromatic, or Young-Helmholtz, theory
- humans can perceive color only within our visible wavelengths of light, ranging from 350-700 nm (violet to red)
- we see a specific color by comparing responses from 3 kinds of cones, each most sensitive to a short, medium, or long wavelength of light
- Ex: blue-green excites short wavelength to 15% of maximum, medium to 65% and long to 40%
Color Vision Cont.
The trichromatic, or Young-Helmholtz, theory cont,
- We have fewer short wavelength cones (blue) so we see red, yellow, and green colors better
- when all 3 cones are equally active we see white or gray
- Incomplete theory, e.g., can’t explain negative color afterimage
—> If you stare at a colorful picture for a minute or so, then look away onto a white wall, you will see the same picture, but with different colors – red will be green; yellow, blue; and black, white. This is the negative color afterimage
The Opponent-Process theory
- brain sees color on a continuum from red to green and another from yellow to blue
- we perceive color in terms of “paired opposites” red-green, black-white and yellow-blue
—> explains why we can’t see reddish green or bluish yellow
—> explains negative color afterimages (partially)
- not a complete explanation since we know we can produce illusion afterimages…so more of the brain must be involved besides the retina
The Retinex theory
– the cortex compares information from various parts of the retina to determine the brightness and color perception for each area
- color perception requires some reasoning
- Thus not just the retina, but the cortex as well is involved in perceiving color and brightness
color constancy
- we see the right colors despite lighting changes, e.g., if the cortex notes a constant amount of green throughout a scene (we have green tinted glasses on), it subtracts some green from each object to determine its true color
- Similarly, brightness requires a comparison with other objects
Color Vision Deficiency
Inability to perceive color differences
-genetic: lack of short- medium- or long-wavelength cones
–> some people lack two types of cones
—> some people have low number of all three
- inability to distinguish red from green is most common deficiency
—> recessive gene on X chromosome
—> 8% in men and 1% in women
- Inability to perceive anything but shades of black and white is extremely rare
Motion Perception
Two temporal lobe areas are consistently and strongly activated by visual motion:
- Cells in area MT (middle temporal cortex) respond selectively to stimulus moving in a particular direction regardless of size, shape or color
—> motion blind people who cannot determine if objects are moving may have damage here
- Cells on MST (medial superior temporal cortex) respond best to expansion, contraction or rotation of large visual scene
Let’s summarize:
- Each cell in the visual system has a receptive field, a portion of the visual field to which it responds
- Each cell in the visual system responds to specific stimulus features in its receptive field, such as shape, color, or movement
- Separate pathways in the visual system attend to different aspects of the visual system
- Certain kinds of brain damage can impair specific aspects of visual perception
