9.2 Flashcards
The Visual System's Functional Anatomy
What is the retina?
The light-sensitive surface at the back of the eye; consists of neurons and photoreceptor cells
What is a photoreceptor?
A specialized neuron that transduces light into neural activity.
What is the fovea?
Area at the center of the retina that is specialized for high acuity.
Allows for color discrimination and making out details.
Depressed part of the retina
What kind of photoreceptor is abundant in the fovea?
Cones. They are the most dense in the fovea.
There are no rods in the fovea.
What is the lens?
Where light in the eye is flipped upside-down and light-to-left
What is the optic disc?
The area of the retina where axons forming the optic nerve leave the eye and where blood vessels enter and leave.
Where is the blind spot in our eye?
The optic disc. There are no photoreceptors because the axons and blood vessels need to be there.
Slightly off center in both our eyes so we don’t have a hole in our vision.
What is papilledema?
Inflammation of the optic disc. Usually causes vision loss.
Photoreceptors convert light energy into what kind of energy?
Chemical (and then into neural activity)
What are the two types of photoreceptors in the eye?
Rods and cones.
How do rods and cones differ in shape?
Rods are longer and cylindrical on one end. Cones are shorter and have tapered ends.
(hence the names…)
What photoreceptor is used mainly for night vision?
Rods. They are sensitive to low levels of light (cones are responsive to bright light)
What photoreceptor is used for color and visual acuity (fine details)
Cones. (remember that the fovea is where they are concentrated and that’s the place with the most visual acuity)
How many types of pigments do cones have?
- Each cone only has one of these three pigments. (Red, green, and blue)
There are equal numbers of red and green cones but fewer blue cones.
What are the three types of cells in the first layer of retinal neurons? (these connect directly to photoreceptors)
bipolar, horizontal, and amacrine
What does a bipolar cell do?
Receive input from photoreceptors
What do horizontal cells do?
Links photoreceptors and bipolar cells
What do amacrine cells do?
Link bipolar cells and retinal ganglion cells (RGCs)
What are retinal ganglion cells?
One of a group of retinal neurons that form to the optic nerve.
Second neural layer of retinal neurons.
What are the two types of RGCs?
Magnocellular (M-cell) and parvocellular (P-cell)
What do M-cells do and where are they located in the eye?
Receive input from rods (so they are sensitive to light). They are found all over the retina, including periphery (so they are sensitive to moving stimuli).
Where do M-cells input to in the LGN of the Thalamus? (what layers)
Layers 1 and 2
What do P-cells do and where are they located in the eye?
Receive input from cones (so they are sensitive to color and fine detail). They are only found in the fovea.
Where do P-cells input to in the LGN of the Thalamus?
Layers 3-6
What are Mueller cells and what do they do?
A type of glial cell in the retina. They channel light to buried photoreceptors and help retina maintain homeostasis.
What’s the secret little third type of RGC and what does it do?
Melanopsin. They are sensitive to blue light and work in waking cycles. Synchronize circadian rhythms, regulate pupil size, regulate melatonin release
What is the optic chiasm?
The junction where the optic nerves partly cross.
What part of the optic nerves cross over to the other side of the brain and what parts stay on the same side?
The nasal (inside) half of the retinas cross over. The temporal (outside) half of the retinas stay on the same side.
What side of the visual field travels to the right hemisphere of the brain?
The left side of the visual field.
What are the three visual pathways in the brain?
Retinohypothalamic tract, geniculostriate system (V1), tectopulvinar system (V2).
What is the route of V1? (Geniculostriate system)
Eye -> optic nerve -> LGN -> layer IV of the striate cortex (aka: primary visual cortex) -> Visual related regions of either the temporal or parietal lobe
What is the purpose of the geniculostriate system?
To process the object’s image
What is the route of the tectopulvinar system? (V2)
M-cells -> superior colliculus -> pulvinar (in the thalamus) -> other visual cortical areas (temporal or parietal lobe)
bypasses occipital visual areas
What is the purpose of the tectopulvinar system?
Detects movements, tell us location of objects, directs eye movement
What do the 2 sub-layers of layer IV of the striate cortex process
(A) processes movement from M-cells
(B) processes color and form from P-cells - from layers 2-6 of the LGN
What is damaged in people with blindsight and why can they still perceive motion?
V1 is damaged. They can detect motion without consciously seeing it because the tectopulvinar pathway bypasses V1
What layer(s) of the LGN does information from the ipsilateral eye go to?
Layers 2, 3, and 5
What does ipsilateral mean?
on the same side of the body as another structure or point (e.g. right eye and right LGN)
What layer(s) on the LGN does information from the contralateral eye go to?
Layers 1, 4, and 6
What does contralateral mean?
Relating to the side of the body opposite to that on which a particular structure occurs (e.g. left eye and right LGN)
What is a cortical column?
Anatomic organization that represents a functional unit six cortical layers deep and approx. 0.5mm square, perpendicular the the cortical surface
How many regions of the occipital lobe are there?
5 (V3 is separated into 2 sublayers as well)
What is the secondary visual cortex?
Visual areas in the occipital lobe outside the striate cortex (V2-V5)
aka: Extrastriate cortex
What are blobs?
Regions in V1 that contain color-sensitive neurons, as revealed by staining for cytochrome oxidase
What do blobs do?
Take part in color perception
Preliminary color processing
What are interblobs?
The regions between the blobs in V1
What do interblobs do?
Participate in perception of form and motion
In V2, where do the thin stripes receive information from?
color-sensitive neurons from V1 (blobs)
In V2, where do thick stripes get their information from?
Movement-sensitive neurons (interblobs)
In V2, where do pale zones get their information from?
form-sensitive neurons in V1 (interblobs)
What is the route of the dorsal visual stream?
LGN -> striate cortex -> V2 -> V3A (form) and V5 (motion) -> parietal lobe (PG)
What is the route of the ventral (what) visual pathway?
LGN -> V1 -> V2 -> V3 (dynamic form) and V4 (color form) -> Temporal lobe (TE)
What’s the pathway of processing form of an object?
LGN -> interblobs (V1) -> pale zones (V2) -> V3A (dorsal stream) and V3+V5 (ventral stream) -> PG and TE
What is the pathway of processing color?
LGN -> blobs -> thin stripes -> V4 (ventral stream) -> TE
What is the pathway for processing motion?
LGN -> interblobs -> thick strips -> V5 (dorsal stream) -> PG
What happens if there is damage to the fusiform face area in the Temporal lobe?
Facial agnosia - inability to recognize faces
What happens if there is damage to the parahippocampal place area (PPA) in the temporal lobe?
Environmental familiarity agnosia - inability to recognize places that should be familiar
What is visual form agnosia (appreciative agnosia)? What part of the brain is damaged?
Inability to recognize objects or line drawings of objects
can draw objects from memory but can’t copy images
Damage to LO region of the temporal lobe
What is simultagnosia?
The inability to see more than one object or aspect in one’s visual field
What is associative agnosia? What part of the brain is damaged?
Inability to identify objects despite apparent perception
can copy drawing accurately but can’t draw an object from memory
Damage from bilateral lesions in areas like the anterior temporal lobe.
What is alexia? What area(s) of the brain is damaged?
Inability to read.
Damage to LEFT fusiform and lingual gyrus ONLY LEFT HEMISPHERE
What happens when there is damage to the Lateral intraparietal area (LIP)?
Inability to focus visual attention (where you should focus your eyes)
What happens when there is damage to the anterior intraparietal area (ALP)?
Inability to grasp things properly; can’t form hand properly
What is optic ataxia? What part of the brain is damaged?
difficulty reaching for objects; there will be tremors and won’t reach in a smooth, straight line.
Damage to the parietal reach region (PRR)
What is ideomotor apraxia? What part of the brain is damaged?
Inability to make coordinated, purposeful movement
e.g. packing a bag, using tools, brushing teeth, mimicking gestures
Neurons that project into the brain from the retina and form the optic nerve are called ______
Retinal ganglion cells
The two major pathways from the retina into the brain are ______ and _______
geniculostriate; tectopulvinar
Damage to the fusiform face area in the temporal lobe can produce ________
facial agnosia
Contrast the paths and functions of the dorsal and ventral streams
The dorsal stream to the parietal lobe processes the visual guidance of movements (the how). The ventral stream to the temporal lobe processes the visual perception of objects (the what)
