Vision Flashcards
The different eye structures help?
to focus the image on the retina
What is the cornea?
where light enters the eye; most of the focusing power, but fixed
What is the pupil and Iris?
the eye’s aperture; controls light level
What is the lens
further focuses light onto the retina. Flexible, allowing the eyes to focus on objects at various distances
What are the ciliary muscles?
change the curvature of the lens (accommodation)
What is the innermost layer of the eye?
the retina
What are the 3 main stages in the retina?
Three main stages:
Outer layers
Photoreceptors
Inner layers
Bipolar cells, horizontal cells, amacrine cells
Ganglion layer
Ganglion cells (the output units of the retina)
What do photoreceptors do?
Transduce photons (light) into neural signals (electrical) – phototransduction
What determines spatial resolution?
The number of receptors determines spatial resolution
Describe Cones
- 3 types of cones with photopigments sensitive to short (S), medium (M), and long (L) wavelengths
- Color vision
- High spatial acuity, but low sensitivity to light
- Active in bright light (daylight vision)
- Cones – low density throughout the retina but clustered in the fovea
How do rods and cones converge differently onto bipolar cells and what effect does this have?
Rods: many-to-one
Cones: one-to-one (fovea) or several-to-one (periphery)
How rods and cones converge onto bipolar cells contributes to differences in the spatial acuity of rod and cone pathways
What do horizontal cells do?
- Modulate synaptic activity of photoreceptor and bipolar cells
- Provide lateral interactions between photoreceptor and bipolar cells
- Thought to be involved in the sensitivity to contrast
What do amacrine cells do?
- Modulate synaptic activity of bipolar and ganglion cells
- Multiple subtypes
- Plays a role in the pathway that transmits information from rods to ganglion cells; also thought to be important for movement and direction detection
How diverse are ganglion cells?
Very diverse. 17 different types of ganglion cells in primates.
Each ganglion cell types support different visual functions (e.g. color, motion, detection of fine details)
What creates visual space?
the receptive fields of a population of ganglion cells together make up the visual space
Which cells have center-surround?
Ganglion, bipolar cells, and LGN neurons
Which type of center-surround type, ON or OFF, is activated when a light is on the center?
ON type
Are photoreceptors depolarized in light or dark?
They depolarize in darkness and hyperpolarize in light.
In ON and OFF type bipolar cells, which type of center-surround is sign conserving and which is sign inverting in the interaction between the photoreceptors and bipolar cells?
Is there ever sign inverting in ganglion cells?
OFF-center bipolar cells are sign conserving
ON-center bipolar cells are sign inverting
The ganglion cells will always respond the same way as the bipolar cells
Describe how horizontal cells work in center-surround systems?
Cone cells in a receptive field center synapse directly with the bipolar cell. Each cell in the surround synapses with a horizontal cell, not a cone cell. In turn, the horizontal cell synapses with cones in the center.
Synapses between horizontal cells and center cone cells are sign inverting.
The bipolar cell, in turn, responds depending on its own polarity, ON or OFF.
Look at neur 385 notes
List the main stream of visual information
Retina optic nerve optic chiasm optic tract lateral geniculate nucleus (LGN) in the thalamus primary visual cortex (striate cortex, V1) extrastriate cortex (V2-V4)
Where are the 4 targets for retinal ganglion cells?
Lateral geniculate nucleus (LGN) of the thalamus, relay visual information further to cerebral cortex
Superior colliculus (SC) in midbrain, mediating eye movements (saccades)
Pretectum: reflex control of pupil size
Suprachiasmatic nucleus (SCN) of the hypothalamus, regulation of circadian rhythms
Describe how the 3 parts of the visual field are processed (ipsi/contralaterally?) and describe retinotopic maps
Retinal projections are both crossed and uncrossed
- Foveal fixation is bilateral
- Binocular (inner) and Monocular (outer) portions of visual field are contralateral.
LGN fibers project to primary visual cortex (V1) and maintain inverted view of visual space. Superior visual field carried ventral path (Meyer’s Loop) through temporal lobe. Inferior visual field carried dorsal path in parietal lobe.
Visual representation in the primary visual cortex remains inverted.
How does the LGN work?
LGN neurons receive visual information from the contralateral side of the visual field.
LGN receive input from both contralateral and ipsilateral retinal ganglion cells, but they are segregated into different layers. So in single cell level, it is monocular.
Midget cells project to parvocellular layers (3 – 6), shape, size, color of object
Parasol cells project to magnocellular layers (1 – 2), location, speed, direction of moving object
Describe the receptive fields of LGN neurons
Retinal ganglion cells and cells in LGN share similar receptive fields:
- Center-surround structure
- Encode contrast instead of absolute intensity
Who did research on cats to show orientation specific neurons?
Hubel and Weisel
What are the 3 input that V1 neurons respond to?
- V1 neurons respond to bars or object edges
Each cell has a preferred orientation response for the bar, max response to preferred orientation, no response to orthogonal orientation. - Some are also tuned to stimulus motion
- Also tuned to the spatial frequency (fineness/courseness)
How is the V1 cortex architecture laid out?
a. Visual cortex is divided into 6 layers based upon cell types
b. LGN neurons project primarily to layer 4
c. V1 output to extrastriate visual cortex (dorsal/ventral streams) from layers 2 and 3
d. V1 output back to LGN from layer 6
e. V1 output to superior colliculus (later lecture) from layer 5
What is special about layer 4 in the visual cortex?
Layer 4 neurons receive segregated monocular input from LGN.
What is the difference between optic nerve and optic tract?
The optic tract (after the optic chiasm), unlike the optic nerve, contains fibers from both eyes.
What is the primary visual cortex called?
V1, or Striate cortex
How does the lens focus an image at various distances?
For long distance, the ciliary muscles relax and the lens is stretched. For closer distance, the ciliary muscles contract to shape the lens
Which part of the visual field is processed in each hemisphere?
The left visual field is processed by the right hemisphere and vice versa. Ganglion cells in the nasal region have axons that go contralateral in the optic chasm. Ganglion cells in the temporal region have axons with ipsilateral connections.
Whats the difference between horizontal and vertical columns of neurons in the visual cortex?
a. Orientation columns: vertical columns have similar orientations in the same receptive field (neurons in column all fire for bars at 125 degrees).
b. Horizontal columns have a a linear progression of orientation selectivity in similar receptive fields
In the visual cortex there are simple and complex cells. How do they differ?
Simple cells: respond to bar or edge in a certain orientation and certain location in the visual field. They Integrate LGN cell receptive fields
Complex cells: respond to bar or edge in a certain orientation, but not sensitive to its location
They Integrate simple cell receptive fields
How does binocular depth perception work?
Depth perception (stereopsis) is produced through fusion of the image by eyes focusing on the same point in space (b) Objects at other distances outside the fovea fixation point (A or C) are not fused in the cortex and cause visual disparity (double vision) Disparity produces disparate images in the cortex and no depth information is perceived
After leaving the visual cortex, the information is split into 2 streams for multi sensory processing (visual cortex is just vision). What is the difference between the dorsal and ventral streams?
Dorsal pathway: extend to posterior parietal cortex, involved in motion perception and spatial vision, from M cells in retina and LGN. M cells project through V2 – V4, then parietal areas MT, MST, VIP, and 7a
Ventral pathway: extend to the temporal lobe, involved in form perception, object recognition, color perception, from P cells in retina and LGN. P cells project through V2 – V4, then temporal areas PIT, CIT, and AIT
What does the image look like in the visual cortex?
The visual image is inverted in V1, but its not actually an image. Its just a population code (neurons).
What is area MT for?
MT encodes visual motion through space and as space moves relative to you. Active and passive motion. Encodes optic flow
What does the dorsal (top) stream do?
The Where pathway
Encodes: visually guided behavior
spatial awareness and guidance of actions
motion perception
fast recognition
What does the ventral (bottom) stream do?
The What pathway
Encodes: object recognition high spatial frequency - detail mainly foveal – small objects object memory color vision
encodes not only the face, but orientation of face.
What do Inferior temporal cortex neurons do?
Respond to complex stimuli (faces).
What happens when you have a lesion in V4? (this is associated more with ventral stream)
Lesions of V4 abolish color vision
Without affecting spatial/motion
vision
What are stereograms?
Researcher: Julesz and Brewster
computers are used to shift patterns of iterated information with respect to each other, resulting in different planes emerging.
Showed that stereoscopy (depth detection) depends on matching information seen by the 2 eyes without any prior recognition of what objects such matching might generate
What is Myasthenia gravis?
an autoimmune disease that targets nicotinic ACh receptors. muscle weakness
What is excitotoxicity?
Excitotoxicity by glutamate destroys dendrites of post synaptic neuron. If abnormally high levels of glutamate accumulate in the cleft, the excessive activation of neuronal glutamate receptors can excite neurons to death. Commonly causes neuronal damage after brain injury because of reduced blood flow to the brain (similar to stroke). The reduced supply of oxygen and glucose presumably elevates extracellular glutamate levels by slowing the energy-dependent removal of glutamate at synapses. Excitotoxicity is also involved in hypoglycemia.
What changes does GABA undergo in the developing brain?
In developing brains GABA is an excitatory neurotransmitter instead of inhibitory. The concentration of intracellular chlorine decreases as your brain matures.
How do psychotropic drugs work?
Most psychotropic drugs (drugs that alter behavior, mood, or perception) selectively affect one or more steps in the synthesis, packaging, or degradation of biogenic amines.
What one interesting fact about addiction?
Interestingly, addiction to heroin or any other agent is not an inevitable consequence of drug use, but depends on the environment. For instance, returning vererans who were heroin addicts in Vietnam typically lost their addiction upon returning to the US. Likewise, patients given morphine for painful conditions rarely become addicts.
What is psychophysics?
Psychophysics Is the Quantitative Study of Sensory Performance
A psychophysical experiment determines the quantitative relationship between a stimulus and a sensation in order to establish the limits of sensory performance (ex. separating 2 probes on back).
Read
Receptors Are Specific for a Narrow Range
of Input
Neurons of the brain and spinal cord do not respond when they are touched or when they ars exposed to sound or light. Each of these forms of energy must be first transduced by specialized cells. In every sensory system, cells that perform such a transduction are called receptors. For each of the fundamental types of stimuli (mechanical or thermal energy, sound, or light) there is a separate population of receptors selec- rive for the particular form of energy.
What are ganglion cells. Read
All receptors transduce the energy to which they are sensitive into a change in membrane voltage. The task of the receptor is to transmit that voltage change by one route or another to a class of neurons-universally referred to as ganglion cells-that send their axons into the brain or spinal cord.Systems vary in the mechanism whereby receptors and ganglion cells interact.
Read
Receptors Have Characteristic Patterns of Position and Density
Receptors are not scattered randomly across the sensory surface. An orderly arrangement of receptors exists along the skin, the basilar membrane, and the retina. In the retina, for example, photoreceptors adopt a hexagonal packing array (Wassle and Boycott, 1991), and in the cochlea, a single row of inner hair cells lines up parallel to three rows of outer hair cells
By far the greatest density of receptor terminals is found at the fingertips and the mouth, whereas recep- tors along the surface of the back are at least an order of magnitude less frequent. Such differences in peri- pheral innervation density are tightly correlated with spatial acuity.
Read
Most commonly, a single ganglion cell receives input from several receptors and, in many cases, a single receptor sends information to two or more ganglion cells. Convergence and divergence
What emerges from a comparison across systems is that convergence and divergence from receptor to ganglion cell vary directly with the demands placed on the system at the specific location. When spatial resolution is a requirement, the convergence of receptor inputs onto individual ganglion cells is low. When detection of weak signals is necessary, convergence is high. When receptor input is used for a complex function or for multiple functions divergence of input from a single receptor onto many ganglion cells occurs.
Read
Axons of sensory relay nuclei of the thalamus project
a single area or a collection of neighboring areas of ~ e cerebral cortex, thereby providing them with a ;: ecise topographic map of the sensory periphery. nese parts of cortex are frequently referred to as :”rimary sensory areas (Fig.22.6).Neighboring areas with - -hich the primary areas communicate directly or by a single intervening relay area are sensory association
eas.
What is nearsightedness and farsightedness?
Myopia (nearsightedness, when you can’t focus on distant objects) is caused when the eyeball is too long and the image focuses in front of the retina rather than on the retina.
Hyperopia (farsightedness)
How do we make up for the information lost from the optic disk?
How can you activate neurons whose receptive fields are in the optic disk?
The visual system fills in the missing part of the scene based on info supplied by the regions surrounding the optic disk.
neurons in optic disk can be activated by stimulating the regions surrounding the optic disk of the contralateral eye
What is Macular Degeneration?
degeneration of the photoreceptors. can be caused by abnormal blood vessel growth under the macula. Or can be caused by a gradual disappearance of the retinal pigment epithelium.
What is Retinitis Pigmentosa?
Progressive vision loss due to degeneration of photoreceptors. The photoreceptors die by apoptosis.
Interesting
Accumulating neuropsychological, electrophysiological and behavioural evidence suggests that the neural substrates of visual perception may be quite distinct from those underlying the visual control of actions.
