Section 3 Visual System Flashcards
Light passes through what before gettign to the rods and cones of the retina?
lens, inner layer of ganglion cells and bipolar cells
Where do the photosensors (rods and cones) of the retina lie?
The outermost part of the retina
What are the output neurons of the retina?
Ganglion cells
Where do ganglion cells lie?
innermost in the retina closest to the lens and front of the eye
T or F? The retina contains interneurons.
T
Are there more rods or cones?
rods, about 20 times more
What color light responds best to short wavelenths?
Blue
What color light responds best to medium wavelengths?
Green
What color light responds best to long wavelengths?
red
Location of the fovea:
center of the eye
What is found in the central region of the fovea?
Only cones, the overlying retinal layers are displaced
T or F? Only a thin layer of rods are found in the fovea.
F. No rods, only cones
Under what conditions does the fovea fxn poorly?
Badly lit conditions
At their highest respective density, which has a higher density, rods or cones?
cones
To look at an object you move your eye so that the object falls:
on the fovea
Under what conditions does the periphery have a higher sensitivity?
Dimly-lit condiitons
Does the periphery have a high or poor resolution of small objects?
poor
What discs contain rhodopsin molecule in their membranes?
Those in the outer segment of rods (about 100 billion molecules per photoreceptor)
What are found in the discs in the outer segment of the rods?
Rhodopsin and CNG channels (cation)
When are high levels of cGMP found in the eye?
In the dark and keep these channels open.
T or F? A negative charge enters the rod
F. positive
What does the entry of a positive charge into the rod lead to?
NT release
How does light shut down NT release?
Light activates rhodopsin, activates transducin (a G protein), activates cGMP phosphodiesterase (PDE) which hydrolyzes cGMP, this closes the external channels and less current enters.
Does more light mean fewer or more channels are open?
Fewer
Under what conditions do the rods release less NT?
When light levels increase and the rod hyper polarizes
When fewer channels are open does the cell hypo or hyperpolarize?
hyper
Rhodopsin is to rods as ___ is to cones.
opsin
This is a hereditary disease of the retina, 10+ mutations in the phototransducation cascade, no cure:
Retinitis Pigmentosa
What does Retinitis Pegmentosa lead to?
Loss of peripheral vision and blindness
This is in clinical trials to treat Retinitis Pegmentosa:
gene therapy to replace a faulty rhodopsin gene
T or F? One photoreceptor synapses onto one postsynapitc interneuron.
F. many postsynapitc interneurons
Where do interneurons synapse?
Onto one antoher and onto ganglion cells
There are about __ million ganglion cells.
One
T or F? The features of the ganglion cells are the same throughout the fovea and periphery.
F.
T or F? The features of the ganglion cells are the same throughout the retina.
T.
Ganglion cells transmit info about:
spatial and temporal contrasts, ie changes in brightness and color or in patterns of light changing
What type of receptor field do most ganglion cells have?
Center-surround receptive field
About 50% of center-surround receptive field ganglion cells are:
ON-center OFF-surround
To what do ON-center OFF-surround ganglion cells respond best?
Small spot of light centered in the receptive field
To what do ON-center OFF-surround ganglion cells respond less?
To a larger spot of light bc it activated inhibitory inputs to the ganglion cell as well as excitatory inputs
To what do OFF-center ON-surround ganglion cells respond best ?
a small dark spot on a light background
Which type of cell repsonds well to an all-bright or all-dark field?
Neither ON-center or OFF-center
T or F? Both OFF-center and ON-center cells signal about contrast regardelss of the absolute light level.
T.
Do ON-center cells respond transiently or with a sustained response?
Either
Do OFF-center cells respond transiently or with a sustained repsonse?
Either
Other types of ganglion cells:
prefer spec combos and distributions of color in enter and suound (ie red vs. green), don’t have center-surround receptive fields, project to superior colliculus and are involved in eye move control, project to the suprachiasmatic nucleus, a region that controls circadian rhythms
Ganglion cells in the center of the fovea are as small as:
a fraction of a degree
Ganglion cell centers in the periphery can be up to:
5 degress in diameter
What determines different receptive field sizes?
different amounts of convergence from photoreceptor to interneurons (bipolar) to ganglion cells
Benefir to lots of convergence in the rod pwy:
spatial resolution is degraded (look fuzzy) but sensitivity to low levels of light is improved
To which portion of the brain doe the eye send info (where is area 17)?
striate cortex
About __ % of optic axons project to the lateral geniculate nucleus.
90%
How do the receptive field properties of the cells in the geniculate compare to those of the retinal ganglion cells?
almost the same
What is different about the layers of the LGN?
diff kinds of ganglion cells project to diff layers (ei color selective vs not will project to diff layers)
What does the lateral geniculate nucleus play a role in?
selective attention
Activation of the LGN comes from:
the cortex, brainstem, and thalamic ret for
When is transmission into the LGN reduced?
sleep and periods of inattentiveness, cells fire spontaneous bursts (why?)
Location of the primary visua cortex:
(Area 17, Striate cortex, V1) - the pole of the occipital lobe on the medial aspect of the hemisphere
What sulcus is adjacent to area 17?
calcarine sulcus
The largest area of the retinotopic map is devoted to:
the fovea
Where do axons from lateral geniculate terminate?
layer 4
To where does info distribute after it leaves area 4?
to excitatory or inhibitory, then lower layer for further processing
What type of arrangement of connections is there within cortical cells of the cortex?
Vertical
This is the basic input layer:
layer 4
Each cell outside of layer 4 responds to the same:
orientation
How ro respond to different orientation:
move to adjacent columns
Why is Layer 4 in the striate cortex exceptional?
because most cells are center-surround, with circular receptive field
What do the vast majority of cells in layers other than 4 respond best to?
straight bars or edges, some dark bars or lines, some light bars or lines
How is the cortex organized?
into columns (30-100 microns in diameter)
What is the basic unit in the striate cortex?
the orientation column
What do all of the cells outside of layer 4 in each column respond best to?
the same orientation, some direction selective, some not
T or F? All of the cells outside of layer 4 in the striate cortex are direction selective.
F. some are, some aren’t
How do the columns in the striate cortex differ?
in the orientation to which they respond best
Adjacent columns usually have cells with:
slightly different orientation preferences
What is the orderly pattern of the orientation columns occasionally interrupted by?
“color blobs”- columns of color-selective cells w center-surround receptive fields
About how many orientation columns are here er group?
20, respond to a small area of the visual field
How is the visual cortex divided?
into groups that respond to a small area of the visual field, so every part of the field is analyzed for stimuli
To how many regions does the striate cortex project?
more than 40
2 areas with different roles:
Parietal (“where”): nuclei about where things are relative to you and relative to each other, spatial aspects of the visual field
Is the “where” or the “what” stream in the temporal lobe?
“what”
in which lobe is the “where” stream?
parietal
Where area:
almost completely dominated by cells related to moving objects, different types of movement that the where stream
What does the “where” stream responds to?
circular or spiralling moves, visual flow (where you are going relative to the world), approaching or receding objects (egocentric)
Lesion in the where stream:
loss of ability to perceive the speed and motion of objects in your environment (ie traffic), ability to use visual info to grasp objects, visual neglect in peripersonal space`
Peripersonal space:
within arms reach
T or F? AD can lead to fxnal deficits where you can’t find things close to you.
T
Lesions of the “what” stream of the temporal cortex:
achromatopsia, can’t recognize colors
Inputs that the what stream recognizes:
color selective cells or cells that respond to complex shapes, ie tools, houses
The inability to perceive colors:
achromatopsia
T or F? Some areas the “what” stream have many cells that are color selective and some that respond to complex shapes.
T
What area of the brain is important for face perception?
The temporal cortex
Aspects of face perception that the temporal cortex recognizes:
face orientation, expression, identity, gaze perception
This area is particularly important for aspect of face perception:
fusiform face area
Prosopagnosia:
face blindness
Occipital (early visual stage) lesions:
can’t make sense of faces or distinguish bw pics of 2 different people
Anterior tempoal lesions:
people understand the characteristics of the face, but can’t identify who they belong to
Lesions of what side would lead to prosopagnosia:
R side lesions
