Visual system Flashcards
What does the macula support?
high acuity site
Where is the blind spot in they eye?
at the optic disc
How do we percieve the blind spot?
We create an actual neural representation of the surround rather than merely ignoring the absence of information from the blind spot (scotoma)
What is the structure of the retina?
A three-neuron chain—photoreceptor, bipolar cell, and ganglion cell—provides the most direct route for transmitting visual information to the brain
Horizontal cells and amacrine cells mediate lateral interactions in the outer and inner plexiform layers
What are the characteristics of rods?
greater number of disks and higher photopigment concentration
1000 times more sensitive to light than cones
Produce a reliable response to a single photon of light
~100 million
What are the characteristics of cons?
responsible for our ability to see color contain one of 3 different photopigments
Response of an individual cone does not saturate at high levels of steady illumination ~5 million
At what lumination do cones start to contribute to perception?
At the lowest levels of illumination, only rods are activated
Cones begin to contribute to perception at about the level of starlight and are the only receptors that function under relatively bright conditions
What are characteristics of the peripheral retina?
much higher ratio of rods to cones
higher ratio of photoreceptors to ganglion cells
So peripheral retina much more sensitive to low light levels
many photoreceptors provide input
What are characteristics of the central retina?
cones only
low ratio of photoreceptors to ganglion cells
So specialized for high resolution vision
a single photoreceptor feeds information to a ganglion cell
What are characteristics of the fovea?
area of retina specialized for high visual acuity in the center of the macula;
Lateral displacement of cells above photoreceptors reduces scattered light so no image blur
high density of cones (less rods -> more thorughout retina)
What are the three types of cones when reffering to colour vision?
short- (blue), medium- (green), and long- (red) wavelength cones
The mixing of red, green, and blue light causes equal activation of the three types of cones, and the perception of “white” results
What is protanopia?
impairment in perception of long wavelengths
What is deuteranopia?
impairment in the perception of medium wavelengths
What do both types of colour blindness have in common?
have difficulties with the discrimination of red and green, and for this reason dichromacy is commonly called red–green color blindness
What is melonin?
absorbs scattered light
How are photoreceptor cells removed by the pigment epithelium?
The tips of the outer segments of photoreceptors are embedded in pigment epithelium. Epithelial cell processes extend down between the outer segments. The disks migrate from the inner to the outer portion of the outer segment over a 12-day period
Expended disks are shed from the outer segment and phagocytosed. Photopigment from the disks enters the pigment epithelium, where it will be biochemically cycled back to “newborn” photoreceptor disks.
What is retinitis pigmentosa?
progressive vision loss due to a gradual degeneration of photoreceptors
damage to periphery
What is macular degeneration?
progressive loss of central vision
Peripheral vision usually remains normal
Explain phototransduction
stimulus - light acts on a receptor to cause a change in protein conformation
G-protein then binds GTP which causes a second messanger to decrease, decreasing Na+ conductance
What is the hyperpolarization response to light of photoreceptors?
Photoreceptors are continuously depolarized in the dark because of an inward sodium current, the dark current.
Dark, sodium enters the photoreceptor through a cGMP-gated channel
Light leads to the activation of an enzyme that destroys cGMP, thereby shutting off the Na+ current and hyperpolarizing the cell
dark -> depolarization
light -> hyperpolarization
Explain the light-activated biochemical cascade
rhodopsin
The activation of rhodopsin by light
Retinal is inactive in the dark, when it absorbs light it undergoes a conformational change that acitvates the opsin
In the dark, cGMP gates a sodium channel, causing an inward Na+ current and depolarization of the cell
The activation of rhodopsin by light energy causes the G-protein to exchange a molecule, which causes the enzyme phosphodiesterase (PDE) to breaks down cGMP and shuts off the dark current
What is the retinoid cycle?
Following photoisomerization, retinal is converted into retinol and is transported by the chaperone protein interphotoreceptor retinoid binding protein (IRBP) into the pigment epithelium
There, in a series of steps, it is converted to retinal and transported back to the outer segment (again via IRBP), where it recombines with opsin
Explain photoreceptor light adaption
Calcium in the outer segment inhibits the activity of guanylate cyclase and rhodopsin kinase, and reduces the affinity of cGMP-gated channels for cGMP
Light-induced closure of channels in the outer segment membrane leads to a reduction in Ca2+ concentration and a reduction in Ca2+-mediated inhibition of these elements of the cascade As a result, the photoreceptor’s sensitivity to photon capture is reduced
What are the two classes of bipolar cells?
ON bipolar cells have G-protein-coupled receptors (mGluR6) and hyperpolarize to glutamate released by photoreceptors. (cascade closes cGMP-gated Na+ channels)
OFF bipolar cells have glutamate-gated cation channels (AMPA, kainate) and depolarize to glutamate release
* OFF and ON refer to whether cells depolarize when the light is OFF (more glutamate) or ON (less glutamate)
What is the direct pathway from photoreceptors to bipolar cells?
An ON-center bipolar cell is depolarized by light in the receptive field center via the direct pathway
What is the indirect pathway from photoreceptors to bipolar cells?
Light in the receptive field surround hyperpolarizes the ON-center bipolar cell via the indirect pathway
bipolar cells recive input from surrounding photoreceptors via horizontal cells
* Because of the intervening horizontal cell, the effect of light on the surround photoreceptors is always opposite the effect of light on the center photoreceptors
How does the number of photoreceptors contributing to the receptive field of a retinal ganglion cell varies depending on location on the retina?
A cell near the fovea receives input from fewer receptors covering a smaller area, whereas a cell farther from the fovea receives input from many more receptors covering a larger area
What are the ON and OFF center ganglion cell responses to stimulation of receptive field centers?
ON and OFF bipolar cells correspond with ON and OFF ganglion cells
Light spot in center of receptive field will cause firing of ON center ganglion cells
dark spot in center of receptive field will cause firing of OFF center ganglion cells
What is the response of bipolar cells and ganglion cells to a light spot in the receptive fiels center?
center cone - hyperpolarize
ON center bipolar cells - depolarize
OFF center bipolar cells - hyperpolarize
ON center ganglion cells - increase firing
OFF center ganglion cells - stop firing
What is the response of bipolar cells and ganglion cells to a dark spot in the receptive fiels center?
center cone - depolarize
ON center bipolar cells - hyperpolarize
OFF center bipolar cells - depolarize
ON center ganglion cells - stop firing
OFF center ganglion cells - increase firing
What is the most effect way to maximize the firing of an ON or OFF center cell?
completely illuminate either the “on area” or the “off area” of its receptive field
* if both areas of a cells receptive field are illuminated together, there is little reaction from the cell
What are mach bands?
illusory bands visible to enhcacne contrast
percieved light between colours
How does lateral inhibition produce contrast enhancement?
caused by the opponent center-surround arrangement of the receptive fields of the retinal ganglion cells
The image of the transition between lighter and darker regions falls across some of these receptive fields
The cells whose centers are located in the brighter region but whose surrounds are located at least partially in the darker region will have the highest rate of firing
What occurs in the retinal cells with light stimulation of the center + surround?
leads to hyperpolarization of the horizontal cells and a decrease in the hyperpolarizing influence of horizontal cell processes on the photoreceptor terminals. The net effect is to depolarize the center cone terminal, offsetting much of the hyperpolarization induced by the transduction cascade in the center cone’s outer segment -> decreased firing of ganglion cells and decreased depolarization of bipolar cells
What are the three components of the non-image forming visual system?
hypothalamus - regulation of circadium rythym
pretectum - reflec control of pupil and lens
superior colliculus - orienting the movements of head and eyes
WHat is the retinohypothalamic tract?
Suprachiasmatic nucleus receives a selective input from the retina that is necessary and sufficient for photic entrainment of circadian rhythms
What is the role of melanopsin ?
it is expressed in a small population of ‘intrinsically photosensitive’ retinal ganglion cells (ipRGC) that respond directly to light
Retinal ganglion cells containing melanopsin innervate the SCN (suprachiasmic nucleus)
Are retinal photoreceptors required for visual image formation are necessary for circadian photoreception
No
What does a pulse of light porduce in ipRGCs?
a burst of AP
What is the pupillary light reflex?
Light shone in either eye elicits pupillary constriction in the same eye (direct response) and in the opposite eye (consensual response)
The pupillary response to light is mediated by the parasympathetic innervation of the iris
Retinal ganglion cells send their axons through the optic tract to the olivary pretectal nucleus
Neurons in this nucleus project to parasympathetic preganglionic neurons in the Edinger-Westphal nucleus
The axons of the preganglionic cells exit with the oculomotor nerve and contact the ciliary ganglion cells, which control the pupilloconstrictor muscle in the iris
What is the afferent pupillary defect?
Failure to elicit a response (either direct or indirect) to stimulation of the right eye if both eyes respond normally to stimulation of the left eye suggests damage to the sensory input from the right eye, possibly to the right retina or optic nerve
What is the efferent pupillary defect?
A direct response in the left eye without a consensual response in the right eye suggests problem with the visceral motor outflow to right eye, possibly as a result of damage to the oculomotor nerve or Edinger-Westphal nucleus in the brainstem
What is the optokinetic response?
a combination of a slow-phase and fast-phase eye movements
It consists of initial slow phases in the direction of the stimulus (smooth pursuits), followed by fast, corrective phases (return saccade)
What are saccades in the optokinetic response?
fast movements of the eyes that present various parts of the visual scene to the fovea—a process called foveation. When we fix our gaze, small saccades that we are unaware of avert photoreceptor adaptation
Explain sensorimotor integration in the superior colliculus
The sensory map of visual space in the superior colliculus is in register with the motor map that generates eye movements
Neurons in a particular region of the superior colliculus are activated by visual stimuli in a limited region of visual space
This activation leads to the generation of a saccade by activating neighboring upper motor neurons that move the eye by an amount just sufficient to align the foveae with the region of visual space that provided the stimulation
What is blindsight?
ability to respond to (behaviour) something outside the visual field
What are the characteristics of P-type ganglion cells?
- 90% population
- small receptive fields
- slow axon conduction velocities
- sustained firing to the presentation of visual stimuli
- can transmit information about color
What are the characteristics of M-type ganglion cells?
- 5% population
- large receptive fields
- transient burst of APs to the presentation of visual stimuli
- fast conduction velocity
- more sensitive to low contrast stimulus
- cannot transmit information about color
What is the function of P-type ganglion cells?
important for high spatial resolution vision the detailed analysis of the shape, size, and color of objects
What is the function of M-type ganglion cells?
high temporal resolution, such as evaluating the location, speed and direction of a rapidly moving object
How are images displayed on the retinal surface?
inverted and left–right reversed
Where do points in the binocular portion of the left visual field fall?
on the nasal retina of the left eye and the temporal retina of the right eye
Where do points in the binocular portion of the right visual field fall?
on the nasal retina of the right eye and the temporal retina of the left eye
Where to the axons of ganglion cells in nasal and tempoal retina go?
The axons of ganglion cells in the nasal retina cross in the optic chiasm, whereas those from the temporal retina do not
As a result, the right optic tract carries information from the left visual field, and the left optic tract carries information from the right visual field
What would result in the visula field from a deficit originating from the right optic nerve?
the nasal and temporal hemiretina of the right eye are affected
What would result in the visula field from a deficit originating from the right optic chiasm?
the nasal hemiretina of both eyes are affected
What would result in the visula field from a deficit originating from the right optic tract?
the nasal hemiretina from the left eye and the temporal hemiretina of the right eye are affected
What is anopsia?
Large visual field deficit
What is the function of the lateral geniculate nucleus?
send information to the primary visual cortex
What is the organization of the lateral geinculate nucleus?
receives inputs from both eyes, but the inputs are segregated in separate layers
What are the two layers in the lateral geniculate nucleus?
magnocellular (1-2)
- M-type ganglion cells
parvocellular layers (3-6)
- P-type ganglion cells
What visual fields do the right and left LGN project?
right - left visual field: left nasal retina and right temporal retina
left - right visual field: right nasal retina and left temporal retina
Where do axons carrying information about the superior portion of visual field travel?
temporal lobe
Where do axons carrying information about the inferior portion of visual field travel?
parietal lobe
What does lesion of the optic radiation fibers that curve into the temporal lobe cause to the visual field?
loss of vision in the upper quadrant of the contralateral visual hemifield in both eyes
What does partial lesions of the visual cortex cause to the visual field?
deficits in portions of the contralateral visual hemifield
The central area of the visual field tends to be unaffected by cortical lesions because of the extent of the representation of the fovea (macular sparing)
How many layers is the striate cortex divided into?
six principal cellular layers
1, 2/3, 4A, 4B, 4C, 5, 6
What is layer 4C of the striate cortex dominated by?
spiny stellate neurons, whose dendrites are confined to this layer
What is the most numerous cell typr in the neocortex of the striate cortex?
Pyramidal cells with prominent apical and basilar dendrites are the most numerous cell type in the neocortex; they are located in all layers except 4C
Where to LGN axons mostly terminate in the striate cortex?
Lateral geniculate axons terminate most heavily in layers 4C and 4A, with less dense projections to layers 1, 2/3, and 6; the terminations in layer 2/3 are “patchy.”
What layer gives rise to axons that terminate more superficaly?
Laminar organization of major intracortical connections. Neurons in layer 4C give rise to axons that terminate in more superficial layers
Where do the parvocellular and magnocellular layers arrive in the visual cortex?
The parvocellular layers project to layer IVCβ and the magnocellular layers to layer IVCα
In addition, the afferents from the ipsilateral and contralateral layers of the lateral geniculate nucleus are segregated into alternating ocular-dominance columns
Where does the mixing of pathways from two eyes first occur?
striate cortex
Axons project from layer IVC to more superficial layers. Most layer III neurons receive binocular input from both left and right eyes. There are layer III neurons with responses dominated by the right eye, left eye, or roughly equally responsive to input from the two eyes
Ocular dominance columns contain neurons with input dominated by one eye, and the columns alternate between left and right eye dominance
What it binocular disparity?
Depth is computed from the positions at which images occur in the two eyes
The image of an object lying in the plane of fixation falls on corresponding points on the two retinas
The images of objects lying in front of the plane of fixation or behind it fall on noncorresponding locations on the two retinas
How are visual cortex neurons are selective for particular ranges of disparity?
Some neurons are tuned to a narrow range of disparities and thus have particular disparity preferences (tuned excitatory or tuned inhibitory neurons), whereas others are tuned broadly for objects in front of the fixation plane (near cells) or beyond the plane (far cells)
How are visual cells in the LGN different from those in the cerebral cortext?
Visual cells in the lateral geniculate nucleus have concentric receptive fields, like those of retinal bipolar cells and ganglion cells
Visual cells in the cerebral cortex are more responsive to bars of light and show orientation specificity
What is orientation selectivity?
The responses of an orientation-selective neuron are monitored as visual stimuli are presented in its receptive field
The visual stimulus is a bar of light
Light bars of various orientations elicit very different responses
What are the receptive fields of neurons displaced along the radial axis of the cortex?
are centered on the same region of visual space and exhibit similar orientation preferences
What are the receptive fields of neurons displaced along the tangential axis of the cortex?
exhibit an orderly progression of receptive field properties
What is direction selectivity?
With a bar stimulus at the optimal orientation, the neuron responds strongly when the bar is swept to the right but weakly when it is swept to the left
What is a simple cell receptive field?
The response of a simple cell to optimally oriented bars of light at different locations in the receptive field
Notice that the response can be ON or OFF depending on where the bar lies in the receptive field
What is a complex cell receptive field?
a complex cell responds best to a bar of light at a particular orientation. However, responses occur to both light ON and light OFF, regardless of position in the receptive field
Explain the model proposed for summation of center–surround inputs to generate the receptive field of a simple cell with orientation selectivity
Activity from simple cells summates to generate the receptive field properties of complex cells, the next level of neuronal processing
What is the area V5 (MT) responsible for?
responsive to visual motion
What are characteristics of neurons beyond V1?
have more complex receptors fields -> repond to different forms of stimulus in the visual field
